Disorders of cranial nerves

The 12 cranial nerves are peripheral nerves except for the optic nerve which is a central nervous system tract. Disorders of particular note include the following: Olfactory (I) nerve—anosmia is most commonly encountered as a sequel to head injury.

Third, fourth, and sixth cranial nerves—complete lesions lead to the following deficits (1) third nerve—a dilated and unreactive pupil, complete ptosis, and loss of upward, downward and medial movement of the eye; (2) fourth nerve—extorsion of the eye when the patient looks outwards, with diplopia when gaze is directed downwards and medially; (3) sixth nerve—convergent strabismus, with inability to abduct the affected eye and diplopia maximal on lateral gaze to the affected side. The third, fourth, and sixth nerves may be affected singly or in combination: in older patients the commonest cause is vascular disease of the nerves themselves or their nuclei in the brainstem. Other causes of lesions include (1) false localizing signs—third or sixth nerve palsies related to displacement of the brainstem produced by supratentorial space-occupying lesions; (2) intracavernous aneurysm of the internal carotid artery—third, fourth, and sixth nerve lesions. Lesions of these nerves can be mimicked by myasthenia gravis.

Pupillary abnormalities—these include (1) constriction (miosis)—due to paralysis of the sympathetic innervation (Horner’s syndrome); (2) dilatation—due to lesions of the third nerve. Trigeminal nerve—pathology causes numbness and tingling of the side of the face and scalp back to the vertex, loss of the corneal reflex and deviation of the jaw to the affected side. May be affected by intramedullary lesions, during the intracranial part of its course, and extracranially. Trigeminal neuralgia is usually due to compression of the nerve by aberrant vessels in the posterior fossa. It is characterized by paroxysms of intense pain strictly confined to the nerve’s distribution and often responsive to carbamazepine.

Facial nerve—in upper (but not lower) motor neuron lesions there is relative preservation of power in the upper facial muscles. In Bell’s palsy, onset is rapid and frequently heralded or accompanied by aching pain in or around the ear: treatment with prednisolone improves the prognosis. Hemifacial spasm is characterized by irregular clonic or simultaneous twitching movements of the facial muscles, usually of insidious onset; injections of botulinum toxin may be helpful. Glossopharyngeal nerve—rarely affected in isolation, when it is very difficult to detect any neurological deficit; usually affected in combination with the vagus nerve.

Vagus nerve—important symptoms of damage relate to pharyngeal and laryngeal innervation producing a bulbar palsy with dysphonia, dysarthria and dysphagia. Causes include brainstem stroke, motor neuron disease, malignant infiltration anywhere along the course of the nerve and cranial polyneuropathy. Spinal accessory nerve—may be affected by lesions, often neoplastic, in the region of the jugular foramen, but more commonly by injuries to the neck or by operations for the removal of cervical glands.

Hypoglossal nerve—may be affected by tumours in the region of the anterior condyloid foramen, or by tumours or penetrating injuries in the neck. The commonest cause of bilateral lesions is the progressive bulbar palsy variant of motor neuron disease.

Multiple cranial neuropathies (cranial polyneuropathy) may be due to inflammatory conditions (such as Guillain–Barré syndrome, Miller Fisher syndrome, vasculitis, Wegener’s granulomatosis, sarcoidosis); infections (such as tuberculosis, Lyme borreliosis); diabetes mellitus; meningeal carcinomatosis or lymphomatosis; or may be mimicked by myasthenia gravis, motor neuron disease, or myopathies. Cranial MRI and cerebrospinal fluid analysis are the key investigations.

Bulbar palsy is the syndrome of abnormality of cranial nerves arising from the medulla oblongata (nerves IX, X, XI, and XII). Tongue wasting and fasciculations, absent gag reflex, and flaccid dysarthria with nasal regurgitation distinguish this lower motor neuron syndrome from pseudobulbar palsy which is an upper motor neuron lesion of the medulla. A unilateral lesion of nerves V, VII, and VIII suggests pathology in the cerebellopontine angle such as vestibular schwannoma.