Cranial nerves

I.      Olfactory nerve

The fine olfactory filaments pass through the cribiform plate to join the olfactory bulb that runs on the undersurface of the frontal lobe. Damage can result from acceleration/decelera­tion injuries causing shifts in the position of the brain, frac­tures of the ethmoid bone and meningioma arising from the floor of the anterior cranial fossa. The sense of smell is impaired, and because of the strong relationship between smell and taste this can considerably affect the enjoyment of food and drink.

II.      Optic nerve

The optic nerve is an outgrowth from the cerebrum and has an investing nerve sheath, enclosing cerebrospinal fluid, which allows intracranial hydrostatic pressure to be trans­mitted to the optic fundus. A rise in intracranial pressure may be manifest by swelling of the optic disc (papilloedema). The optic nerve may be damaged as it leaves the skull and glial tumours may arise within the substance of the optic nerve, particularly in children. By testing the visual field, it may be possible to infer the site of intrinsic or extrinsic lesions affecting the optic pathways.

III.      Oculomotor nerve

A complete lesion of this cranial nerve causes total paralysis of the levator palperae superioris, resulting in ptosis. A proptosis will occur because of loss of tone of the extraocular muscles which normally exert traction on the globe. Owing to the unopposed action of the sixth and seventh cranial nerves, the eye is deviated downwards and outward and, when the lid is lifted, diplopia will occur. Because of the unopposed action of sympathetic fibres there is dilatation of the pupil, which is unresponsive to both light and accommodation. The length of the intracranial course of the third cranial nerve exposes the nerve to damage either intracranially, as it leaves the skull, or within the orbit. In circumstances of raised intracranial pressure, herniation of the uncus of the temporal lobe through the tentorial notch leads to pressure on the third cranial nerve and a dilated pupil. This is a late and serious sign of raised intracranial pressure.

IV.      Trochlear nerve

The fourth cranial nerve supplies the superior oblique muscle and is rarely involved by itself. It is associated with mild diplopia.

V.      Trigeminal nerve

This nerve has a sensory portion, conveying sensation from the face and a motor root, supplying the muscles of mastica­tion. There are three divisions of the sensory part of the nerve: the ophthalmic, the maxillary and the mandibular (Fig. 34.13). Large tumors in the cerebellar pontine angle may affect the trigeminal nerve. However, the commonest clinical manifestation of trigeminal nerve dysfunction is trigeminal neuralgia. This condition occurs predominantly in the middle aged and elderly, with a female predominance. It is characterised by severe, dagger-like pain within one or more divisions of the trigeminal nerve. Frequently the pain is triggered by any movement or stimulus to the face. It is becoming increasingly recognised that ectatic vascular loops may cause compression of the fifth nerve, producing these symptoms. Management of trigeminal neuralgia is, in the first instance, with carbamazepine. However, if this fails surgery may be considered to relieve vascular compression in the posterior fossa or to disrupt the trigeminal ganglion using percutaneous thermocoagulation.

VI.      Abducent  nerve

This nerve supplies the lateral rectus muscle and, when it occurs in isolation, results in diplopia due to the unopposed action of the medial rectus muscle. Because of its long intra­cranial course, the sixth nerve may be affected by fractures of the skull base or, alternatively, a supratentorial mass lesion may result in traction of the nerve as it passes over the petrous tip.

  VII.    Facial nerve

The seventh cranial nerve gives a motor supply to the mus­cles of facial expression and its sensory branch, the chorda tympani, carries taste from the anterior two-thirds of the tongue.

In clinical practice the motor supply is of most importance. Paralysis of the facial muscles may result from upper or lower motor neuron lesions. Because of bilateral cortical represen­tation of motor supply to the upper half of the face, upper motor neuron lesions, such as those caused by cerebrovascu­lar events, will result in weakness of the face with preserva­tion of eye closure and forehead movement. In a lower motor neuron lesion, all muscles innervated by the facial nerve will be affected and this results in complete facial weakness with loss of resting tone and of facial expression (Fig. 34.14).

The causes of facial nerve damage include:

cerebellar pontine angle lesions, such as an acoustic neuroma;

Bell’s palsy, a mononeuritis may be related to viral infection;

trauma to the nerve during surgery on the parotid gland.

  Damage to the nerve in the face should be repaired as with other peripheral nerves. If damage or a defect occurs during the course of a cerebellar pontine angle operation, such as removal of an acoustic neuroma, then the facial nerve may be reconstituted using either a nerve graft or a piece of freeze—thawed skeletal muscle. When repair is not possible a nerve transfer of the hypoglossal to the facial nerve or cross face transfer may be carried out. Alternatively, plastic surgical procedures may be used to improve the resting state of the face.

VIII.    Vestibulocochlear nerve

The eighth cranial nerve carries information from the vestibular apparatus and organ of Corn (hearing). The surgical significance of the eighth nerve is that it may be involved in fractures of the middle cranial fossa or be affected by tumors such as an acoustic neuroma.

IX.    Glossopharyngeal nerve

The motor supply of the ninth cranial nerve is to the stylo­pharyngeus muscle, which cannot be tested clinically. There­fore, for the purposes of examination, the ninth cranial nerve carries sensation from the soft palate and the posterior third of the tongue. It mediates the sensory component to the gag reflex. It may be affected by fractures of the skull base or by pathology involving the lower cranial nerve roots as they leave the brainstem.

X. Vagus nerve

This nerve has a small sensory supply to the ear canal, with motor innervation to the palate and vocal cords. Although the nerve may be affected throughout its course, damage to the recurrent laryngeal nerve, which may occur during a thyroid operation as a result of intraoperative traction, divi­sion or postoperative haematoma formation, is of particular note. A complete recurrent laryngeal nerve palsy results in paralysis of both abductors and adductors of the correspond­ing vocal cord, which therefore adopts a halfway position, the so-called cadaveric position. The opposite vocal cord can compensate, closing the glottis, but the range of the voice is impaired. Partial recurrent laryngeal nerve involvement has a predilection to the muscles of abduction leading to adduction of the vocal cord on the affected side. If this is bilateral, stridor can result, which rarely may require tracheostomy.

  XI.    Accessory nerve

This is the nerve to the sternocleidomastoid and trapezius. It may be damaged by base-of-skull fractures, but it is more commonly affected in its cervical course. It is particularly at risk during operations on the posterior triangle, for example biopsy of lymph nodes. Division of the nerve in the anterior triangle will produce paralysis of the sternocleidomastoid and trapezius muscles (Fig. 34.15). Damage in the posterior triangle will affect only the trapezius, resulting in a drooping shoulder with wasting of the trapezius and often a consider­able amount of pain. If the injury is recognised early then direct repair or nerve grafting will usually allow some recovery.

XII.    Hypoglossal nerve

This is the motor nerve to the tongue, and damage results in wasting, weakness and fasciculation on the affected side. On protrusion of the tongue, deviation occurs towards the side of the lesion. The twelfth nerve may be involved with intra­cranial pathology but is more often injured distally, partic­ularly during operations such as those on the submandibular gland.