Cranial Nerve Lesions

See cranial nerve examination record for a detailed reminder on how to examine the nerves.

Try to think systematically.

• Is it muscle (eg a dystrophy)?
• Neuromuscular junction (eg myasthenia)?
• Cranial nerve lesion outside the brainstem (eg compression)?
• Cranial nerve lesion within the brainstem (eg MS)?

Cranial nerves may be affected singly or in groups and knowledge of which nerves are involved helps locate the lesion. Some of the causes of cranial nerve lesions are given below, after a reminder of the anatomical course of the nerve.

• Anatomy Olfactory cells are a series of bipolar neurones which pass through the cribriform plate to the olfactory bulb.
• Signs Reduced taste and smell, but not to ammonia which stimulates the pain fibres carried in the trigeminal nerve.
• Causes Trauma; frontal lobe tumour; meningitis.

• Anatomy The optic nerve fibres are the axons of the retinal ganglion cells. At the optic chiasm, only the fibres derived from the nasal parts of the retina decussate, join with the non-decussating fibres and pass backwards in their respective optic tracts to the visual cortex.

Signs and causes

• Visual field defects:
  • Field defects start as small areas of visual loss (scotomas).
  • Monocular blindness: Lesions of one eye or optic nerve eg MS, giant cell arteritis.
  • Bilateral blindness: Methyl alcohol, tobacco amblyopia; neurosyphilis.
  • Bitemporal hemianopia: Optic chiasm compression eg internal carotid artery aneurysm, pituitary adenoma or craniopharyngioma
  • Homonymous hemianopia: Affects half the visual field contralateral to the lesion in each eye.
    Lesions lie beyond the optic chiasm in the tracts, radiation or occipital cortex eg stroke, abscess, tumour.
• Pupillary Abnormalities see pupillary abnormalities article.
• Optic neuritis (pain on moving eye, loss of central vision, afferent pupillary defect, papilloedema). Causes: demyelination; rarely sinusitis, syphilis, collagen vascular disorders.
• Optic atrophy (pale optic discs and reduced acuity): MS; frontal tumours; Friedreich's ataxia; retinitis pigmentosa; syphilis; glaucoma; Leber's optic atrophy; optic nerve compression.
• Papilloedema (swollen discs):
  1. Raised ICP (tumour, abscess, encephalitis, hydrocephalus, benign intracranial hypertension);
  2. Retro-orbital lesion (eg cavernous sinus thrombosis);
  3. Inflammation (eg optic neuritis);
  4. Ischaemia (eg accelerated hypertension).

• Anatomy This nerve emerges from the brainstem on the medial aspect of the crus cerebri and then passes forwards between the posterior cerebral and superior cerebellar arteries, very close to the posterior communicating artery. It pierces the dura near the edge of the tentorium cerebelli, passes through the lateral part of the cavernous sinus with the IV and VI nerves to enter the orbit.
• Signs The initial sign is often a fixed dilated pupil which doesn't accommodate; then ptosis develops and then a complete internal ophthalmoplegia (masked by ptosis). Unopposed lateral rectus causes outward deviation of the eye. If the ocular sympathetic fibres are also affected behind the orbit, the pupil will be fixed but not dilated.
• Causes of a single III lesion Diabetes mellitus; giant cell arteritis; syphilis; posterior communicating artery aneurysm; idiopathic; Raised ICP if causes uncal herniation through the tentorium - this compresses the nerve. Third nerve palsies without a dilated pupil are due to diabetes mellitus or another vascular cause. Early dilatation of a pupil implies a compressive lesion. Diplopia from a third nerve lesion may cause nystagmus .

See also Combined cranial nerve lesions (below).

• Anatomy Passes backwards in the brainstem, decussates in the anterior medullary velum and emerges to pass round the cerebral peduncle between it and the temporal lobe, passing over the tentorium to enter the cavernous sinus with II and VI, and enters the orbit to supply the superior oblique.
• Signs Diplopia due to weakness of downward and inward eye movement. Commonest cause of a pure vertical diplopia. Patient tends to compensate by tilting head towards the affected side.
• Causes of a single IV lesion Rare and most commonly due to trauma to the orbit. May also occur in diabetes or infarction secondary to hypertension.

See also Combined cranial nerve lesions (below).

• Anatomy Forms three trunks: ophthalmic, maxillary and mandibular divisions. The latter contains both sensory and motor fibres. There may be considerable individual variation in the exact areas of skin supplied.
  • Ophthalmic division lies with III, IV and VI in the cavernous sinus and supplies the skin over the medial nose, forehead, eye (including corneal reflex).
  • Maxillary division passes through the inferior part of the cavernous sinus and the foramen rotundum and joins with parasympathetic fibres to form the sphenopalatine ganglion (lacrimation). It then enters the orbit as the infraorbital nerve, eventually supplying the skin of the upper lip, cheek and triangle of skin extending from the angle of eye and mouth to an apex in the mid temporal region.
  • Mandibular division Leaves the skull through the foramen ovale carrying sensory fibres from the skin of the lower lip and chin up to and including the tragus and upper part of the pinna; mucus membranes of floor of the mouth, cheek and anterior two-thirds of the tongue (taste fibres joining it from the chorda tympani branch of the facial nerve). Motor fibres supply the masseter, temporalis, pterigoids.
• Signs Reduced sensation or dysasthesia over affected area. Weakness of jaw clenching and side to side movement. If there is a LMN lesion, the jaw deviates to the weak side when the mouth is opened. There may be fasiculation of temporalis and masseter.
• Causes of a single V lesion See also Combined cranial nerve lesions
  • Sensory: Trigeminal neuralgia, Herpes zoster, nasopharyngeal carcinoma.
  • Motor: Bulbar palsy, acoustic neuroma.

• Anatomy From the nucleus in the floor of the forth ventricle fibres pass forward in the pons and emerge to follow a long extracerebral course on the base of the brain, across the apex of the petrous temporal, through the posterior fossa near the dorsum sellae to enter the cavernous sinus and thence to the orbit and lateral rectus.
• Signs Inability to look laterally. Eye is deviated medially because of unopposed action of medial rectus.
  
  
  The failure of lateral rectus to move the left eye is very obvious. Ignore the eccentric reflection of the light as the patient is not trying to follow it.
• Causes of a single VI lesion MS, pontine CVA. It is considered a false localizing sign (because of long extracerebral course) in raised ICP.

See also Combined cranial nerve lesions (below).

• Anatomy Mainly motor (some sensory fibres from external acoustic meatus, fibres controlling salivation and taste fibres from the anterior tongue). Fibres loop around the VI nucleus before leaving the pons medial to VIII and passing through the internal acoustic meatus. It passes through the petrous temporal in the facial canal, widens to form the geniculate ganglion (taste and salivation) on the medial side of the middle ear whence it turns sharply (and the chorda tympani leaves), to emerge through the stylomastoid foramen to supply the muscles of facial expression.
• Signs Facial weakness. In a LMN lesion the forehead is paralysed - the final common pathway to the muscles is destroyed; whereas the upper facial muscles are partially spared in an UMN lesion because of alternative pathways in the brainstem. There appear to be different pathways for voluntary and emotional movement. CVA's usually weaken voluntary movement often sparing involuntary movements (eg spontaneous smiling). The much rarer selective loss of emotional movement is called mimic paralysis and is usually due to a frontal or thalamic lesion.
• Causes of a single VII lesion
  • LMN: Bell's palsy, polio, otitis media, skull fracture, cerebello-pontine angle tumours, parotid tumours, Herpes zoster (Ramsay-Hunt syndrome), Lyme Disease
  • UMN: (spares the forehead - bilateral innervation) Stroke, tumour.

See also Combined cranial nerve lesions (below).

• Anatomy Carries two groups of fibres, those to the cochlea (hearing) and to the semicircular canals, utricle and saccule (balance and posture). They pass, together with the facial nerve, from the brainstem across the posterior fossa to the internal acoustic meatus.
• Signs Unilateral sensorineural deafness, tinnitus. Slow growing lesions seldom present with vestibular symptoms as compensation has time to occur.
• Causes of a single VIII lesion loud noise; Paget's disease; Ménière's disease; Herpes zoster; neurofibroma, acoustic neuroma, brainstem CVA; lead; aminoglycosides; furosemide (frusemide); aspirin. See also Combined cranial nerve lesions

• Anatomy Contains sensory, motor (stylopharyngeus only) and parasympathetic fibres (salivary glands). Passes across the posterior fossa, through the jugular foramen and into the neck supplying tonsil, palate and posterior third of tongue.
• Signs Unilateral lesions do not cause any deficit because of bilateral cortico-bulbar connections. Bilateral lesions result in pseudo-bulbar palsy. These nerves are closely interlinked.
• Causes (single nerve lesions exceedingly rare) Trauma, brainstem lesions, cerebello-pontine angle and neck tumours, polio, Gullain-Barre.

• Anatomy The vagus nerve, "the wanderer", contains motor fibres (to the palate and vocal cords), sensory components (posterior and floor of external acoustic meatus) and visceral afferent and efferent fibres. It leaves the skull through the jugular foramen, passes within the carotid sheath in the neck (giving off cardiac branches, and the recurrent laryngeal nerves supplying the vocal cords), through the thorax supplying lungs, and continues on via the oesophageal opening to supply the abdominal organs.
• Signs Palatal weakness can cause "nasal speech" and nasal regurgitation of food. The palate moves asymmetrically when the patient says "ah". Recurrent nerve palsy results in hoarseness, loss of volume and "bovine cough".
• Causes (single nerve lesions exceedingly rare) Trauma, brainstem lesions, tumours in the cerebello-pontine angle, jugular foramen and neck; polio, Gullain-Barre.

• Anatomy Motor to sternocleidomastoid and trapezius.
• Signs weakness and wasting of these muscles.
• Causes as vagus above.

• Anatomy It passes briefly across the posterior fossa, leaves the skull through the hypoglossal canal and supplies motor fibres to the tongue and most of the infrahyoid muscles.
• Signs A LMN lesion produces wasting of the ipsilateral side of the tongue, with fasiculation; and on attempted protrusion tongue deviates towards affected side, but the tongue deviates away from the side of a central lesion.
  
  
  Note the wasted left side of the tongue and deviation to the left suggesting a left lower motor neurone lesion.
• Causes of a single XII lesion: Rare. Polio, syringomyelia TB, median branch thrombosis of the vertebral artery.

• VII, VIII, then V and sometimes IX: cerebellopontine angle tumours.
• V, VI (Gradenigo's syndrome): lesions within the petrous temporal bone.
• Combined III, IV, VI: stroke, tumours, Wernicke's encephalopathy, aneurysms, MS, myasthenia gravis, meningitis, muscular dystrophy, myotonic dystrophy, cavernous sinus thrombosis, Guillain-Barre, cranial arteritis, trauma and orbital pathology.