Having relatively short courses, the peripheral parts of cranial nerves are not subject to the kind of injuries that commonly afflict peripheral nerves in the limbs. Those most commonly affected by trauma are the first, second, third, sixth and seventh, but tumours, ischaemia of nerve trunks and aneurysms of adjacent vessels are among the more usual afflictions of these and other cranial nerves.
Olfactory nerve
Head injury may tear olfactory nerve filaments passing through the cribriform plate of the ethmoid, especially if a fracture involves this part of the anterior cranial fossa. Such a fracture usually causes leakage of CSF through the nose (CSF rhinorrhoea), from tearing of the meningeal sleeves that ensheathe the olfactory nerve bundles. If all filaments on one side are torn there will be complete anosmia on that side. Test by closing one nostril (with finger pressure) and sniffing familiar substances like coffee or oranges. Most deficiencies of smell are due to affections of nasal mucosa rather than neurological disease. Smell is an essential component of taste, and complaints about lack of taste may be due to loss of smell. Olfactory hallucinations, which are usually unpleasant, arise from the uncus of the temporal lobe (the cortical centre for smell).
Optic nerve
Assessment of the visual fields tests the integrity of the visual pathways from retina to cortex, and lesions at different points along the path give rise to characteristic defects, as illustrated in Figure 7.9. The defects are conventionally described with reference to the visual fields and not to the retina. The numbers below correspond to the sites of the lesions in Figure 7.9A; clinically the most common lesions are at the chiasma (2) and in the optic radiation (4). The visual field defects are indicated in Figure 7.9B.
1. A complete lesion of the left optic nerve gives rise to complete blindness in the left eye.
2. Compression of the optic chiasma, as by a pituitary tumour, causes bitemporal hemianopia (blindness in the temporal half of both visual fields) because the nasal fibres from both retinas are interrupted. This effectively narrows the outer part of each visual field, so that the patient complains of bumping into the sides of a doorway or into people on each side.
3. A lesion of the left optic tract gives a right homonymous hemianopia, due to interruption of fibres from the same (left) sides of both retinas (hence homonymous, meaning same-sided). The field defects are therefore right-sided.
4. A lesion of the lower fibres in the left optic radiation (as from an abscess in the temporal lobe from infection spreading upwards from the middle ear) causes a right upper quadrantic homonymous hemianopia, because the lower fibres in the optic radiation are from the lower part of the retina.
5. Similar to (4), a lesion of the upper fibres in the left optic radiation (as from a parietal lobe lesion, and in practice very rare) gives a right lower quadrantic homonymous hemianopia.
6. A lesion of the anterior part of the left visual cortex (as from occlusion of the posterior cerebral artery) gives a right homonymous hemianopia similar to the optic tract lesion in (3), but there may be sparing of the macular (central) vision when the most posterior part of the visual cortex at the very tip of the occipital lobe, where macular vision is represented, is (sometimes) supplied by the middle cerebral artery.
7. Traumatic damage to the tip of the left occipital lobe, i.e. to the macular area, gives a right homonymous macular defect.
Oculomotor, trochlear and abducens nerves
Ocular nerve palsies are described on page 404. The major signs are as follows.
Oculomotor nerve: ptosis. When the lid is lifted up, the eye is looking down and out. The diplopia disappears on looking outwards. The pupil is dilated and does not react to light or on accommodation; ptosis with a large pupil thus suggests an oculomotor nerve lesion (but with a small pupil suggests Horner's syndrome; see p. 408).
Trochlear nerve: the eye cannot look down as far as it should when turned in. The head is tilted towards the opposite shoulder to compensate for extorsion.
Abducens nerve: the eye cannot look outwards.
Trigeminal nerve
Affections of the motor part of the fifth nerve, whose fibres run in the mandibular branch, are very unusual. Test for contraction of masseter.
The most common condition affecting the sensory part of the nerve is trigeminal neuralgia (tic doloureux), characterized by pain in the distribution of the maxillary and/or mandibular branches. The ophthalmic branch is rarely involved. With the maxillary nerve affected the pain is usually felt deeply in the face and nose between the mouth and orbit, and with the mandibular nerve from the mouth up to the ear and the temporal region. Compression of the trigeminal nerve adjacent to the pons by contact with a vessel (usually a branch of the basilar artery, such as the superior cerebellar or anterior inferior cerebellar artery) is considered to be a cause of trigeminal neuralgia, and patients resistant to medical therapy have been relieved of their pain surgically by the placement of a pad between the vessel and the nerve. Sclerosant injection or electrocoagulation of the trigeminal ganglion have also been utilized to abolish the pain.
The afferent side of the corneal reflex (see p. 406) depends on the ciliary branches of the nasociliary part of the ophthalmic nerve. Disappearance of the reflex is often the first sign of a lesion of the ophthalmic nerve; test by gently touching the cornea (not the conjunctiva) with cotton wool.
Facial nerve
Facial nerve paralysis is the most common of all cranial nerve lesions, and the most frequent type is Bell's palsy, a sudden onset of facial paralysis of uncertain aetiology. Paralysis of facial muscles causes asymmetry of the corner of the mouth, flattening of skin folds and inability to close the eye or wrinkle the forehead on the affected side. Test by asking the patient to show the teeth forcibly, screw up the eyes and wrinkle the forehead.
A lesion higher in the facial canal, above the origin of the chorda tympani, will add to the facial paralysis a loss of taste in the anterior part of the tongue. Test by holding the tongue out and applying sweet, sour and salt substances on cotton wool to the sides of the dorsum, asking the patient to point to the appropriate flavour written on a card. The accompanying interference with secretion from the submandibular and sublingual glands is very difficult to detect or test. A lesion higher still, above the origin of the nerve to stapedius, will give hyperacusis.
The above are all lesions of the lower motor neuron (infranuclear). A typical upper motor neuron lesion (supranuclear) paralyses the lower part of the face but not the upper (forehead and orbicularis oculi) because the upper part of the facial nerve nucleus which innervates the upper musculature is supplied by the cerebral cortex of both sides, whereas the lower part innervating the lower face only receives contralateral cortical fibres. However, emotional (as opposed to voluntary) movements of the lower facial muscles, as in smiling and laughing, are still possible with supranuclear lesions (so presumably there must be alternative pathways through the cerebrum).
Vestibulocochlear nerve
Acoustic neuromas on the extracerebral part of the eighth nerve are among the most common intracranial tumours, but other lesions of this nerve are rare. In any loss of hearing, it must be determined whether it is conduction deafness from lesions of the external or middle ear, or nerve deafness from a cochlear lesion.
The simple tests for hearing involve asking the patient to listen to whispering, the ticking of a watch, and the rubbing together of a thumb and forefinger (with the opposite ear closed by finger pressure on the tragus, and after examining the external acoustic meatus and tympanic membrane with an auroscope). Both ears are similarly tested. The basis of the two common tuning fork tests is that air conduction, which involves the amplifying effect of the ossicles across the middle ear cavity, is better than solid bone conduction. In Weber's test a vibrating tuning fork is placed on the midline of the forehead; in conduction deafness the sound is heard better in the deaf ear, and in nerve deafness it is better in the good ear. In Rinne's test the vibrating fork is held on the mastoid process until the sound is no longer heard and then quickly transferred to the external acoustic meatus; in a normal ear or with nerve deafness the sound will be heard again, but in conduction deafness further sound will not be heard. For more precise information audiometric tests are necessary.
Diseases of the internal ear or its central connections (as in Ménière's disease where there are degenerative changes in the utricle and saccule) form one group of causes of muscular incoordination or ataxia (labyrinthine ataxia; the other varieties of ataxia are cerebellar and sensory; see p. 489). The semicircular ducts of the vestibular apparatus can be tested with the head in different positions by irrigating the external acoustic meatus alternately with warm (44°C; 110°F) and cool (30°C; 86°F) water (caloric tests); this stimulates convection currents in the endolymph causing vertigo (giddiness) and nystagmus (an involuntary oscillatory movement of the eyes, which may be horizontal, vertical or rotatory). Internal ear disease gives an exaggerated response with the head in a particular position, whereas vestibular nerve damage gives an exaggerated response in any head position.
Glossopharyngeal nerve
Isolated glossopharyngeal nerve lesions are extremely rare. The last four cranial nerves may, however, be affected together (e.g. by a posterior cranial fossa tumour). Since the motor part of the glossopharyngeal nerve supplies only one small muscle, stylopharyngeus, it is impossible to test. If necessary, taste sensation on the posterior part of the tongue can be tested, with difficulty. The gag reflex (palatal elevation and pharyngeal contraction on tactile stimulation of the posterior tongue or oropharynx) tests both glossopharyngeal (afferent arc) and vagal (efferent arc) function.
Vagus nerve
Through its pharyngeal and laryngeal branches the motor component of the vagus nerve (nucleus ambiguus) supplies the pharynx, palate and larynx. Recurrent laryngeal nerve palsies are most commonly due to malignant disease (25%) and surgical damage (20%) during operations on the thyroid gland, neck, oesophagus, heart and lung. Because of its longer and partly intrathoracic course, lesions of the left nerve are more frequent than those of the right. Test the motor innervation of the vocal cords by listening to the patient's speech and cough and by inspecting the cords through a laryngoscope.
High lesions of the vagus nerve which affect the pharyngeal and superior laryngeal as well as the recurrent laryngeal branches cause difficulty in swallowing as well as vocal cord defects. The motor innervation of the soft palate can be tested by asking the patient to say a prolonged ‘Ah’ and observing the upward palatal movement; if paralysed on one side the unaffected side will rise further and pull the uvula towards the normal side.
Accessory nerve
The spinal part of the accessory nerve may be damaged in the posterior triangle, paralysing the trapezius. Test by asking the patient to shrug the shoulder; the paralysis may not be complete on account of some motor innervation occasionally reaching the muscle through cervical nerves. A lesion higher up before the nerve enters sternocleidomastoid will paralyse that muscle also; test whether the patient can turn the face to the opposite side against resistance.
Hypoglossal nerve
Paralysis of this nerve makes its own half of the tongue immobile and if of long standing the tongue will be wasted on that side. Test by asking the patient to put out the tongue; it will deviate towards the affected side due to the unopposed action of the normal half.