Wilkinson. Essential Neurology 2005

Nuclei, intermedullary nerve fibre pathways, cranial nerve, sensory ganglion and the three main branches of the trigeminal nerve (motor in dark grey; sensory in green)

Cerebrum

3

4

9

10

12

Spinal

cord

Introduction

Disorders of the cranial nerves usually produce clear abnormalities, apparent to both patient and doctor alike. The specialists who become involved in the management of patients with cranial nerve problems are neurologists, neurosurgeons, ophthalmologists (cranial nerves 2–4, 6), dentists (cranial nerve

5)and ENT surgeons (cranial nerves 1, 5, 7–10, 12).

Cranial nerves 1, 2 and 11 are a little different from the others.

Nerves 1 and 2 are highly specialized extensions of the brain, for smell and sight, in the anterior cranial fossa and suprasellar region. Nerve 11 largely originates from the cervical spinal cord, rises into the posterior fossa only to exit it again very quickly, to supply muscles of the neck and shoulder.

It is useful to remember that the other cranial nerves (3–10 and 12; Fig. 8.1) can be damaged at three different points along their paths. The lesion may affect the nucleus of the cranial nerve within the brainstem, where its cell bodies lie. Alternatively, the lesion may damage the axons travelling to or from the nucleus but still within the brainstem. In both these situations there is commonly damage to nearby pathways running through the brainstem, so that in addition to the cranial nerve palsy, the patient will often have weakness, sensory loss or ataxia in the limbs. Finally the lesion may affect the nerve itself outside the brainstem as it passes to or from the structure which it supplies. This causes either an isolated cranial nerve palsy, or a cluster of palsies arising from adjacent nerves. Examples of these clusters include malfunction of 5, 7 and 8 caused by an acoustic neuroma in the cerebellopontine angle, or malfunction of 9, 10 and 11 due to malignancy infiltrating the skull base.

Fig. 8.1 Lateral aspect of the brainstem, and cranial nerves 3–10 and 12 (seen from the left).

Olfactory (1) nerve (Fig. 8.2)

Patients who have impaired olfactory function complain that they are unable to smell and that all their food tastes the same. This reflects the fact that appreciation of the subtleties of flavour (beyond the simple sweet, salt, acid, meaty and bitter tastes) is achieved by aromatic stimulation of the olfactory nerves in the nose. This is why wine tasters sniff and slurp.

The commonest cause of this loss is nasal obstruction by infective or allergic oedema of the nasal mucosa. Olfactory nerve function declines with age and with some neurodegenerative diseases. Olfactory nerve lesions are not common. They may result from head injury, either involving fracture in the anterior fossa floor, or as a result of damage to the nerves on the anterior fossa floor at the time of impact of the head injury. Sometimes, the olfactory nerves stop working on a permanent basis for no apparent reason, i.e. idiopathic anosmia. Very occasionally, a tumour arising from the floor of the anterior fossa (e.g. meningioma)maycauseunilateralorbilaterallossofolfactoryfunction.

Anterior cranial fossa

Nasal cavity

Fig. 8.2 Olfactory nerve and bulb on the floor of the anterior cranial fossa, and olfactory nerve bundles penetrating the thin cribiform plate to innervate the mucosa in the roof of the nasal cavity.

Visual field

Eye

Optic nerve

Optic chiasm

Optic tract

Lateral geniculate body

Optic radiation

Visual cortex

Ipsilateral monocular blindness

Bitemporal hemianopia

Contralateral homonymous

hemianopia

Fig. 8.3 The anatomy of the visual pathways, and the three common types of lesion occurring therein.

CRANIAL NERVE DISORDERS

When recording visual field defects, the convention is to show the field from the left eye on the left, and the right eye on the right, as if the fields were projecting out of the patient’s eyes and down onto the page

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Monocular blindness

Monocular visual disturbances occur transiently in the prodromal phase of migraine (see pp. 214–15), or as a consequence of thrombo-embolism in the ophthalmic artery, as a result of ipsilateral carotid artery atheromatous disease or embolism from the heart. Transient visual loss due to embolization often commences ‘like a curtain descending over the vision’. Infarction of the optic nerve or retina, with permanent monocular visual loss, is relatively uncommon in patients with thrombo-embolic disease, though common in untreated patients with giant cell arteritis (see p. 218). Monocular visual loss occurs in patients with optic neuritis as part of multiple sclerosis (see p. 103).

Rarely, impairment of vision in both eyes occurs as a result of bilateral simultaneous optic nerve disease:

•bilateral optic neuritis due to multiple sclerosis;

•methanol poisoning;

•Leber’s hereditary optic neuropathy;

•tobacco–alcohol amblyopia;

•longstanding papilloedema due to untreated intracranial hypertension.

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Homonymous hemianopia

Homonymous hemianopia, for example due to posterior cerebral artery occlusion, may or may not be noticed by the patient. If central vision is spared, the patient may become aware of the field defect only by bumping into things on the affected side, either with his body, or occasionally with his car! If the homonymous field defect involves central vision on the affected side, the patient usually complains that he can see only half of what he is looking at, which is very noticeable when reading.

Though posterior cerebral artery occlusion and infarction of the occipital cortex is the commonest cerebral hemisphere lesion causing permanent visual loss, other hemisphere lesions do cause visual problems:

•an infarct or haematoma in the region of the internal capsule may cause a contralateral homonymous hemianopia, due to involvement of optic tract fibres in the posterior limb of the internal capsule. Contralateral hemiplegia and hemianaesthesia are commonly associated with the visual field defect in patients with lesions in this site;

•vascular lesions, abscesses and tumours situated in the posterior half of the cerebral hemisphere, affecting the optic radiation (between internal capsule and occipital cortex), may cause incomplete or partial homonymous hemianopia. Lesions in the temporal region, affecting the lower parts of the optic radiation, cause homonymous visual field loss in the contralateral upper quadrant. Similarly, by disturbing function in the upper parts of the optic radiation, lesions in the parietal region tend to cause contralateral homonymous lower quadrant field defects.

More subtle dysfunction in the visual pathways may cause difficulty in attending to stimuli in one half of the visual field, effectively a lesser form of contralateral homonymous hemianopia. In this situation the patient can actually see in each half of the visual field when it is tested on its own. When both halffields are tested simultaneously, for example by the examiner wiggling her fingers to either side of a patient who has both eyes open, the patient consistently notices the finger movements on the normal side and ignores the movements on the affected side. This phenomenon, which is common after strokes, is referred to as visual inattention or visual neglect.

Left

Left

Left

CHAPTER 8

Right

Right

Right

Upper motor neurone

Supranuclear gaze palsies

Right eye muscles

Lower motor neurone

in cranial nerves 3, 4 and 6

Left eye muscles

Centres and pathways for conjugate gaze, and cranial nerve nuclei 3, 4 and 6, in midbrain and pons

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Supranuclear gaze palsy

•Site of lesion: cerebral hemisphere.

•Common.

•Common causes: massive stroke; severe head injury.

•Movement of the eyes to the left is initiated by the right cerebral hemisphere, just like all motor, sensory and visual functions involving the left-hand side of the body. Each cerebral hemisphere has a ‘centre’ in the frontal region, involved in conjugate deviation of the eyes to the opposite side. Patients with an acute major cerebral hemisphere lesion are unable to deviate their eyes towards the contralateral side. This is the commonest form of supranuclear gaze palsy (right cerebral hemisphere lesion, and paralysis of conjugate gaze to the left in the diagram).

•The centres for conjugate gaze in the brainstem and the cranial nerves are intact. If the brainstem is stimulated reflexly to induce conjugate eye movement, either by caloric stimulation of the ears, or by rapid doll’s head movement of the head from side to side, perfectly normal responses will occur. Paralysis of voluntary conjugate gaze, with preserved reflex conjugate eye movement, is the hallmark of supranuclear gaze palsy.

•Supranuclear vertical gaze palsy, i.e. loss of the ability to look up or down voluntarily, is occasionally seen in neurodegenerative diseases.

Gaze palsy

At the midbrain level

•Uncommon.

•The programming of the 3rd and 4th cranial nerve nuclei for conjugate vertical eye movement, and for convergence of the two eyes, occurs in centres in the midbrain. The paralysis of voluntary and reflex eye movement which occurs with lesions in this region is known as Parinaud’s syndrome.

At the pontine level

•Uncommon.

•Conjugation of the two eyes in horizontal eye movements is achieved by an ipsilateral pontine gaze centre, as shown in Fig. 8.6. A lesion in the lateral pontine region (on the right in the diagram) will cause voluntary and reflex paralysis of conjugate gaze towards the side of the lesion.

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R L

Eyes deviated to the right because of conjugate gaze palsy to the left

R L

Eyes won't move up or down in the vertical plane

Eyes won't converge

There may be associated ptosis and pupil abnormality

R L

Eyes deviated to the left because of conjugate gaze palsy to the right

CRANIAL NERVE DISORDERS

Fig. 8.6 Brainstem centres and pathways for conjugate horizontal movement. Voluntary gaze to the left is initiated in the right cerebral hemisphere. A descending pathway from the right cerebral hemisphere innervates the left pontine gaze centre. From there, impulses pass directly to the left 6th nerve nucleus to abduct the left eye, and (via the medial longitudinal fasciculus) to the right 3rd nerve nucleus to adduct the right eye.

Fig. 8.7 Normal eye movements in terms of which muscle and which nerve effect them. Diagram shows the right eye viewed from the front.

R L

Complete ptosis

Eye is deviated 'down and out' in the primary position

Dilated, non-reactive pupil

Normal abduction

Rotation of globe on attempted down-gaze

No

No

No other movement

R L

There may be some inturning of the eye and double vision in the primary position (because of weakness of right eye abduction in this diagram)

There may be compensatory head turning (to the right in this case) to obtain single vision whilst looking forward

No abduction of the eye

Third nerve palsy

•Common.

•Common causes:

posterior communicating artery aneurysm (painful); mononeuritis in diabetes (pupil usually normal);

pathology beside the cavernous sinus, in the superior orbital fissure or in the orbit (adjacent nerves commonly involved, e.g. 4, 6, 5a, and 2 if in the orbit).

•The parasympathetic innervation of the eye is supplied by the 3rd nerve.

•The diagram shows a complete right 3rd nerve palsy. The lesion can be incomplete of course, in terms of ptosis, pupil dilatation or weakness of eye movement.

Sixth nerve palsy

•Common.

•Common causes:

as a false localizing sign in patients with raised intracranial pressure;

multiple sclerosis and small cerebrovascular lesions within the pons;

pathology beside the cavernous sinus, in the superior orbital fissure or orbit (adjacent nerves commonly involved, e.g. 3, 4, 5a, and 2 if in the orbit).