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Superior laryngeal nerve paralysis

Introduction:
Anatomically superior laryngeal nerve is one of the branches of vagus nerve.  Paralysis involving this nerve is frequently overlooked because of complex
clinical picture.  Functionally speaking the superior laryngeal nerve function can be divided into sensory and motor components.  The sensori function
provides a variety of afferent signals from supraglottic larynx.  Motor function involves motor supply to ipsilateral cricothyroid muscle. 

Role of cricothyroid muscle in phonation:
Contraction of cricothyroid muscle tilts the cricoid lamina backward at the cricothyorid joint causing lengthening, tensing and adduction of vocal folds causing
an increase in the pitch of the voice generated.

Causes of superior laryngeal nerve paralysis:

1. Thyroid surgery

2. Skull base tumors

3. Oesophageal tumors

4. Trauma

Diagnosis of superior laryngeal nerve paralysis is based largely on symptomatology and clinical suspicion.

Symptoms:

1. Raspy voice
2. Voice fatigue
3. Volume deficit
4. Loss of singing volume







Investigations:

Laryngeal electromyographic examination:

Potentials generated by the contracting cricothryoid muscle can be recorded by placing electrodes in appropriate sites.  A paralysed
muscle usually generates injury potentials / no potential. 
Patient should assume supine position, with head slightly extended by keeping a shoulder roll. 
Laryngeal electromyography is performed using a 37mm monopolar electrode. 

Reference electrode - is placed over sternum.

Ground disk electrode - is placed over clavicle.

Settings:

Low frequency setting - 20Hz

High frequency setting - 10KHz

Identity of strap muscles is confirmed by slight head elevation without phonation.  The brisk signal is seen during this maneuver is from
the strap muscles.

The cricothyroid muscle is located by inserting the needle 5 - 10 mm off midline a the cricothyroid membrane level and angling it laterally
towards the cricoid cartilage.  Recordings from both sides are made.

Kymograph:

This is one very useful modality by which vibrations of the vocal folds can be clearly studied.  Movements of vocal folds are recorded using strobe light.
Kymogram of normal human phonation displays a characteristic rhomboid shape with symmetry.  The glottic excursions on both sides begin in the midline,
travel equal distances at equal rates and return back to the midline. 

Kymogram of a patient with Left superior laryngeal nerve paralysis displays the opening of the glottic cycle beginning on the right of the midline glottic longitudinal
axis opposite to the paralyzed side.  As the glottic space enlarges and reaches its maximum width it begins to shift across the glottic midline towards the paralysed
left side.  The resulting kymogram has a skewed orientation.



Figure showing a normal kymograph

Management:

Speech therapy will go a long way in restoring a near normal voice.

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