Vagus nerve (and cranial part of accessory)
This nerve has an extensive distribution but testing is based on its motor supply to the soft palate and to the larynx.
1. Ask the patient to open the mouth wide and say ‘aah’. Observe the movement of the soft palate. In a normal person the soft palate is elevated. When one vagus nerve is paralysed the palate is pulled towards the normal side. When the nerve is paralysed on both sides the soft palate does not move at all.
2. In injury to the superior laryngeal nerve the voice is weak due to paralysis of the cricothyroid muscle. At first there is hoarseness but after some time the opposite cricothyroid compensates for the deficit and hoarseness disappears.
3. Injury to the recurrent laryngeal nerve also leads to hoarseness, but this hoarseness is permanent. On examining the larynx through a laryngoscope it is seen that on the affected side the vocal fold does not move. It is fixed in a position midway between adduction and abduction. In cases where the recurrent laryngeal nerve is pressed upon by a tumour it is observed that nerve fibres that supply abductors are lost first.
4. In paralysis of both recurrent laryngeal nerves voice is lost as both vocal folds are immobile.
5. It may be remembered that the left recurrent laryngeal nerve runs part of its course in the thorax. It can be involved in bronchial or oesophageal carcinoma, or by secondary growths in mediastinal lymph nodes.