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)
Along with the radial nerve, the axillary nerve originates from the posterior cord of the brachial plexus. The axillary nerve is composed primarily of C5–C6 fibers, running through the upper trunk and posterior cord of the plexus. The nerve leaves the axilla through the quadrilateral space, which is formed by the humerus and the teres minor, teres major, and long head of the triceps muscles. Posteriorly in the quadrilateral space, it often divides into two major trunks. The posterior trunk always supplies the teres minor before terminating as the superior lateral brachial cutaneous nerve (i.e., axillary sensory nerve). The teres minor aids in the external rotation of the shoulder, while the deltoid is principally a shoulder abductor. The axillary sensory nerve supplies an oval-shaped area over the lateral shoulder. The anterior trunk travels deep to the fascia of the deltoid and always supplies the middle and anterior heads of the deltoid as well as a deep sensory branch to the shoulder joint. The posterior head of the deltoid is most commonly supplied by the posterior trunk, but some variations exist wherein it is supplied by the anterior trunk alone, and in others by a combination of the anterior and posterior trunks.
Axillary neuropathies typically result from trauma, especially dislocation of the shoulder and fracture of the humerus. Less commonly, athletes participating in contact sports have developed axillary neuropathies as a result of injury, typically a direct blow to the anterolateral deltoid area. Similar to suprascapular neuropathy, axillary neuropathies have been reported in professional volleyball players. Rare cases of entrapment in the quadrilateral space have been reported but are exceptional. Quadrilateral space syndrome results from compression of the axillary nerve and posterior humeral circumflex artery. Patients with axillary neuropathies have a well-defined oval area of numbness over the lateral shoulder, along with partial weakness of shoulder abduction and external rotation. The degree of weakness varies from patient to patient. The weakness is only partial because other muscles also contribute to shoulder abduction (i.e., the supraspinatus) and external rotation (i.e., the infraspinatus).
The major goal of electrodiagnosis is to demonstrate abnormalities of axillary-innervated muscles and rule out cervical radiculopathy, brachial plexopathy, or involvement of other proximal nerves. Unfortunately, there is no routine sensory nerve conduction study for the axillary nerve. However, because the axillary nerve originates from the posterior cord and upper trunk, sensory nerves that run through the posterior cord or upper trunk of the brachial plexus should be studied. These include the radial and lateral antebrachial cutaneous sensory nerves and the median sensory nerve, especially when recording the thumb. To detect mild abnormalities, a comparison with the contralateral asymptomatic nerve is suggested, even if the studies are within the normal range on the symptomatic side. Abnormalities of any of these sensory studies suggest a more widespread brachial plexopathy. Axillary motor nerve conduction studies can be performed, stimulating the axilla and Erb’s point and recording with a monopolar needle or surface electrode over the deltoid. A surface reference electrode is placed distally over the deltoid tendon. To calculate a conduction velocity, distances must be measured with calipers. CMAP amplitude can be compared both from side to side, to assess the amount of axonal loss, and between the axilla and Erb’s point on the symptomatic side, to look for conduction block. These studies can be technically difficult to perform, however, especially obtaining supramaximal stimulation, and are best used to assess axonal loss by comparing the symptomatic side to the asymptomatic side. Because these usually are axonal loss lesions, motor studies generally do not increase the yield of localizing the lesion beyond what is obtained from routine needle EMG. In axillary neuropathies, needle EMG is used to demonstrate denervation, reinnervation, or both in the two axillary innervated muscles: the deltoid and the teres minor. All three heads of the deltoid are easily accessible to needle EMG; the teres minor is more difficult to study. If abnormalities are found in any of these muscles, it is essential to sample other muscles innervated by the upper trunk and posterior cord of the brachial plexus to ensure that the abnormalities found in the axillary-innervated muscles are not part of a more widespread brachial plexus lesion or cervical radiculopathy. Muscles that are important to check include the biceps, supraspinatus, infraspinatus, triceps, and brachioradialis. In addition, the cervical paraspinals should be sampled to help exclude a C5–C6 radiculopathy.