Brachial plexus Anatomy: 

• Formed by the ventral primary rami (spinal nerves or roots) from the cervical segments at C5, C6, C7, C8, and T1

  • Prefixed plexus (C4-C7; when C4 contributes a branch to the brachial plexus) seen in approximately 2/3rds of cases.

  • Postfixed plexus (C6-T2).

The brachial plexus is located between the lower neck and axilla, running behind the scalene muscles proximally and behind the bony clavicle and the pectoral muscles distally. 

• Parts of the brachial plexus: mnemonic (Robert Taylor Drinks Cold Beer) - Root, Trunks (upper, middle, and lower), Divisions (anterior and posterior), Cord (lateral, posterior, and medial), Branches.

  • 5 roots, 3 trunks, 6 division (two from each trunk), and 3 cords.

• Roots and peripheral nerves are not considered part of the plexus proper.

Roots:  

• Two nerves that innervate the upper extremity branch off the nerve roots.

  • The dorsal scapular nerve, which innervates the rhomboids and levator scapulae, arises from the C5 nerve root. 

  • The long thoracic nerve, which innervates the serratus anterior, arises from the C5, C6, C7 roots.

  • Assessment of these two muscles is important both clinically and electrically, in differentiating a severe lesion at the level of the plexus from one affecting these roots. 

Trunks: 

• The ventral rami of the C5 to T1 nerve roots join  together above the level of the clavicle to form the the 3 trunks of the brachial plexus. 

• Upper (or superior) trunk:

  • Formed from the C5-C6 nerve roots, gives off  two branches: the suprascapular nerve, that innervates the supraspinatus and infraspinatus muscles, and the nerve to subclavius muscle. 

  • The point where the C5 and C6 nerve roots meet is called Erb’s point. 

• Middle trunk:

  • Formed from the C7 root. There are no branches off the middle trunk. 

• Lower (inferior trunk):

  • Formed from the C8 and T1 roots. There are no branches off the inferior trunk. 

The trunks then divide into anterior and posterior divisions. 

• Total: 6 divisions - 2 from each trunk.

Cords and nerves:

• Located below the level of clavicle.

• The cords are named according to their position to the axillary artery. 

• Lateral cord is formed from the anterior divisions of the upper and middle trunk, and therefore carries fibers from C5 to C7. 

  • The lateral cord gives rise to the lateral pectoral nerve, which innervates the pectoralis major muscle. 

  • The lateral cord ends as two nerves, the median nerve (which also receives a contribution from the medial cord), and the musculocutaneous nerve, which contains the lateral antebrachial cutaneous (lateral cutaneous nerve of forearm) nerve. 

• Posterior cord:

  • Formed from the union of the posterior divisions of the upper, middle, and lower trunk. It carries fibers from C5 to C8. 

  • Three nerves arise from the posterior cord: 

    1. Upper subscapular nerve, which contains predominantly C7 and C8 fibers and innervates the subscapularis muscle

    2. Lower subscapular nerve, which contains C5 and C6 fibers and innervates the teres major and the lower part of the subscapularis muscles

    3. Thoracodorsal nerve, which contains C6, C7, and C8 fibers and innervates the latissimus dorsi muscle. 

    4. The posterior cord ends as two nerves, the axillary nerve, and the radial nerve. 

• Medial cord:

  • It is a continuation of the anterior division of the lower (inferior) trunk. The medial cord gives off three nerves: 

• • • Medial pectoral nerve, which contains predominantly C8 and T1 fibers and innervates the pectoralis minor muscle.

    • Medial brachial (arm) cutaneous nerve, which provides sensory innervation to the medial arm

    • Medial antebrachial (forearm) cutaneous nerve, which provides sensory innervation to the medial forearm. 

    • The medial cord gives fibers to the median nerve and then continues as the ulnar nerve.

• 5 most important nerve branches of brachial plexus: Radial, axillary, musculocutaneous, median, and ulnar (RAMMU)

• Posterior cord, nerve branches: mnemonic (ARTS) - axillary, radial, thoracodorsal, subscapular (upper and lower)

• Muscles innervated by musculocutaneous nerve (C5-C7): mnemonic (BBC): biceps, brachialis, coracobrachialis.  It continues as the lateral antebrachial cutaneous sensory nerve.

• Intrinsic muscles of hand: Thenar, hypothenar, lumbricals, interossei

  • Thenar: Opponens pollicis, abductor pollicis brevis, adductor pollicis, flexor pollics brevis (superficial and deep heads).

  • Hypothenar: Opponens digiti minimi, abductor digiti minimi, flexor digiti minimi

  • Intrinsic muscles of hand are innervated by ulnar nerve except LOAF (mnemonic) muscles  which are innervated by the median nerve after it passes through the carpal tunnel:

  • Palmar and dorsal interossei are innervated by ulnar nerve

• • • 3 Palmar interrosei adduct the fingers

    • 4 Dorsal interossei abduct the fingers

• • C8-T1: to all intrinsic muscle of hand

• Different thumb actions:

  • Radial: Thumb abduction in the plane of palm (abductor pollicis longus)

  • Ulnar: Thumb adduction in the plane of palm (adductor pollicis)

  • Median: Thumb abduction perpendicular to the plane of palm (after the nerve passes through the carpal tunnel), opposition of thumb to other fingers (opponens pollicis), and flexion of thumb (flexor pollicis longus - superfical head) 

• T1 nerve root lesion causes a deep aching sensation in the shoulder joint, axilla, and medial side of the upper arm down to the olecranon. There is a loss of intrinsic hand muscles, including the abductor pollicis brevis, which differentiates T1 nerve root lesions for ulnar nerve lesions. In ulnar nerve lesions, all intrinsic muscles of the hand are affected except the abductor pollicis brevis (innnervated by median nerve).

Radial nerve

• Radial nerve: receives contribution from all 3 trunks of the brachial plexus and, correspondingly, gets contribution from each of the C5-T1 nerve roots. 

  • It derives from the posterior cord in the distal axilla and exits the axilla as the radial nerve.  It descends into the arm, runs between the long and medial heads of the triceps muscles, and curves around the humerus in the spiral groove.   Prior to reaching the spiral groove, it gives off the inferior lateral cutaneous nerve of the arm and the posterior cutaneous nerve of the arm.  It then gives off the posterior cutaneous nerve of the forearm and motor branches to the 3 heads of the triceps brachii (medial, long, and lateral) and anconeus (extension of the medial head of the triceps) but a separate muscle.  The radial nerve then wraps around the spiral groove over the posterior aspect of humerus, to descend into the anterior aspect of the arm by piercing the lateral intermuscular septum.  It gives of muscular branches to the brachialis (this muscle is predominantly innervated by the musculocutaneous nerve), brachioradialis and extensor carpi radialis longus (ECRL) muscles.  Next, 3 - 4 cm distal to the lateral epicondyle, it bifurcates into two separate nerves: superficial radial sensory nerve and the deep radial motor branch.  

  • The superficial radial sensory nerve descends distally into the forearm over the radial bone to supply sensation over the dorsolateral aspect of the hand as well as the thumb and dorsal proximal phalanges of the index, middle, and ring fingers. The nerve can be felt running over the extensor tendons to the thumb at the anatomical snuffbox.

  • The deep radial motor branch first supplies the extensor carpi radialis brevis (ECRB) and supinator muscles before entering the supinator muscle under the Arcade of Frohse, which actually is the proximal border of the supinator.  The arcade of Frohse can be tendinous or a fibrous aponeurosis in some cases. Once the radial nerve enters the supinator (after supplying it) it is  referred to as the posterior interosseous nerve (PIN).

  • Radial nerve/PIN (mnemonic): BEATS 6E-A: 

• • • Radial: Triceps (long, lateral, and medial heads), brachioradialis, ECRL, ECRB, anconeus, supinator

    • Posterior interosseous nerve (PIN) branch of radial nerve: EDC, ECU, APL, EIP, EPL, EPB.  

      • PIN is considered to be purely motor nerve, but not in the strictest sense as it supplies sensory fibers that perceives deep sensation from the interosseous membrane and joints between radial and ulnar bones. 

Median Nerve

• Median nerve/AIN (mnemonic): It is formed in the distal axilla by the joining of the lateral and medial cords of the brachial plexus. Upon exiting the axilla, the terminal median nerve becomes the median nerve.  It descends along the medial aspect of the arm to the antecubital fossa.  There are no branches in the arm.  In the antecubital fossa, the median nerve travels medially to the brachial artery.  As it enters the forearm, it runs first beneath the lacertus fibrosus (bicipital aponeurosis), a thick fibrous band that runs from the medial aspect of the biceps tendon and blends with the fascia covering the proximal forearm flexor muscles.  The median nerve then runs between the two heads of the pronator teres muscle. It gives off branches to the pronator teres, FCR, PL, and FDS.  5 – 8 cms distal the medial epicondyle, after it has passed between the two heads of PT, the median nerve gives off the AIN, which is the largest branch of the median nerve. The AIN innervates the FPL, FDP to digits 2, and 3, and PQ.  It is a motor nerve, in that it does not carry any cutaneous sensory fibers, however it carries deep sensory fibers to the interosseous membrane and to the wrist joint. The median nerve runs distally, passing under the sublimis bridge (proximal aponeurotic tendinous edge of the FDS muscle.) 

  • Median nerve in forearm: 2Ps and 2Fs: pronator teres, palmaris longus, FCR, FDS

  • It then gives off the anterior interosseous nerve: Remember O.K sign.  FPL, FDP (2nd and 3rd digit), pronator quadratus and then continues distally in the forearm.  Just proximal to the wrist, it gives off the palmar cutaneous nerve (innervates the skin overlying the thenar eminence) before the median nerve traverses the carpal tunnel.  After exiting the carpal tunnel, it gives off the recurrent thenar nerve, which innervates the thenar eminence muscles.  The remaining sensory and motor axons form a number of nerves.  The first common palmar digital nerve gives off three sensory branches (two to the volar aspect of the thumb and one to the radial aspect of the index finger) and one motor branch to the first lumbrical muscle.  The second common palmar digital nerve provides sensory branches to the adjacent  sides of the index and middle fingers and motor branch to the second lumbrical muscle.  The third common palmar digital nerve provides sensory branches to the adjacent sides of the middle and ring fingers.  This is the traditional pattern of median innervation (i.e. fourth digit splitting).  

  • Median nerve in hand: LOAF muscles:

• • • Lumbrical (I, II) 

    • Opponens pollics

    • Abductor pollicis brevis

    • Flexor pollicis brevis (superficial head)

  • Median nerve: Derived from the lateral and medial cords of the brachial plexus.

• • Lateral cord: C6-C7

    • Sensory fibers to thenar eminence, thumb, index, and middle fingers

    • Motor fibers to proximal median forearm muscles

  • Medial cord: C8-T1

    • Sensory fibers to lateral half of the ring finger.

    • Motor fibers to median/AIN innnervated (FDP-II and III, FPL, and PQ) of the distal forearm and hand (LOAF).

  • Median sensory nerve fibers from the thumb and index finger travel through the upper trunk.

• Median motor nerve fibers innervating the APB travel through the lower trunk.

Ulnar nerve

• The ulnar nerve is essentially derived from the C8 and T1 roots, although some anatomic dissections have also demonstrated a minor component from C7.  Accordingly, nearly all ulnar fibers travel through the lower trunk of the brachial plexus and then continue into the medial cord.  The terminal extension of the medial cord becomes the ulnar nerve.  The medial brachial and medial antebrachial cutaneous sensory nerves and a large contribution to the median nerve are derived from the medial cord as well.    The medial cord gives off the ulnar terminal nerve, which exits the axilla as the ulnar nerve and descends along the medial aspect of the arm towards the elbow.  It does not branch in arm.   It pierces the the medial intermuscular septum in the mid-arm and then passes through the arcade of Struthers, which is composed of deep fascia, muscle fibers from the medial head of the triceps, and the internal brachial ligament. The ulnar nerve then travels medially and distally toward the elbow.  As it approaches the elbow, it enters a groove between the medial epicondyle and the olecranon process, which it traverses to enter the cubital tunnel.  The roof of the tunnel consists of the aponeurosis that extends from the medial epicondyle to the olecranon process and the floor of the tunnel consists of the medial ligament of the elbow.  It then passes between the two heads of the FCU muscles, at which point it lies on top of the FDP.  At this level, it gives off motor branches to the FCU and FDP (3, and 4) and then continues distally down the forearm.  In the distal distal half of the forearm, it gives off the palmar cutaneous branch (supplies the skin overlying the hypothenar eminence), followed by the dorsal ulnar cutaneous branch (supplies the dorsomedial aspect of the hand and the dorsal aspect of the medial 4th and 5th digits.  It enters the wrist, gives off the superficial terminal branch (supplies the medial aspect of the palm, distally, and the palmar aspects of the medial 4th and 5th digits).  It then continues as the deep muscular branch which enters the Guyon's canal (between the pisiform bone medially and the hook of hamate laterally) and innervates the ulnar hand intrinsic muscles. 

  • 3 Fs in forearm: FCU, FDP (4th and 5th digits)

  • LOAF-OAF in hand: Lumbricals III and IV, 4 Dorsal interosseous (FDI), 3 palmar interossei, Adductor pollicis, Flexor pollicis (deep head), opponens digiti minimi, abductor digiti minimi (quinti), flexor digiti minimi.

Brachial plexus, upper trunk (Erb-Duchenne Palsy):

• Loss of function of C5-C6 innervated muscles due to injury to C5-C6 nerve roots or upper trunk.

• Obstetric brachial plexopathy most commonly involves the C5 and C6 nerve fibers (Erb’s palsy), with adduction and internal rotation of the arm and pronation and extension of the forearm. This position reflects weakness of muscles belonging to the C5 and C6 myotomes (supraspinatus, infraspinatus, deltoid, teres minor, supinator, biceps).  A flail arm occurs with C5 through T1 nerve fiber involvement.  Finger and hand movements are relatively spared

  • Waiter's tip.

• Common causes: 8% - 23% of shoulder dystocia (obstetric brachial plexopathy) cases; motor cycle accident; radiation, radiation to neck, idiopathic brachial plexitis (neuralgic amyotrophy or Parsonage-Turner syndrome).  If the arm is located at the side of the patient at the time of impact or is distracted away from the torso, the upper roots and plexus are preferentially injured. 

  • 80% of brachial plexus injuries.

• DDx: traumatic avulsion of C5 and C6 nerve roots, C5 and C6 radiculopathy

• The upper trunk of the brachial plexus is best assessed with at least 2 motor and 3 sensory (median, ulnar, and radial) nerves.  Whenever the superficial radial response is reduced it is indicative of axonal loss.  Because the sensory axons subserving the superficial radial nerve derives the C6 and C7 DRG, the potential lesion sites include the superficial radial nerve, the radial nerve, the posterior cord, and the sensory elements of the upper and middle plexus.  Add the LABC and median D1 NCS to shorten the list of potential localization sites.  Always check the contralateral side as well.  The cell bodies of the lateral antebrachial cutaneous nerve and median sensory nerve to the thumb are located in the C6 dorsal root ganglion.  If the LABC and median-D1 responses are absent, their involvement eliminates the superficial radial nerve, radial nerve, posterior cord, and middle plexus as potential lesion localization sites.  Thus, at this point the lesion is axon loss in nature and involves the upper plexus (for the sensory fibers, this includes the upper trunk, C6 APR, and C6 DRG).  Whether the C5 DRG derived sensory axons are also affected is unclear because there are no sensory NCS available to assess the C5 sensory axon traversing the upper plexus.  At this point, the motor NCS can be performed.  Given the upper plexus localization and the need to better determine lesion severity, in addition to the routine motor NCS (median and ulnar), the axillary-deltoid and musculocutaneous-biceps NCS are added bilaterally.  At this point, the needle EMG study can be performed.  Additional muscles in the upper plexus domain will be added, along with some contralateral muscles for comparison purposes.  Muscles tested: Deltoid, biceps, triceps, pronator teres, FCR, brachioradialis, FPL, FDI, EI, infraspinatus, rhomboids, lower cervical paraspinals, high thoracic paraspinals.  Also test contralateral muscles to include brachioradialis, PT, and infraspinatus.

• Commonly affected motor studies include the axillary nerve with recording at the deltoid muscle and the musculocutaneous nerve with recording at the biceps muscle. The cell bodies of the axillary and musculocutaneous motor nerves are located in the anterior horn cells of C5 and C6.  Because the sensory axons subserving the median D2 sensory NCS are only abnormal in 20% of upper plexus lesions, they are not useful for screening this element of the brachial plexus.  The musculocutaneous and axillary motor NCS are useful for estimating lesion severity with upper plexopathy.  If the infraspinatus and rhomboids are spared it suggest that the lesion involves the upper trunk because the suprascapular nerve exits the upper trunk just distal to it formation, and, thus, is often spared with upper trunk lesions.  Because the dorsal scapular nerve (like the long thoracic nerve) exits from the APR level of the plexus, the muscles that it innervates are spared with the upper trunk lesions. 

Brachial plexus, lower trunk (Dejerine-Klumpke's Palsy):

• Traction injury to arm especially arm traction in an abducted position. Ifthe arm is elevated above the head, the lower trunks and roots are most vulnerable.  Neoplastic plexopathy due to local tumor invasion like Pancoast's tumor of the lung or metastases, breast cancer, lymphoma; thoracic outlet syndrome. Common peripheral nervous system complication of CABG. Rare plexus injury.  

• Weakness of C8-T1 innervated muscles

• Hand and finger extension weakness from weakness of the intrinsic hand muscles, forearm extensor, and wrist and finger flexors.

  • Claw-like deformity, elbow is flexed, forearm is supinated, and wrist is extended.

• Atrophy of hypothenar muscles, sensory loss of the ulnar aspect of the hand and forearm.

• If T1 is damaged proximal to the sympathetic trunk, there may be associated Horner's syndrome.

• Pain is prominent in neoplastic plexopathy

• DDx: ulnar neuropathy, C8-T1 radiculopathies.

• Test median nerve (motor) to APB, to distinguish between a lower trunk and an ulnar nerve lesion

• The median and ulnar nerves are derived from the C8-T1 segments and are helpful in assessing the lower trunk of the brachial plexus. 

• Test the radial innervated muscles, EDC and EIP to distinguish lesions of the lower trunk from medial cord.

• Generally hand weakness suggest C8 and T1 motor nerve fiber involvement.  Perform sensory nerve conduction studies to include median-D2, ulnar-D5, superficial radial.  The left ulnar sensory responses if absent indicates an axonal loss lesion involving the ulnar nerve, medial cord, or C8 fibers of the lower plexus.  If the other responses are normal, define the locus of the lesion by performing additional NCS.  Whenever the ulnar-D5 sensory responses abnormal, add the MABC NCS to best assess the medial cord and lower plexus to assess the more proximal potential locations of the lesion.  When the MABC responses normal, the DUC NCS is done to better define its most distal potential location.  Also add the contralateral ulnar-D5, if the ipsilateral MABC response is abnormal, the contralateral MABC NCS needs to be added.  Absence in the MABC response indicates axonal loss process lies proximal to the ulnar nerve.  Does the lesional localizes to the medial cord or the lower plexus.  Since more accurate localization is not available through the sensory NCS because it does not differentiate between a medial cord localization and a lower trunk localization, add motor NCS using radial-EI NCS for if it is abnormal, excludes a medial cord lesion (because it assesses the posterior cord, not the medial cord), and when it is normal, it does not further localize the lesion (it is normal with the medial cord lesion and with partial lower plexus lesion).  Adding the radial-EI NCS ipsilaterally (for localization purposes) and, for severe reassessment, the contralateral radial-EI, contralateral ulnar-ADM, and bilateral ulnar-FDI NCS is performed.  The left median, both ulnar, and radial responses are severely reduced in amplitude consistent with an axon loss process.  Involvement of the radial motor response excludes medial cord localization, thereby localizing the lesion to the lower plexus.  At this point, the needle EMG study can be performed.  APB, FDI, EI, FPL, and triceps are abnormal.  The EMG identifies abnormalities in the muscles of the lower plexus domain.

Traumatic plexopathies and time for intervention

• Treatment of traumatic brachial plexus injuries depends on the type and severity of injury, the location, and the time frame. 

  • Traumatic neuropathies due to blunt trauma (non-open, non-lacerating injuries) that fail to demonstrate clinical or electrodiagnostic evidence of reinnervation are considered for surgical exploration and repair 3 weeks to 3 months after injury. 

  • Injuries associated with sharp penetrating trauma are more likely associated with severing of nerves and should be repaired within 72 hours if possible. 

  • Most closed injuries are due to neuropraxia or axonotmesis and may recover spontaneously.  As a result, they are initially treated conservatively with physical and occupational therapy.  Patients are followed closely with serial clinical and electrodiagnostic assessments to assess for recovery.  If patients show does not show signs of recovery after 2-3 months in upper trunk lesions or 4-5 months for middle or lower trunk lesion, then surgical intervention should be considered. 

• Surgical repair performed greater than 1 year after injury is less successful due to resultant denervation muscle atrophy. 

• Neuropathies with nerve laceration or transection that occur following trauma may warrant open repair soon after injury.

Brachial Plexus  EDx Table

Traumatic Brachial Plexus Root Avulsion With Pseudomeningocele: A Case Series - PMC (nih.gov) 

Pearl regarding FCR:

• FCR is a C6/7 median nerve-innervated muscle.  The C7 derived motor axons innervating this muscle traverse the middle trunk and its anterior division, the lateral cord, the lateral head of the median nerve, and the median nerve.  Thus, this muscle would be expected to be involved with a complete middle trunk lesion and spared with a complete posterior cord lesion.

Anomalous innervations

The Martin-Gruber anastomosis:

• Occurs in 15% to 30% of the population. 

• It consists of a communicating branch from the median nerve, or from the anterior interosseous nerve that pass into the ulnar nerve in the mid-forearm. 

• It consists of motor nerve fibers only.

• The median nerve fibers that have crossed over to the ulnar nerve, run along with it, to innervate the following ulnar innervated muscles:

  • First dorsal interosseous, adductor pollicis, flexor pollicis brevis (deep head) and abductor digiti minimi. 

• There are 3 types of MGA:

  • The most common form of MGA, by far, is for the anastomosis to innervate the FDI, (Type II).

    • Recording at FDI and stimulating the ulnar nerve at wrist site results in large CMAP amplitude when compared to stimulation of ulnar nerve at below elbow site, giving an impression of a conduction block between wrist and elbow nerve segment.  Stimulation above elbow is also the same.

    • To demonstrate this MGA, stimulate the median nerve at wrist and AF while recording at FDI.

      • The CMAP evoked with median nerve stimulation at wrist is smaller than CMAP evoked with stimulation of median nerve at AF.

  • CMAP recorded at ADM with stimulation at ulnar nerve at wrist site shows higher CMAP recorded than CMAP recorded with stimulation of ulnar nerve below-elbow site.  This gives the impression of a conduction block at below elbow site, (Type I).

    • This does not usually cause diagnostic confusion during routine ulnar NCS, which are recorded from hypothenar muscles. 

    • To demonstrate a MGA, stimulate the median nerve at the wrist and AF while recording at ADM.

      • There is no CMAP with median nerve stimulation at wrist

      • There is CMAP evoked with median nerve stimulation at AF.

  • CMAP recorded at ABP with stimulation of median nerve at wrist site results in a CMAP which is lower in amplitude than the CMAP which results from stimulation of median nerve at AF, (Type III), least common.

    • To demonstrate a MGA, one must then stimulate the ulnar nerve at wrist and below elbow sites while recording from APB.  

    • If no MGA exists:  Normally, ulnar stimulation at the wrist with recording at APB would evoke a CMAP, usually with an initial positive deflection ("dip").  The dip reflects the ulnar innervated muscles in the thenar eminence.  If no MGA exists the ulnar nerve stimulation at below elbow site will evoke a CMAP with the same morphology and amplitude as the CMAP evoked with ulnar nerve stimulation at wrist.

    • If MGA is present, the CMAP amplitude with ulnar nerve stimulation at wrist will be larger than the CMAP amplitude with ulnar nerve stimulation at below elbow.  

    • In a patient with CTS and a MGA, the median nerve stimulation at AF results in CMAP recorded from the APB with an initial positive deflection ("dip") while stimulation of median nerve at wrist site does not.

• A prolonged distal motor latency following median nerve stimulation at the wrist occurs because of the CTS.

• The median nerve activation at the elbow (Ante-cubital fossa) results in action potentials traversing nerve fibers contained in the median nerve (AIN) which cross-over to the ulnar nerve at the mid-forearm.  These action potentials generated with stimulation of the median nerve at the elbow, therefore, traverse the ulnar nerve and reach the ulnar innervated thenar muscles (AP and FPB - deep head), before the arrival of the action potentials down the median nerve innervating the thenar muscle (APB).  This delay occur because of the CTS induced median nerve fibers' delay. Therefore, a CMAP is first generated from the ulnar innervated thenar muscles producing a CMAP with an initial positive dip because the active recording electrode is positioned over the APB's motor point and not the underlying initially activated ulnar innervated thenar muscles.  Shortly thereafter, the muscles supplied by the median nerve generate a temporally delayed CMAP that summates with the ulnar induced CMAP.  The summated CMAP is a compound motor unit action potential  of ulnar and median innervated thenar muscles, accounting for the typically larger CMAP with elbow stimulation when compared to the CMAP generated from median nerve stimulation at the wrist. 

• A higher, sometimes very high, CV is recorded in the forearm.  This is not because of faster conduction along axons but is an artifact of how the CV is calculated.  CV is calculated as: 

  • CV = Distance / (Prox latency - Distal latency).

• In the setting of CTS with MGA, the distal latency is markedly prolonged (e.g. 6 ms).  However, the proximal latency is not prolonged because it can bypass the carpal tunnel along the ulnar nerve rather than the median nerve (e.g. 7 ms).  This difference in latencies is thus very short (1 ms), and therefore the calculated CV becomes erroneously high (e.g., 250 mm which is the distance between the distal proximal stimulation sites, divided by the difference in latencies between proximal and distal stimulation sites, which in this case is 1, results in 250/1 = 250 m/s).

Riche-Cannieu anastomosis is a communication in the palm between the recurrent motor branch of the median nerve and the deep branch of the ulnar nerve. The result is dual innervation of some intrinsic hand muscles such as the first dorsal interosseous, adductor pollicis, and abductor pollicis brevis. "All ulnar hand" results and affects only motor nerves. Stimulating the ulnar nerve at wrist, while recording at thenar results in all motor response.

Accessory deep peroneal nerve: Extensor digitorum brevis muscle is normally innervated by deep peroneal nerve.  In 28% of people (present bilaterally in 57% of these individuals), the superficial peroneal nerve supplies an extension to innervate the lateral slip of EDB muscle, in the form of communicating branch called an accessory peroneal nerve (it travels posterior to the lateral malleolus to innervate the EDB). When present, stimulation at the anterior ankle will result in CMAP that is smaller than when stimulated below the fibular head. Stimulation behind the lateral malleolus will result in a CMAP which is roughly equal to the difference between the anterior ankle and fibular head stimulation sites. 

DDx of withered hand - Wasting of muscles of hand

A LMN lesions is a strong possibility unless there is an arthropathy leading to disuse atrophy.  

C8-T1 involvement.  

Check joint positions in MCP, and ICP joints

Claw hand deformity

Striatal hand of PD

Corticobasal degeneration

Lesions of AHC (C8-T1):  

• MND (fasic, spasticity, paresis, tongue fasic, no sensory signs)

• Syringomyelia: dissociated sensory loss (impaired pain and temp sensation, and sparing of vibration and position sensations), fasic are not prominent

• CMT (distal)

• Old polio

• Tumor

• Syphilis

Root lesion (C8-T1)

• Cervical sponydlotic myeloradiculopathy

• Tumors

Brachial plexus lesions (lower trunk/medial cord):  sensory and motor deficits

• NOS

• Pancoast tumor

Combination of median/ulnar nerve lesions:

• Arthritis: wasting out of proportion to weakness. 

http://neuromuscular.wustl.edu/nanatomy/cervanat.htm 

Neurologic Thoarcic outlet syndrome (TOS)

• Caused by angulation and stretching of the lower plexus axons by a taut fibrous band extending from the first thoracic rib to a C7 bony anomaly such as a rudimentary rib or elongated transverse process and, rarely, by scalene hypertrophy (scalenus anticus syndrome).

• Rare occurrence

• Because the lower plexus is angulated from below, the T1 axons are affected more than the C8 axons.

• The typical electrophysiologic findings are those that indicate a lower trunk brachial plexopathy.  Classically, these findings consist of low median and ulnar motor amplitudes and a low ulnar sensory nerve action potential amplitude.  However, in very mild or early cases, the most sensitive nerve conduction study may be the medial antebrachial sensory.  

• Weakness and sensory loss in the distribution of C8-T1 distribution, medial antebrachial cutaneous nerve. Numbness and paresthesia in medial arm and forearm; hand and fingers.

• Weakness and atrophy of thenar and intrinsic muscles of hand on affected side. 

• Symptoms may be increased by raising and externally rotating the arm, which may also cause decreased brachial arterial pulses.

• CXR to see for cervical rib.

• Nerve conduction studies in thoracic outlet syndrome (TOS).  A distinctive pattern of motor and sensory conduction studies occurs in TOS.  C8 and especially T1 fibers passing through the lower trunk are affected in true neurogenic TOS.  On sensory conduction studies, ulnar and medial antebrachial cutaneous sensory nerve action potentials (SNAPs) are abnormal, but median SNAPs are spared.  Whereas both ulnar and medial antebrachial cutaneous sensory fibers travel through the lower trunk, median sensory fibers are derived from the upper and middle trunks. A different pattern is seen on motor conduction studies. Although both median- and ulnar-innervated motor fibers travel through the lower trunk, the median-innervated thenar muscles typically are more affected than are the ulnar-innervated hypothenar muscles, probably reflecting greater T1 innervation of thenar compared to hypothenar muscles.

Case Vignette:  

A 26-year-old, right-hand-dominant woman is referred to the EMG lab for electrodiagnostic assessment of left upper extremity pain and numbness.  According to the patient, she has a several year history of intermittent numbness along the medial aspect of her left forearm and hand that is often precipitated by the supine position.  She also reports a 10-year history of aching pain along the medial aspect of her left arm and forearm and that her friend noted that the base of her thumb looked funny.  Her left thenar eminence is severely atrophied, there is weakness of left thumb abduction > thumb tip flexion and index finger extension, as well as diminished sensation along the radial aspect of the left forearm and hand.  

The distribution of pain and tingling along the medial aspect of the arm, forearm, and hand suggest C8 and T1 nerve fiber involvement in the distribution of weakness suggests the supraclavicular localization (e.g., lower plexus or root level).  At this point, the screening sensory NCS can be performed, beginning with the left upper extremity:  NCS (sensory): Median-D2-C6,7 (PkLat: 3.1 ms, SNAP 51 mcV); ulnar-D5-C8 (2.7, 16); superficial radial-C6, 7 (2.2, 59).  The screening sensory NCS appear normal.  However, it is important to assess the relationship of the sensory response amplitudes with each other to avoid missing a relative abnormality.  Notice that the amplitude value of the median response is approximately 2.5 times the lower limit of normal.  The amplitude value of the ulnar response, however, is about 1.25 times the lower limit of normal, and that the amplitude value of the superficial radial response is more than 3 times the lower limit of normal.  Comparison of these values in relation to the cutoff value for normal suggests a possible relative abnormality of the ulnar response.  Thus, the ulnar-D5 NCS should be performed on the contralateral side.  Right ulnar-D5-C8 (2.6, 41.7).  The amplitude value of the contralateral ulnar responses nearly 3 times larger than that of the ipsilateral response, identifying a relative abnormality.  This abnormality indicates an axonal loss process involving the ulnar nerve, the medial cord, or the C8 fibers of the lower plexus.  In this setting, the MABC NCS is performed bilaterally.  Left MABC-T1 (NR).  Right MABC-T1 (2.5, 15.8).The absent MABC shortens the list of potential localization sites to the medial cord of the lower plexus.

Another important feature of these values is the variation in the degree of involvement of 2 abnormal responses-the ulnar response is relatively abnormal and the MABC response is absent.  This suggest that the lesion might be at the APR level and affect the T1 APR to a greater extent than the C8 APR, which is a feature of true neurogenic thoracic outlet syndrome (TN-NOS).   With TN-TOS a fibrocartilaginous band extends from the first thoracic rib to the C7 vertebral body (either to an elongated C7 transverse process or to a rudimentary C7 rib).  This causes the inferior aspect of the brachial plexus to be stretched over the band, producing a traction injury at the APR level or the proximal lower trunk level that affects the more inferior fibers (i.e. the T1 nerve fibers) more than the more superior fibers (i.e. the C8 nerve fibers).

The sensory nerve fibers studied by the MABC NCS emanate from the T1 DRG, whereas those studied by the ulnar-D5 NCS emanate from the C8 DRG.  Thus, disorders involving the T1 APR to a greater extent than the C8 APR or involving the inferior aspect of the lower trunk affect the MABC sensory response to a greater extent than the ulnar sensory response.  At this point, the motor NCS can be performed, adding the bilateral radial-EI, bilateral ulnar-FDI, and the contralateral ulnar-ADM NCS to the routine NCS (for localization and severity assessment).  Left median-APB (wrist): 3.6: 2.2.  Left median-APB (elbow): 2.1, 2.1.  CV: 51.  Right median-APB (wrist): 3.5, 12.4.  Right median-APB (elbow): 12.4.  CV: 52.  Left ulnar-ADM (wrist): 2.7, 12.3.  Left ulnar-ADM (above elbow): 12.1.  CV: 53.  Right ulnar-ADM (wrist): 2.7, 14.1.  CV: 52.  Right ulnar-ADM (above elbow): 14.  CV: 56.  Left ulnar-FDI (wrist): 4.2: 10.1.  Left ulnar-FDI (above elbow): 10.  CV 54.  Right ulnar-FDI (wrist): 4.1, 15.3.  Right ulnar-FDI (above elbow): 15.1.  CV: 54.  Left radial-EI (forearm): 1.6, 2.1.  Left radial-EI (elbow): 2.1.  52.  Right radial-EI (forearm): 1.7, 4.6.  Right radial-EI (elbow): 4.6.  53.  The median motor response is very low in amplitude, indicating a severe axonal loss process.  The 2 ulnar motor responses are normal, but clearly lower than the contralateral side (especially the ulnar-FDI response).  However, they do not meet criteria for relative abnormality (i.e., <50% of the contralateral side).  The amplitude value of the radial motor response is abnormal.  Its involvement excludes medial cord localization.  Thus, at this point, there is an axon loss process involving the lower plexus. 

Similar to the range of severities noted on the sensory NCS, where the MABC response (T1 DRG-derived sensory axons) was affected to a much greater degree than the ulnar response (C8 DRG-derived sensory axons), the median-APB response (T1 > C8) is affected out of proportion to the 2 ulnar responses (C8 > T1).  Again, this is the typical pattern of NCS abnormalities observed in TN-TOS.  At this point, the needle EMG study can be performed, adding muscles in the muscles domain of the lower plexus and determining whether the APB muscle is affected to a greater degree than the other lower plexus innervated muscles (i.e. looking for the same T1 > C8 pattern).

Left APB, FDI, EIA, FPL: Abnormal.  Left APB severely abnormal.  Left PT, biceps brachii, triceps, lower cervicals paraspinals, high thoracic paraspinals and  right APB, FDI, EI, triceps: Normal.

The abnormal muscles are in the muscle domain of the lower plexus with the APB muscle iaffected to a greater degree than the other affected muscles.  The fibrillation potentials were low in amplitude, consistent with a chronic process, as indicated by the presence of long duration MUAPs in the affected muscles.

Electrodiagnostic study impression: Left lower plexopathy (suspect TN-TOS).

Left lower plexopathy is axon loss in nature and severe in degree.  The abnormality is localized to the lower plexus and the pattern of abnormalities indicate that the T1 sensory and motor nerve fibers are affected to a greater extent than the C8 sensory and motor nerve fibers.  This constellation of electrodiagnostic abnormalities is considered essentially pathognomonic of TN-TOS.  Since its severity is slowly progressive disorder it permits reinnervation to keep pace with denervation.  For this reason, it does not respond to conservative therapy and, therefore is treated surgically by a neurosurgeon specializing in brachial plexus disorders.

Pancoast's syndrome

• Apical lung tumor - SCLC

• Tumor extends into the lower brachial plexus.

• Horner's syndrome

• Recurrent laryngeal nerve involvement resulting in hoarseness

• Inflitration of the entire plexus results in insensate plegic arm.

• Metastatic plexopathy often presents with severe pain and more often invades the C8-T1 nerve roots, or lower trunk, because of the proximity of axillary lymph nodes. Lung and breast cancer are common malignancies associated with metastatic plexopathy.

• Pain is prominent in neoplastic plexopathy

Neoplastic/noneoplastic tumors causing Brachial plexopathies

• Pancoast tumors of the lung may spread and invade the plexus directly.

• Lymphomas, breast cancer, and lung cancer are the most frequent causes. Lymphomas and leukemia can also infiltrate nerve directly, in the absence of a mass lesion.

• Rarely, primary nerve sheath tumors (e.g., schwannomas, neurofibromas, or neurofibrosarcomas) may affect the brachial plexus.

• In unusual cases, non-neoplastic mass lesions, such as hematomas and unusual vascular anomalies (e.g., aneurysm, arteriovenous malformation), can compress the brachial plexus.

Classic postoperative paralysis

• Initially described in 1894, is considered to be a traction or compressive injury that characteristically presents in the immediate postoperative setting as a unilateral upper plexopathy or, much less frequently, as a supraclavicular plexopathy that involves the upper plexus disproportionately. 

• This entity is related to multiple factors, including patient positioning, loss of muscle tone from anesthesia, and the unconscious state (impedes weight-shifting). 

• The underlying pathophysiology typically is demyelinating conduction block, and, thus, the prognosis for a full recovery is excellent.

• DDx:  HNPP

Parsonage-Turner syndrome/Brachial neuritis/Neuralgic amyotrophy:

• First described by Parsonage and Turner in 1948, and then described in detail with respect to its natural history more than 20 years later.  

• Neuralgic amyotrophy, which also is referred to as Parsonage-Turner syndrome, brachial neuritis, and many other terms, frequently follows an event such as a medical procedure (e.g., surgery) and a flu-like illness.   Individuals of all ages are affected, and their is male predominance.  The two most distinctive features are sudden onset, severe shoulder pain mainly in the periscapular (forequarter) region.  Pain generally starts improvig  2-3 weeks later, however, som e feel a dull achy sensation in the arm.  There is weakness of the affected arm with worsening after the pain lessens, associated with muscle atrophy.  The weakness may involve the brachial plexus in a patchy fashion, for example affecting one or more trunks or single peripheral nerves.   This presumably autoimmune disorder has a predilection for those peripheral nervous system elements of the forequarter region of the body (i.e., the superior torso, shoulder girdle, and upper extremity) that contain solely or predominantly motor nerve fibers. For this reason, the most commonly affected nerves are the suprascapular, long thoracic, axillary, musculocutaneous, anterior interosseous nerves, as well as individual motor nerve branches (e.g., the motor nerve branch to the flexor pollicis longus). This motor nerve fiber predilection explains why it is so frequently mislocalized to the upper trunk of the brachial plexus.  When sensory nerves are affected, the most common one is the lateral antebrachial cutaneous nerve.  Bilateral involvement is seen in 1/3rd of cases, usually asymmetrically. 

• This disorder which is presumable autoimmune in origin is more common in young adults and often follows physical exertion or other physical stress, including upper respiratory tract infection, vaccination, surgery, or childbirth. 

• Most cases occur sporadically. 

• Typically, idiopathic brachial plexitis presents with severe usually unilateral neck and shoulder pain followed by weakness and muscle atrophy.  The distribution is in the forequarter region of the body (torso, shoulder girdle, and arm) and contains solely or predominantly motor nerves.

• Movement of the arm is avoided. The arm is held in flexion at the elbow and adducted at the shoulder.

• Muscles innervated by long thoracic nerves, suprascapular, axillary, musculocutaneous, AIN, and nerve to FPL are most commonly affected. 

  • Mnemonic: L. SAM AFib. 

  • Most commonly affected muscle is the infraspinatus (>70%).

  • Muscles of the forequarter (imagine a quadruped animal like a cow) of the body. 

• Unilateral or bilateral phrenic nerve paralysis may occur. 

• Spinal accessory nerve may also be affected. 

• In 30% of cases, the other side is affected simultaneously or within a few weeks. 

• Loss of muscle stretch reflexes

• The prognosis is generally good with a slow but progressive recovery over 6-18 months.

• EDX testing and MRI of the brachial plexus is helpful in diagnosis.

• Electrodiagnostic studies may reveal a pattern of brachial plexus involvement not readily localizable to one or more specific trunks, divisions, cords, or peripheral nerves.  The patchy or multifocal involvement is common and is a hallmark of this syndrome. 

• Labs: ESR, ANA, ANCA, paraneoplastic-ab, CSF.

• MRI and US can show torsion of the nerve and hour-glass fascicular constrictions.

• The role of corticosteroids is not certain, although they often help alleviate pain considerably.  An observational study of 50 patients with Parsonage-Turner syndrome treated with a 13-day course of oral prednisolone within 1 month of symptom onset, compared with 203 untreated patients, showed remarkable improvements in pain duration (12.5 versus 20.5 days), improved strength recovery (full recovery in 12% within 1 year versus 1%), and improved longterm outcome (44% good or higher versus 10.7%).

• A small percentage of patients have an autosomal dominant inherited form of the disorder. 

• Criteria for the diagnosis of classic phenotype, neuralgic amyotrophy.   Subacute or acute onset initial pain equal or greater than 7/10 for days to weeks. Multifocal distribution focusing on the upper trunk, long thoracic nerve, and suprascapular nerve.  Monophasic course with slow recovery or no recovery over months. No other obvious explanation for the neurological deficit

https://practicalneurology.com/articles/2015-nov-dec/how-to-know-it-when-you-see-it-diagnosing-neuralgic-amyotrophy-parsonage-turner-syndrome/pdf

Hereditary Neuralgic amyotrophy

Hereditary neuralgic amyotrophy (HNA) is an autosomal-dominant disorder associated with recurrent, episodic, painful, brachial neuropathy. The gene for HNA has been mapped to chromosome 17q25. Characteristic features including hypotelorism, short stature, and cleft palate occur in some patients.  Unusual skin folds and creases were observed on the necks of several individuals as well as on the scalp: cutis verticis gyrata. In three families, deep skin creases were present on the limbs of infants and toddlers who were subsequently affected with HNA.

Some cases of hereditary neuralgic amyotrophy has been linked to SEPT9 gene on chromosome 17q25.3 which is a member of cytoskeleton-related septin family.   It is genetically heterogenous and has been linked to a variation or duplication in the SEPT9 gene in only 55% of affected families.  Presentation is similar to idiopathic neuraglic amyotrophy but with recurrent attacks and autosomal dominant inheritance pattern.   Other distinguishing features include family history, earlier age of onset, higher rate of recurrence, and the presence of dysmorphic features.  

The European CMT Consortium diagnostic guidelines for HNA, supplemented by Alfen et al:

1. Acute, unilateral, or bilateral brachial plexopathy

2. Severe pain precedes onset of weakness by days or weeks

3. Predominantly motor deficits

4. Number of episodes variable (1-20)

5. Precipitating factors: infection, immunizations, surgery, pregnancy, parturition, usually strenous exercise of the affected limb, and exposure to cold.  

Onset of HNA usually occurs in the 2nd or 3rd decade of life, though earlier or later onset is possible.  HNA can run 2 distinct courses: a relapsing/remitting course with symptom-free intervals or incomplete recovery, characterized by persistent neurologic deficits after repeated attacks in the same limb.  Dysmorphic features include long, narrow face, small mouth, hypotelorism (close-set eyes), shortened palpebral fissures, epincanthal folds, cleft palate, minor syndactyly, circular skin creases, long-nasal bridge, and short stature.  

There is no standardized approach for treatment.  Some case series suggest early corticosteroid therapy or IVIg may benefit in the acute phase of neuralgic amyotrophy.  

https://www.neurology.org/doi/10.1212/WNL.57.11.1963

https://www.researchgate.net/figure/Ten-year-old-male-with-recurrent-hereditary-neuralgic-amyotrophy-HNA-Courtesy-of_fig1_278680338

https://onlinelibrary.wiley.com/doi/abs/10.1111/j.1399-0004.2008.01022.x

Neuralgic amyotrophy : Current Opinion in Neurology (lww.com) 

History taking  in NA/PTS:

What went wrong with your arm and when? 

How much did it really hurt in the beginning (on a scale of 0 to10)? 

Did the pain disturb your sleep? 

What did you take for your pain? did it help at all? 

How long did that initial pain score last? 

When did you notice that certain movements or postures became difficult? 

Has moving or using your arm overhead become difficult for you? 

Did you see or feel your shoulder blade protrude? 

Did you notice any tingling or numb sensations? 

Did you have other symptoms like hoarseness or shortness of breath lying down or bending over? 

Was the onset preceded by an infection, surgery, pregnancy or giving birth, stress etc? Course and recovery.

Do you still have pain? If so, is it different from that at onset?  Can you provoke it? How? (stretch sensitive or postexercise muscular).

 Have you noticed any recovery yet? (e.g. is the shoulder blade back against the chest wall at rest etc.)

Are there certain activities in daily life that you cannot do anymore? How about work and sports?  

Have you stopped working / have you tried to work again 

Have you ever had this before? 

Or did you ever have a painless wasting of muscles or a protruding shoulder blade? 

Is there anyone in your family who had something like this? 

Do you or did you have a carpal tunnel syndrome or dropfoot? Anyone in your family? 

Update on Neuralgic Amyotrophy

It is actually an extra plexitis and a peripheral vascular axonal neuropathy.

Check brachialis muscle.  Limited series, pronator quadratus, flexor carpi radialis and flexor pollicis longus.

MRI and ultrasound must include above the elbow as these are common anatomical positions where the median nerve can be affected.

Vascular immunology Mechanism.  There is sudden onset resulting in perineural edema followed by scarring in the healing phase.  The scarring is mostly in the collagen connective tissue supporting the perineurium and this results in fascicular constrictions.

Surgical interventions in the acute and chronic phases. 

Idiopathic brachial neuritis, neuralgic amyotrophy (NA) or Parsonage-Turner syndrome can mimic cervical radiculopathies clinically and should be part of the differential diagnosis for those presenting with limb symptoms, particularly in the shoulder girdle region.  The most commonly involved nerves are the suprascapular, long thoracic, anterior interosseous, and axillary nerves.  More than one nerve may be involved.  Because of the prominent C5-C6 innervated muscle patterns, neuralgic amyotrophy may mimic upper trunk plexopathy or radiculopathy.  Patient with neuralgic amyotrophy give history of shoulder region pain, described as sudden in onset, severe in degree, and transient (typically lasting 1 to 2 weeks); and presence of muscle weakness and wasting in the forequarter region of the body (typical a severe degree, frequently limited to a single muscle or muscle head, and usually becoming apparent when the decrease in pain permits the patient to use the arm).  In addition, majority of patients have an antecedent event known to be associated with neuralgic amyotrophy and a latent period between the trigger and the onset of the pain.

Is NA an autoimmune disorder?

In my opinion: Yes.  However, literature says it is not clear.  The exact onset and pathophysiology of this condition is still obscure.  I favor the current Vascular immunology Mechanism:. There is sudden onset resulting in perineural edema followed by scarring in the healing phase.  The scarring is mostly in the collagen connective tissue supporting the perineurium and this results in fascicular constrictions.  

Most evidence points to NA is an auto-immune disorder, in which several independent predisposing factors will lead to the occurrence of an actual episode.  An intrinsic factor is presumed present that makes patients more vulnerable than the general population. Mechanical factors also seem to play a role as strenuous activity or local trauma can trigger an attack, and NA patients are more often physically active than the general population.  Mechanical strain is suspected to cause a focal disturbance of the fascicular perineural blood-nerve barrier, especially in nerve segments that routinely undergo large mechanical deformation, such as the brachial plexus and certain arm nerves. The final step leading to the onset of an attack seems to involve activation of the immune system, that in the context of a ‘leaky’ blood-nerve barrier leads to an auto inflammatory response with subsequent damage of the nerve segments. Scant histologic evidence suggests that an attack is the result of an aspecific activation of the innate immune system at the level of the blood-nerve barrier, resulting in focal inflammatory infiltrates, severe pain caused by the release of inflammatory mediators, and ischemia of the nervi nervorum, and acute damage to the paranodal regions of large nerve fibers, as an early conduction block has been described in proximal nerve segments in NA. If left untreated, this inflammatory response will progress into axonal loss and denervation, and perineurial and epineurial fibrosis with the formation of constrictive bands that constrict the nerve and hamper recovery.

Would it be triggered by an agent that could activate the immune system?

It is usually a monophasic phenomenon, but it is not absolute.  There is also a recurrent hereditary form which he does not have. 

Any immune-related factor can trigger NA, including infection, vaccination, immunotherapy such as interferon or immune-checkpoint inhibitors, recovery from surgery, pregnancy or childbirth, trauma, or psychological distress. 

For your reading pleasure: 

https://journals.lww.com/co-neurology/Fulltext/2021/10000/Neuralgic_amyotrophy.2.aspx

Neuralgic amyotrophy is inflammatory, patchy, and often multifocal.  The strict paradigms of “preganglionic versus postganglionic” when this disorder is on the differential does not fit.  Also, one should not think it as a “plexopathy” at all.  In particular, fascicles of the long thoracic, phrenic, dorsal scapular nerves, and even (nerves to the) paraspinal muscles are frequently involved with this entity (all of which branch proximal to the trunks of the brachial plexus and would, absent this awareness, mislead someone toward an impression of radiculopathy). One should not rely on the rhomboids for ruling in or ruling out neuralgic amyotrophy; I don't think that muscle is as guiding as it is for structural or traumatic brachial plexopathy, wherein we are looking for the localization of a single lesion. The 2006 study by van Alfen et al. offers data on distribution (rhomboid involvement was quite prevalent in their sample; > 50% from among cases where this muscle was examined).

Musculocutaneous Neuropathy

• C5-C7 derived terminal branch of the lateral cord.

• Innervates corachobrachialis first and then biceps brachii and brachialis.  

• Continues past the elbow as a pure sensory nerve (lateral antebrachial cutaneous nerve) to supply sensation to the lateral aspect of the forearm (anterior and posterior divisions) from elbow to the thenar eminence. 

• May be injured by overly vigorous elbow flexion (weight lifter's palsy)

• Weakness of elbow flexion with the forearm supinated and marked weakness of supination.

• The semi-pronated forearm can still be flexed by the brachioradialis.

• There is a relatively small area of sensory loss on the lateral surface of the forearm.

• Biceps reflex is diminished or absent.

• Preservation of the axillary, dorsal scapular, and suprascapular nerve functions differentiate musculocutaneous palsy from an upper trunk lesion and C5 radiculopathy.

• Preservation of forearm pronation and lateral hand sensation, median nerve functions, distinguishes from a lateral cord lesion and C6 radiculopathy.

Axillary neuropathy

• Dislocation or fracture of proximal humerus can compress axillary nerve, stab wounds, missile injuries, blunt force injuries, stretch injuries (hyperabduction during sleep, surgery), injection injury, Parsonage-Turner syndrome, soft tissue or peripheral nerve tumor, ischemia (vasculitis), multifocal motor neuropathy, MADSAM.

• Deltoid weakness, and numbness in the shoulder.

• EDx: Check superficial radial SNAP.  This would be expected to be normal in an axillary neuropathy and can help distinguish an axillary neuropathy from a posterior cord lesion or upper trunk lesion.  Furthermore EMG should show evidence of denervation in the deltoid and teres minor muscles with sparing of the radial innervated muscles in an isolated axillary neuropathy.  In addition, a normal EMG of the supraspinatus, infraspinatus, rhomboids, biceps brachii, pronator teres, and brachioradialis suggest that the lesion is distal to the C5-C6 roots or upper trunk when combined with denervation of the deltoid.  Axillary CMAPs may be recorded from the deltoid muscle following supraclavicular stimulation of the brachial plexus at Erb's point to see if there is asymmetrical loss of amplitude on the affected side or a disconnect between the amount of movement and the size of the CMAP. 

• DDX: C5 radiculopathy (axillary neuropathy does not involve biceps, whereas C5 radiculopathy does)

Medial brachial fascial compartment syndrome

• Associated with transaxillary arteriography.

• Extends from the clavicle to the elbow.

• Proximally, it contains the terminal branches of the brachial plexus and the axillary vessels. 

• Lesions located in this compartment (pseudoaneurysms, hematomas, and other lesion causing mass effect) impede the microcirculation of the nerve fibers. 

• The median nerve it the most affected, followed by the ulnar nerve.  These lie near the axillary artery at the site of its cannulation.  The radial, musculocutaneous, and axillary nerves may also get involved. 

• Patient presents with pain and paresthesias in the distribution of these nerves, followed by weakness. 

• Prompt decompression is needed, within 4 hours. 

Radial neuropathy

• Largest nerve in UE

• Compression in the axilla, causes "Crutch palsy or Honey-moon palsy" 

• Compression in the spiral groove:  It can commonly occur in a person whose arm is draped over the back of the chair or bench, during a state of deep sleep or while the person is intoxicated, hence, referred to as "Saturday night palsy."  Other prolonged immobilization postures that compresses the nerve in the spiral groove, against the humerus.

• Fracture of the humerus damaging the radial nerve in the spiral groove (most common site of injury), strenuous muscular activity, or infarction from vasculitis.

  • Finger drop (weakness of EIP) wrist drop (weakness of EDC, ECU, and ECRL), weakness of elbow flexion with forearm semi-pronated (weakness of brachioradialis), and weakness in supination of forearm (weak supinator).  Elbow extension is preserved as the radial nerve branches to the triceps and anconeus muscles originate proximal to the spiral groove.

  • Sensory abnormalities may occur over the distribution of superficial radial sensory nerve (dorsum of the hand, thumb, index finger, middle finger, and lateral border of ring finger.)

  • Finger abduction and adduction is normal in wrist drop from isolated radial neuropathy at spiral groove, as ulnar (abduction/adduction), and flexion (median) are unaffected. 

• Compression in the axilla results in radial neuropathy similar to radial neuropathy at the spiral groove, except that the triceps and anconeus are also affected (weakness), hence arm extension is weak.  There is sensory loss over the extensor surface of the forearm and lateral half of the arm and over the triceps owing to involvement of the posterior cutaneous nerve of the forearm and the lower lateral cutaneous and posterior cutaneous nerve of the arm.

• PIN Neuropathy:

  • A posterior interosseous neuropathy typically is a motor neuropathy involving the radial nerve.  It occurs due to entrapment of the PIN in the tendinous arcade of Frohse when passing through the supinator muscle.  

  • Mass lesions like ganglion cysts or tumors can also compress the PIN at this site.

  • All of the finger extensor muscles will be weak in a complete PIN lesion, resulting in a "finger drop."  

    • Thus, PIN neuropathy causes finger drop and weakness of wrist extension. 

    • Thumb abduction and extension of the IP joint of thumb are also weak.  

    • Elbow extension and the sensation of the distribution of radial motor an d  superficial radial nerve , respectively are  spared.  

    • There is no sensory loss.  Deep forearm pain is present from involvement of deep sensory fibers of the PIN that supply the interosseous membrane and joint capsules. 

  • There is sparing of radial-innervated muscles above the takeoff of the PIN (brachioradialis, long and short head of ECR, triceps). The extensor carpi radialis longus muscle gets branches directly from the radial nerve, and is spared in PIN lesions.  Thus, patients are able to perform wrist extension with radial deviation.

  • The extensor carpi ulnaris muscle gets branches from PIN, so will be weak when the nerve is lesioned. 

    • When the wrist is extended, it is extended weakly and deviates radially due to weakness of the extensor carpi ulnaris (innervated by PIN) and sparing of the extensor carpi radialis longus and brevis (which are innervated by the main trunk - radial nerve).  

• Radial dislocation injuries in the elbow and injury at the wrist (due to handcuffs - Wartenberg's syndrome).

• Sensory loss in the distribution of radial nerve, back of the hand.  Lateral dorsum of the hand, part of thumb, and dorsal proximal phalanges of the index, middle, and ring fingers.

• DDx: Posterior cord lesion; deltoid also weak. PIN; isolated finger drop, C7 radiculopathy

  • The brachioradialis muscle is a key muscle that needs to be tested when trying to distinguish C7 radiculopathy from a proximal radial nerve lesion (axilla). It is supplied by C5-6 nerve roots by way of the radial nerve. It would not be affected in a C7 radiculopathy. The extensor carpi radialis longus, extensor digitorum communis, and triceps muscles could all be affected in both a C7 radiculopathy in a proximal radial mononeuropathy.  If the C7 radiculopathy is severe enough to cause muscle weakness, other non-radial C7-innervated muscles also should be weak (PT, FCR), leading to weakness of arm pronation and wrist flexion. Sometimes these muscles are spared and it become difficult to differentiate C7 radiculopathy from proximal radial mononeuropathy.

  • Distinguishing a radial neuropathy at the spiral groove from a radial neuropathy at the arcade of Frohse:

    • The superficial radial is involved in a spiral groove lesion but not involved in lesions distal to the elbow.

  • Lesions of posterior cord of the brachial plexus results in weakness of radial-innervated muscles, the deltoid (axillary nerve) and latissimus dorsi (thoracodorsal nerve) should also weak.

• EDX findings:

  • PIN: 

    • Axonal loss lesion: 

      • SNAPs: Superficial radial sensory SNAPs are normal

      • CMAPs: Distal radial CMAPs are low

      • EMG: EIP, EDC, ECU

    • Demyelination lesion:

      • SNAPs: Superficial radial sensory SNAPs are normal

      • CMAPs: Distal radial CMAPs are normal; CB between forearm and elbow stimulation sites.

      • EMG: EIP, EDC, ECU

    • Mixed (axonal and demyelinating) lesions:

      • SNAPs: Superficial radial sensory SNAPs are normal

      • CMAPs: Distal radial CMAPs are low; CB between forearm and elbow stimulation sites.

      • EMG: EIP, EDC, ECU

  • Radial neuropathy at spiral groove:

    • Axonal loss lesion:

      • SNAPs: Superficial radial sensory SNAPs are reduced

      • CMAPs: Distal radial CMAPs are low.  No CB across spiral groove.

      • EMG: EIP, EDC, ECU, ECRL, brachoradialis, supinator

    • Demyelinating lesion:

      • SNAPs: Superfical radial sensory SNAPs are normal

      • CMAPs: Distal radial CMAPs are normal with CB across spiral groove.

      • EMG: EIP, EDC, ECU, ECRL, brachioradialis, supinator

    • Mixed (axonal and demyelinating) lesions:

      • SNAPs: Superficial radial sensory SNAPs are low

      • CMAPs: Distal radial CMAPs are low with CB across the spiral groove.

      • EMG: EIP, EDC, ECU, ECRL, brachioradialis, supinator.

  • Radial neuropathy at the axilla

    • Axonal loss lesion:

      • SNAPs: Superficial radial sensory SNAPs are reduced

      • CMAPs: Distal radial CMAPs are low.

      • EMG: EIP, EDC, ECU, ECRL, brachioradialis, supinator, anconeus, triceps

    • Demyelinating lesion:

      • SNAPs: Superficial radial sensory SNAPs are normal

      • CMAPs: Distal radial CMAPs are normal with normal CMAPs above spiral groove (no CB, here, but may be higher which is not possible to test)

        • May try Erb's point to see evidence of CB, but there is possibilty of overstimulation and volume conduction, hence not reliable.

      • EMG: EIP, EDC, ECU, ECRL, brachioradialis, supinator, anconeus, triceps

• Tx: Splint

Wrist drop: Possible Anatomical Locations

• PIN

• Radial nerve at spiral groove

• Radial nerve at axilla

• Posterior cord of brachial plexus

• C7 root

• CNS

Causes of Wrist Drop and Normal Superficial Radial SNAP

• PIN

• Demyelinating lesion of radial nerve at the level of spiral groove or axilla

• C7 radiculopathy

• CNS lesion

• Hyperacute axonal loss lesion of the main radial nerve (<4 days old)

The terminal nerves exit the medial brachial fascial compartment syndrome in the following order (proximal to distal): supraclavicular, musculocutaneous, axillary, and radial. The median and ulnar nerves remain confined within the medial brachial fascial compartment until the elbow. This anatomic arrangement explains why the median and ulnar nerves are disproportionately affected in the syndrome. 

Median mononeuropathies

Median neuropathy in the axilla:

• Median neuropathies in axilla are commonly associated with damage to ulnar and radial nerves.

  • Median n. lesions in axilla or upper arm result in paresis of all muscles innervated by median n, with sensory loss in distribution of both palmar cutaneous and palmar digital branches. 

• Etiologies: crutch palsy, sleep paralysis, penetrating trauma, shoulder dislocation, AVF, sheath H'ge. Fracture of humerus or distal radius can injure the median nerve.

  • Atrophy of thenar eminence causes recession of the MC bones of thumb to the plane of other MC bones, giving rise to simian hand or ape hand. The appearance is as a result of unopposed action of Extensor pollicis longus (radial n) and Adductor pollicis (ulnar n). Because the 2nd finger cannot be flexed and the 3rd finger can be flexed only partially, when the person attempts to make a fist, these fingers remain extended. The hand then takes on the appearance of that of a clergyman offering benediction (benedictine sign, preacher's hand or orator's hand pose).

  • Paresis of forearm pronation

  • Paresis of radial wrist flexion, distal flexion of thumb, palmar abduction, and of opposition of thumb and of flexion of 2nd and, 3rd fingers. 

Lesions at the ligament of Struthers:

• Seen in ~ 1% of population, an anomalous bony spur 3 to 5 cm above the medial epicondyle on the anteromedial humerus. A fibrous tunnel formed by a ligament (ligament of Struthers) connects this spur to the medial epicondyle. This is a site where the median nerve can be compressed by this ligament. 

• Sx are similar to compression of median n in axilla.

  • The Struthers’ ligament and the arcade of Struthers are two anatomical structures that are often confused. The Struthers’ ligament was described by anatomist John Struthers in 1854; it is a fibrous band that extends from a bone spur located on the anteromedial surface of the lower third of the humerus, known as the supracondylar process, and is inserted in the medial humeral epicondyle. The Struthers’ ligament passes over the median nerve and the brachial artery, which may cause compression of these structures. It may be observed even in the absence of the supracondylar process; even when present, it may not cause the compression of these structures. The supracondylar process of the humerus has been described by anatomists and anthropologists; it is phylogenetically considered as a vestige of the supracondylar foramen, found in reptiles, marsupials, and some mammals. Its occurrence in humans is very rare, being observed in 0.7% to 2.5% of the population. However, there is no dispute as to the existence of this ligament. In contrast, the arcade of Struthers was first described in 1973 by Kane et al. It is sometimes defined as a thickening of the brachial fascia, and sometimes as an aponeurotic or musculoaponeurotic structure extending from the medial intermuscular septa to the medial head of the triceps brachii muscle at a variable distance above the medial humeral epicondyle. 

Pronator Teres Syndrome:

• Median nerve is compressed in the proximal forearm between the two heads of pronator teres muscle, a fibrous arcade of the flexor digitorum superficialis muscle, or the bicipetal aponeurosis. 

• Repetitive pronating movements of the forearm can cause it. 

• May be associated with medial epicondylitis "golfer's elbow."

• Vague aching pain in the volar aspect of the elbow and forearm, beginning or worsening during activities involving grasping or pronation or both. 

• Paresthesias of the palm of hand, mimicking CTS but without the nocturnal symptoms. 

• Resistance to pronation produces pain in the proximal forearm.

• Tenderness on deep pressure over the pronator teres muscle.

• Branches from the median nerve that innervate the pronator teres arise proximal to this muscle (before the nerve passes under this muscle), pronator teres strength is intact in this syndrome.

• Sparing of muscles innervated by anterior interosseous nerve.

• Atrophy and paresis of the median thenar musculature.

• median motor or sensory slowing across forearm with normal distal latencies (same as CTS)

• EMG abnormal in median distribution (FPL, FCR, FDP, APB) 

Anterior interosseous syndrome:

• It s the largest branch of the median nerve and is a pure motor nerve.

• Compressed by fibrous bands attached to the flexor digitorum superficialis muscle, an anomalous muscle such as accessory head of the flexor pollicis longus (Gantzer muscle), or by an external compression such as with anterior elbow dislocations or complex elbow fractures.

• It arises from the median nerve distal to motor branches to Pronator teres, Flexor carpi radialis, Flexor digitorum sublimis, and plamaris longus in the upper forearm.

• It innervates the flexor pollicis longus, pronator quadratus, and flexor digitorum profundus muscles of the index and middle fingers.

• Pain in the forearm and arm lasting hours to days.

• Weakness of forearm pronation after flexion of the forearm (due to weakness of pronator quadratus)

• Weakness of flexion of the terminal phalanx of the thumb (due to FPL weakness) and the terminal phalanges of 2nd and 3rd digits (due to weakness of the FDP I and II). 

• Normal sensation, as the AIN is a pure motor nerve.

• A patient with anterior interosseous syndrome is unable to make a nice round circle using the thumb and index finger (OK or pinch sign) due to weakness of the FPL and FDP.  The circle looks squashed in AIS.

  • Kiloh-Nevin syndrome: When the patient is asked to pinch a sheet of paper between the thumb and index fingers using only the fingertips, the ulnar innervated adductor pollicis and also the FDI come into play and hold the paper between the extended index finger and adducting thumb. 

• Sensory and motor nerve conduction studies of the median nerve are usually normal. 

• Needle EMG reveals denervation in muscles innervated by the anterior interosseous nerve.

• EMG: Limited to FPL, FDP, PQ, and median motor distal latency prolonged to PQ

Carpal tunnel syndrome:  

Carpal tunnel:

• • Carpal bones forms the floor and sides of the carpal tunnel

  • Transverse carpal ligament forms the roof

    • Contents of the carpal tunnel: median nerve + 9 flexor tendons (FDP: 4 tendons, FDS: 4 tendons, FPL: 1 tendon).

• Most common entrapment syndrome: 3.4%

• Clinical symptoms and signs:

  • Pain

  • Pain/paresthesiae associated with driving or holding a phone, book, or newspaper

  • Nocturnal paresthesias awakening patient from sleep.

  • Shaking or ringing the hands alleviates the paresthesiae.

  • Sensory disturbance of digits 1, 2, 3, and radial half of 4th digit.

  • Weakness/wasting of thenar eminence

  • Phalen's maneuver reproduces symptoms (usually in the 3rd digit) within 30 seconds to 2 minutes in CTS, and is more sensitive than Tinel's sign.

  • Tinel's sign over the median nerve at the wrist.

    • 50% sensitive and 77% specific

  • Compare sensation over the lateral ring finger (median innervated) to that over the medial ring finer (ulnar innervated).

  • The neck is not affected.

  • Little finger is spared

  • Sensation over the thenar eminence is spared.

• Causes: 

  • Repetitive stress, occupational, hypothyroidism, acromegaly, diabetes, RA, ganglia, lipoma, schwannoma, neurofibroma, hemangioma, persistent median artery, congenital small tunnel, anomalous muscles (palmaris longus, FDS), sarcoid, histoplasmosis, septic arthritis, lyme, TB, Colles' fracture, H'ge, spasticity, HD, amyloidosis, pregnancy.

• Sensation is spared over the thenar eminence because the palmar sensory branch that innervates the thenar eminence travels outside the carpal tunnel.

• DDx: C6, C7 radiculopathy, brachial plexus, and compression of the median nerve proximal to the carpal tunnel. 

• EDX: 

  • slowing of sensory and motor conduction across carpal tunnel.  Distal latency is prolonged on the median sensory study before motor NCS are abnormal.  However, routine nerve conduction studies may be normal.  Specific testing for focal slowing or conduction block of median nerve fibers across the carpal tunnel may be performed.  For instance, the median nerve can be compared to the ulnar nerve, such as with the palmar mixed study, on inching may be performed (segmental stimulation of the median nerve across the carpal tunnel)

  • CTS is typically due to demyelination, but there may be secondary axonal loss.

  • Velocity <44m/s indicates slowing across CT

  • Difference in sensory distal latency of more than 0.5ms between ulnar and median indicates CTS

  • Decreased amplitude of SNAP and CMAP

  • Severity:  Different grading scales are used to grade the severity of median neuropathies at the wrist - Mayo Clinic Standard: 

    • Mild CTS: Prolonged sensory or mixed (palm-wrist) distal latency, +/- amplitude reduction

    • Moderately severe CTS:  Prolonged sensory or mixed (palm-wrist) distal latency, +/- amplitude reduction PLUS prolonged median motor distal latency

    • Severe CTS: Absent sensory response OR Low amplitude CMAP

    • Very Severe CTS: Absent routine sensory and thenar motor responses.

    • Documenting an electrophysiologic grade of the degree of the median neuropathy at the wrist is useful in the EMG report, because it may guide the referring physician’s decision on treatment.

• Tx: Wrist splint by maintaining wrist in a neutral position at night, surgery definitive treatment

Identification of Aberrant Muscle Bellies in the Carpal Tunnel using Sonography - PMC (nih.gov) 

https://www.ncbi.nlm.nih.gov/pmc/articles/PMC7460039/

DHIS

https://www.ncbi.nlm.nih.gov/pmc/articles/PMC3782223/#:~:text=The%20etiology%20of%20DHIS%20is,gangrene%5B1%2D3%5D.

The intrinsic hand muscles including the APB muscle are traditionally known to be innervated by both T1 and C8 nerver oots/segments.  However, current understanding confirms that the principal innervation of APB muscle is the T1 root/segment.  EDx findings supporting predominant T1 root involvement include a low median CMAP amplitude recorded from the APB.  Although the medial brachial and antebrachial cutaneous nerves also shareT1 innervation, their sensory nerve action potentials are preserved inT1 radiculopathy owing to the extraspinal location of the dorsal root ganglion. 

https://www.neurology.org/doi/pdfdirect/10.1212/WNL.0000000000209561

Ulnar neuropathies

Ulnar nerve entrapment sites:

• Arcade of Struthers- fascia from the upper surface of the triceps to the interarticular septum to the medial triceps. 

• Struthers' ligament (rare)

• Intercondylar ulnar nerve pressure:  Single or repetitive, chronic subluxations, pressure from bony deformity, masses, and rarely from anconeus epitrochlearis muscle.  

• Cubital tunnel syndrome:  Proximal edge of FCU aponeurosis or arcuate ligament.

• Tardy ulnar nerve palsy:  Late sequelae of bony injury and hence deformity.

• Dorsal ulnar cutaneous neuropathy: Typically not entrapment. 

• Ulnar neuropathy at wrist.

Ulnar nerve entrapment at the elbow (UNE): 

• 2nd most common entrapment neuropathy

• C8 and T1 roots via the lower trunk and medial cord of the brachial plexus.

• The ulnar nerve gives no branches in the arm.

• Ulnar groove - cubital canal entrapment, post-traumatic; compression as in leaning on elbows, fracture at medial epicondyle.

• Weakness of wrist flexion and adduction (ulnar flexion), finger adduction and abduction, flexion of 4th and 5th digits, opposition and abduction of the 5th digit, thumb adduction.

• Sensory loss in ulnar nerve distribution, little finger and ulnar half of ring finger. Numbness and paresthesia

• Froment sign: To compensate for adductor pollicis weakness during an attempt to pinch a piece of paper between the thumb and index fingers, the flexor pollicis longus, a median nerve–innervated muscle, becomes involuntarily active and flexes the distal phalanx of the thumb.

• Weakness of the third palmar interosseous muscle results in abduction of the little finger, which may get caught when the patient tries to put the hand in a pocket (Wartenberg sign). 

• Atrophy of the hypothenar eminence and fasiculations in severe cases.

• Claw hand deformity due to weakness of 4th and 5th lumbricals

  • Hyperextension of the 4th and 5th digits at the MCP jt and flexion at the IP jt.

• Weakness of the FDI

• DDx: C8-T1 radiculopathy, pancoast's syndrome, thoracic outlet syndrome, or other lesions of the brachial plexus inferior trunk or medial cord

• Ulnar nerve lesions at the elbow should be distinguished from ulnar nerve lesions at the wrist, lower brachial plexus lesions (lower trunk or medial cord), and C8 radiculopathy. 

  • Sensory loss that extends more than 3 cm above the wrist into the medial forearm and arm, the territories of the medial brachial and antebrachial cutaneous nerves, is inconsistent with an ulnar neuropathy at the elbow and suggest a more proximal lesion of the lower plexus or C8 or T1 roots.

• Compression of the ulnar nerve in the hand as it passes over the hamate bone in Guyon's canal can occur from prolonged leaning forward and cycling.

• The ulnar nerve contains three sensory branches:  (1) ulnar digital sensory branches supply the volar fifth and medial fourth fingers;  (2) palmar cutaneous branch supplies the proximal volar medial hand, arising 1–2 cm proximal to the wrist; and  (3) dorsal ulnar cutaneous sensory branch arises 5–7 cm proximal to the wrist and supplies the dorsal medial hand and the dorsal medial fourth and fifth fingers.   Lesions at the elbow may be associated with abnormalities in all three territories; lesions at the wrist never involve the dorsal ulnar territory or the proximal volar ulnar palm.  In other words, because the palmar cutaneous and dorsal cutaneous branches leave the ulnar nerve in the distal forearm and do not enter the Guyon canal, sensation in the proximal hypothenar region and the dorsum of the little and ring fingers is not impaired in all cases of ulnar nerve lesions at the wrist or hand. The sensory loss, if present, is confined to the palmar surface of the ulnar-innervated fingers (the little finger and usually the ulnar half of the ring finger) and the distal hypothenar region.  

• EDX: focal slowing of nerve conduction at the elbow. 

• Tx: elbow pads to avoid further injury. Surgery definitive treatment.

https://www.ncbi.nlm.nih.gov/pmc/articles/PMC5908985/pdf/WJO-9-65.pdf

• Ultrasound includes dynamic imaging to check for ulnar nerve subluxation or dynamic compression from a "snapping" medial head of triceps muscle resulting in extra-articular snapping of the elbow.  Intra-articular snapping is caused by annular ligament pathology, lateral meniscal remnants, or hypertrophic synovial plica.  Ultrasound may help elucidate these pathologies if present. 

Ulnar nerve entrapment at wrist:

• 6 - 8 cm proximal to the ulnar styloid, the ulnar nerve gives off the DUC (dorsal ulnar cutaneous sensory nerve), which is always spared in UNW and therefore, has localizing value. 

• 1 - 2 cm above the wrist, the palmar cutaneous branch arises.  It innervates the proximal volar medial hand.  It also is spared in UNW and it too has localizing value.

• Ulnar nerve enters the Guyon's canal in the wrist at the level of the distal wrist crease.  The canal is formed distally by the hook of the hamate, and proximally by the pisiform bone, the floor is formed by a combination of the thick transverse carpal ligament and the adjacent hamate and triquetrum bones.  The roof is loosely formed.  A thick band is present at the outlet that runs from the hook of the hamate to the pisiform bone; the pisohamate hiatus.

•  In the Guyon's canal the nerve splits into a deep branch and a superficial branch.

  • Before exiting the pisohamate hiatus, motor fibers are given off the deep branch, a.k.a deep palmar motor branch of the ulnar nerve, to supply 3 of 4 hypothenar muscles (ADM, FDM, and ODM).   It then exits the canal and goes on to innervate the 3rd and 4th lumbricals, the 4 dorsal interossei, and 3 palmar interossei, the adductor pollicis, and the deep head of the FPB.

  • The superficial terminal branch innervates the palmaris brevis muscle and also supplies sensation to the volar 5th and medial 4th digits, after it exits at the level of hiatus or Guyon's canal.

• Etiology:

  • This includes occupational trauma, cycling, edema, wrist fracture, ganglion cyst,  calcium deposits, tumor, fibrous band, true or false aneurysm, rheumatoid synovial cyst, Dupuytren's disease, or anomalous muscle. 

  • Lipoma, or nerve sheath tumor.  Degenerative arthritis, rheumatoid arthritis.  Trauma (fracture to metacarpals, pisiform, hamate, dislocation of distal ulna).

  • Rarely the nerve is entrapped within the median nerve in the carpal tunnel or by handcuffs. 

• Clinical types:

  • Type 1: Proximal (main trunk) of ulnar nerve just proximal to or within Guyon's canal. 

    • Atrophy and weakness of ulnar innervated intrinsic hand muscles occur.  Sensory loss (due to superficial terminal branch); sparing dorsal aspect of hand. 

    • Causes: Ganglion, lipoma, external pressure, rheumatoid synovial cyst, chondroma, schwannoma, anomalous muscle. 

  • Type 2: Deep terminal branch proximal to branch to hypothenar muscle groups.  Atrophy and weakness of all ulnar innervated hand muscles and hypothenar group.  There is no sensory deficit; the superficial terminal branch of the ulnar nerve containing the sensory fibers and motor innervation to the palmaris brevis is not affected. 

    • Causes: External pressure, ganglion, ligamentous compression, nerve and other tumors, scleroderma/calcinosis

  • Type 3: Deep palmar arcade distal to hypothenar muscles.  Atrophy and weakness of ulnar innervated  interossei, lumbrical(s) and 1st dorsal interosseous muscles, except the hypothenar muscles.  There is no sensory deficit; the superficial branch containing the sensory fibers and motor innervation to the palmaris brevis is not affected. 

    • 75% of cases of UNW are from type 2 and type 3 motor lesions.

    • "Palmaris brevis sign" is seen in the UNW (proximal and/or deep palmar distal motor lesions), as the superficial branch of the ulnar nerve which innervates the palmaris brevis muscle is not affected. Contraction of the palmaris brevis muscle in the presence of wasting of the other ulnar innervated intrinsic muscles of the hands, results in puckering of the skin along the proximal medial border of the hand, when the the 5th digit is abducted. 

    • Causes: Ganglion, external pressure, giant cell tumor, ligamentous compression, anomalous muscle.

  • Type 4.  Superficial terminal branch.  There is no atrophy or weakness.  Sensory deficit in ulnar cutaneous distribution sparing the dorsal aspect of the hand.  Also the palmaris brevis muscle is affected, but this is not clinically apparent.

    • Causes: Ununited fracture hook of hamate and ulnar artery aneurysm. 

• EDX findings:

  • The proximal Guyon's canal entrapment causes ulnar palmar sensory loss and weakness of all the intrinsic hand muscles.

  • The distal Guyon's canal entrapment causes weakness of all the intrinsic hand muscles without ulnar palmar sensory loss and without palmaris brevis muscle being affected (as it is innervated by the superficial terminal branch).

  • Entrapment at the mid-palm affects the distal deep palmar motor branch and affects the causes weakness of adductor pollicis, weakness of interossei.  It spares the 4th DI, hypothenar muscles, and sensory function. 

  • Proximal canal lesions

    • Hypothenar muscles may be abnormal on EMG with prolonged DML and SNAP abnormalities.  

  • If the lesion is distal, affecting only the deep palmar motor branch after the take-off to the hypothenar muscles, then the routine ulnar sensory study, recording the fifth digit, and the routine ulnar motor conduction study, recording the ADM, will be normal.   In suspected UNW, additional nerve conduction studies must always be performed in order to detect abnormalities that may not be present on routine ulnar motor and sensory studies.

  • Median innervated thenar muscle (APB), forearm (FPL), FDP 3 and 4 (ulnar innervated at forearm), radial innervated EIP are spared.

  • MABC sensory innervated territory (medial forearm), DUC sensory innervated territory (dorsal medial hand, dorsal 5th and 4th digits), and palmar cutaneous sensory branch territory (proximal volar medial hand) is spared.

  • Ulnar Motor Studies Recording the FDI: 

    • Performed in all cases of suspected UNW. 

    • In lesions of the distal deep palmar motor branch, the latency to the FDI may be prolonged with a decreased CMAP amplitude.

    • Comparison with the contralateral asymptomatic side often is helpful as well. 

    • In cases where the lesion is more proximal, affecting the hypothenar branches, the DML to the ADM also may be prolonged, with a decreased CMAP amplitude.  

    • However, one of the patterns highly suggestive of UNW is preferential involvement of the distal deep palmar motor branch, whereby the ulnar motor study recording the FDI is affected out of proportion to the ulnar motor study recording the ADM.  Comparison of their relative distal motor latencies often can be helpful:

    • Normal Values:

      • DML to FDI: ≤4.5 ms

      • DML comparing FDI to ADM: ≤2.0 ms difference

      • DML comparing symptomatic FDI to contralateral FDI: ≤1.3 ms difference

  • Dorsal Ulnar Cutaneous Sensory Study

    • In cases of suspected UNW where the routine ulnar sensory conduction to digit 5 is abnormal, it is important to study the dorsal ulnar cutaneous sensory nerve.  As the dorsal ulnar cutaneous sensory nerve arises 5 to 8 cm proximal to the wrist, it is expected to be normal in all cases of UNW.  A normal antidromic response is greater than 8 μV, but, as in other uncommonly performed sensory nerve conduction studies, comparison with the contralateral asymptomatic side frequently is helpful.   Any potential that is less than 50% of the amplitude of the contralateral asymptomatic side likely is abnormal as well, even if the absolute amplitude is greater than 8 μV.

  • Lumbrical-interossei comparison study.

    • This study is used most often in the diagnosis of carpal tunnel syndrome but can be equally helpful in the diagnosis of ulnar neuropathy at the wrist. The median nerve is stimulated at the wrist while the second lumbrical muscle is recorded ; the ulnar nerve is stimulated at the wrist, using the same distance, while the interossei muscles are recorded. 

    • In normal controls, latencies are similar.   In a patient with ulnar neuropathy at the wrist, the interossei latency is prolonged compared with the second lumbrical.

  • Wrist and Palmar Stimulation (most sensitive study)

    • To perform this study, the ulnar nerve is stimulated 3 cm above the wrist and 4 cm distal to the distal wrist crease in the palm, recording the FDI.

    • UNW can be localized either by finding a conduction block between the wrist and palm stimulation sites or by finding conduction velocity slowing across the wrist.   

    • In UNW, any conduction velocity less than 37 m/s is considered abnormal and is of localizing value. 

• Needle EMG:  FDI, ADM, APB, EIP, and FCU 

Claw hand

Cervical Plexus

• C1-C5  

• C1:  Ansa cervicalis superior root.  

• C2 + C3:  Ansa cervicalis inferior root gives rise to less occipital nerve, greater auricular nerve, and transverse cervical nerve.  

• C3 + C4: supraclavicular nerve.  

• C3 + C4 + C5:  Phrenic nerve

Radiation induced neuropathies in head and neck cancer

https://www.ncbi.nlm.nih.gov/pmc/articles/PMC7685771/

https://link.springer.com/article/10.1007/s13760-014-0402-z

https://www.physio-pedia.com/Upper_Limb_Tension_Tests_(ULTTs)

https://www.ncbi.nlm.nih.gov/pmc/articles/PMC137445/

https://evtoday.com/articles/2010-may/brachial-and-axillary-arterial-access

Arteriovenous access and hand pain: the distal hypoperfusion ischemic syndrome

https://pubmed.ncbi.nlm.nih.gov/17699402/

Hand ischemia in the setting of an arteriovenous access on the ipsilateral side was for many years referred to simply as steal syndrome. Although steal syndrome is an important clinical entity, it is one of many potential causes of hand ischemia. Although most arteriovenous accesses show evidence of arterial steal, ischemic symptoms are rarely found. 3 Based on this observation, one can conclude that peripheral hypoperfusion and ischemia are more fundamental problems than arterial steal per se.

Distal hypoperfusion ischemia syndrome can occur in 2 specific entities that should not be described as steal syndrome: (1) focal arterial disease and (2) diffuse distal arteriopathy. Previous reports have highlighted that substantial (≥50%) arterial stenoses are common in dialysis patients who present with symptoms of hand ischemia or vascular access dysfunction. 45 Such lesions may be present anywhere within the arteries of the upper extremities, including the proximal arteries. Diffuse arteriopathy caused by vascular calcification and diabetes may also be a key contributor to the evolution of symptoms of distal hypoperfusion ischemia syndrome. 6

True steal syndrome can only be diagnosed if the mentioned clinical scenarios are ruled out. The syndrome is defined by high blood flow volume through an arteriovenous anastomosis that steals blood from the forearm arteries, which otherwise have no pathology. In the setting of true steal syndrome, arteriovenous access ligation has historically been the treatment of choice. 3 Such an approach leads to loss of an arteriovenous access. Although open surgical banding and a variety of other highly invasive techniques have been used, they are limited by their invasiveness and increased risk for access thrombosis due to the reduction in access flow. 7 The goal of therapy must focus on augmenting arterial flow to the hand without compromising access flow to a degree that would induce thrombosis.

In addition, the shunting of blood from the distal extremity back to the venous circulation, and subsequently the right ventricle, has been implicated in the exacerbation of congestive heart failure (CHF). 8 In patients with end-stage renal disease, CHF contributes to morbidity and mortality. CHF is commonly linked to low cardiac output, but it has also been reported in high-output states (ie, cardiac output > 8 L/min or cardiac index > 3.9 L/min/m 2 ). 8 Several conditions are associated with increased cardiac output, including Paget disease, chronic hypercapnia, hyperthyroidism, sepsis, Beriberi heart disease, and congenital or intentionally created fistulas. 9 AVFs have been reported to be associated with reduced peripheral resistance with a consequent increase in venous return and preload, which leads to decreased left ventricle compliance as early as 3 to 14 days after access creation. 10

In a recent review of arteriovenous access and high-output heart failure, Wasse and Singapuri 11 suggested that fistula ligation is associated with mixed results, with some but not all studies showing improvement in heart failure symptoms. All the studies cited by Wasse and Singapuri 11 were limited by a shared methodological flaw in that they did not include real-time assessment of the possible effects of the intervention on hemodynamic and clinical outcomes. 12 A recently published case series reported real-time hemodynamic and clinical testing at the time of potential access ligation. 12 Swan-Ganz hemodynamic data were acquired through transfistula access for assessment of the possible impact of access ligation on hemodynamic status. Invasive real-time hemodynamic monitoring verified a high-output state (cardiac output of 6 L/min), low systemic vascular resistance, and a hyperdynamic left ventricle on the echocardiogram with ejection fraction of 80%. Access flow in one of the cases 12 was 2.4 L/min with estimated cardiopulmonary recirculation of 34%, which corresponds to an effective cardiac output of 3.6 L/min. Bedside compression of the patient’s arteriovenous graft led to in a marked elevation in blood pressure and improvement in tachycardia (ie, the Nicoladoni-Israel-Branham sign). 13

Recently, a minimally invasive (MILLER procedure) technique was described that combined an access diameter reduction procedure with direct angiographic evidence of flow determination in the fistula and distal arteries as a potential treatment of both distal hypoperfusion ischemia syndrome and CHF. 2 The technique was evaluated in 183 patients; of these, 114 presented with hand ischemia (steal), whereas 69 had clinical manifestations of pathologic high-access flow (eg, CHF). Technical success and symptom amelioration, primary and secondary access patency, and primary band patency were the primary outcomes studied. In all, 229 bandings were performed and technical success was attained in 225 patients. Complete relief of symptoms was achieved in 109 patients with steal and in all patients with high flow. 2 Follow-up duration averaged 11 months, and at 6 months, the primary band patencies in patients with steal and high flow were 75% and 85%, respectively. 2 Secondary access patency was 90% at 2 years, with thrombotic event rates of 0.21, 0.10, and 0.92 per access-year for upper-arm fistulas, forearm fistulas, and grafts, respectively. 2

Surgical interventions such as banding, DRIL (distal revascularization-interval ligation), and RUDI (revision using distal artery as inflow) have been applied to patients with hand ischemia. 14151617 Surgical banding has been commonly used to manage patients with distal hypoperfusion ischemia syndrome; however, the procedure is associated with a 6-month patency of only 9%. 14 The DRIL technique creates a bypass that begins upstream from the arteriovenous anastomosis and connects to the brachial artery just distal to the anastomosis. 1516 To block retrograde flow into the arteriovenous access, the brachial artery is subsequently ligated immediately below the fistula anastomosis. In a report of 23 patients who underwent the DRIL procedure, symptoms resolved in 19 patients. 15 Patency of the bypass was 96% at 2 years, whereas fistula patency was 73% and 46% at 1 and 2 years, respectively. Another larger report (n = 52) demonstrated that at a mean follow-up of 16 months, 90% of patients presenting with hand ischemia had complete or significant improvement in their symptoms after the DRIL procedure. 16 RUDI, an alternative procedure, has also been used to manage hand ischemia. 17 While the surgical procedures clearly provide additional options, these are rather extensive interventions (particularly DRIL) and require surgical expertise with these interventions.

Based on the minimally invasive nature and excellent patency rates, the current case was treated with the MILLER procedure. The symptoms of hand ischemia resolved after the procedure and the fistula has remained functional at a follow-up of 9 months without episodes of thrombosis.

The MILLER procedure should be recognized as a potential therapeutic option in the treatment of distal hypoperfusion ischemia syndrome instead of simply referring all patients with this clinical entity for access ligation. The procedure can be successfully performed on an outpatient basis under local anesthesia and sedation. Additionally, the procedure allows direct visualization of the access and distal arterial flow. Finally, ultrasonographic determination of access flow can be easily obtained during or after the procedure.