One of the vagus nerve’s functions is to carry parasympathetic fibers to the heart, and to glands and smooth muscle, both in the head and neck, and the alimentary tract as far as the splenic flexure. The vagus is involved in cardiorespiratory reflexes such as the Valsalva maneuver. Its somatic motor fibers pass to the palate, pharynx and larynx, and these are most easily examined. In the neck, the nerve can be damaged by malignant infiltration, and surgery of the thyroid gland, the carotid bifurcation, airway and pharynx. An auricular branch supplies the posterior aspect of the external acoustic meatus and can provide the afferent pathway of cough and vomiting reflexes.
The motor component of the vagus nerve produces movement of the soft palate and, with the accessory, produces swallowing and speech. The position of the uvula can be unreliable but ask the patient to say ‘ah’ to assess symmetry (figure 40). Observe or gently palpate the larynx while the patient is swallowing a glass of water (see – Thyroid), and note the pitch and power of speech.
Laryngeal and pharyngeal muscles are bilaterally innervated and dysarthria and dysphagia require lower motor neuron or bilateral upper motor neuron denervation. Unilateral damage to the recurrent laryngeal nerve, which may be produced by neoplasia or surgery of the thyroid gland, interferes with coughing (producing a bovine cough, without the explosive element produced by tight apposition of the vocal cords) and the subject is unable to sing a high pitched ‘ee’.