Cranial Nerves: General

Published on 09/04/2015 by admin

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Last modified 22/04/2025

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Cranial Nerves

General

BACKGROUND

Abnormalities found when examining the ‘cranial nerves’ may arise from lesions at different levels (Fig. 5.1) including:

a. the central nervous system pathways to and from the cortex, diencephalon (thalamus and associated structures), cerebellum or other parts of the brainstem

b. lesions in the nucleus

c. lesions to the nerve itself

d. generalised problems of nerve, neuromuscular junction or muscle.

When examining the cranial nerves, you need to establish whether there is an abnormality in cranial nerve function, the nature and extent of the abnormality and any associations.

COMMON MISTAKES

Sometimes, when summarising neurological examination, people divide it into ‘cranial nerves’ and examination of the ‘peripheral nervous system’. This distinction misleads. By thinking of the examination in this way, you can forget that you are examining not just the cranial or peripheral nerves but also their central nervous system connections. To prevent ensnaring yourself in this trap, it is useful to think of examination of the ‘head and neck’ rather than the ‘cranial nerves’, and the ‘limbs’ rather than the ‘peripheral nervous system’. Tradition is so strong that this book continues to describe examination under the heading ‘cranial nerves’, but you know better…

Figure 5.1 Location of cranial nerve abnormalities (see text for key)

More than one cranial nerve may be abnormal:

• if there is a lesion where several cranial nerves run together either in the brainstem or within the skull (e.g. cerebellopontine angle or cavernous sinus)

• when affected by a generalised disorder (e.g. myasthenia gravis)

• following multiple lesions (e.g. multiple sclerosis, cerebrovascular disease, basal meningitis).

Abnormalities of cranial nerves are very useful in localising a lesion within the central nervous system.

Examination of the eye and its fields allows the examination of a tract running from the eye to the occipital lobe that also crosses the midline.

The nuclei of the cranial nerves within the brainstem act as markers for the level of the lesion (Fig. 5.2). Particularly useful are the nuclei of the III, IV, VI, VII and XII nerves. When the tongue and face are affected on the same side as a hemiplegia, the lesion must be above the XII and VII nucleus respectively. If a cranial nerve is affected on the opposite side to a hemiparesis, then the causative lesion must be at the level of the nucleus of that nerve. This is illustrated in Figure 5.3.

Figure 5.2 Level of cranial nerve nuclei in the brainstem, indicated by roman numerals

Figure 5.3 Flow chart: multiple cranial nerve abnormalities

Multiple cranial nerve abnormalities are also recognised in a number of syndromes:

• Unilateral V, VII and VIII: cerebellopontine angle lesion.

• Unilateral III, IV, V₁ and VI: cavernous sinus lesion.

• Combined unilateral IX, X and XI: jugular foramen syndrome.

– Combined bilateral X, XI and XII:

– if lower motor neurone = bulbar palsy

– if upper motor neurone = pseudobulbar palsy.

• Prominent involvement of eye muscles and facial weakness, particularly when variable, suggests a myasthenic syndrome.

• Multiple cranial nerve palsies can reflect basal meningitis—either malignant meningitis, chronic infective or inflammatory meningitis.

The most common cause of intrinsic brainstem lesions in younger patients is multiple sclerosis, and in older patients it is vascular disease. Rarer causes include gliomas, lymphomas and brainstem encephalitis.

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