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.
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.
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.
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.