THE VESTIBULOCOCHLEAR NERVE (EIGHTH CRANIAL NERVE) Medical Assignment Help

This nerve has two parts-cochlear and vestibular. Cochlear nerve Auditory fibres from the spiral organ (of Corti) in the cochlea pass to the cochlear nuclei in the pons. Fibres from these nuclei cross the midline and pass upwards through the medial lemnisci to the medial geniculate bodies and the temporal gyri.

The symptoms of a cochlear nerve lesion are deafness and tinnitus. The signs are of hearing loss, with bone conduction decreased as well as air conduction. This is called sensorineural or perceptive deafness.

Vestibular nerve

Vestibular fibres from the three semicircular canals, the saccule and the utricle pass to the vestibular nuclei in the pons. Vestibular nerve fibres also pass directly to the cerebellum.

The vestibular nuclei are connected to the spinal cord, the cerebellum, the nuclei of the ocular muscles and the centre for lateral The maintenance of balance and posture depends in part upon impulses passing between the neck and spinal muscles and the vestibular system. The main symptom of a vestibular lesion is vertigo.

Vomiting frequently accompanies acute vertigo of any cause. Nystagmus is the principal physical sign, often with loss of balance.

Vertigo 

Vertigo, the definite illusion of movement of the subject or surroundings, indicates a disturbance of vestibular, brain stem or, rarely, cortical function. The principal causes are given in Table 18.13.

Deafness and tinnitus accompanying vertigo indicate that its origin is from the ear or the eighth cranial nerve.

Nystagmus

Nystagmus is a rhythmic oscillation of the eyes. It is a sign of disease of either the ocular or the vestibular system and its connections. Nystagmus is described as either ‘pendular’ or ‘jerk’.

For true nystagmus to be present it must be sustained and demonstrable within binocular gaze.

PENDULAR NYSTAGMUS. Pendular movement means movements to and fro which are similar both in velocity and amplitude. Pendular nystagmus is almost always binocular, horizontal and present in all directions of gaze. It is seen when there is poor visual fixation (e.g. longstanding, severe visual impairment) or as a congenital lesion, when it is sometimes associated with headnodding. It very rarely occurs in ‘neurological’ diseases.
JERK NYSTAGMUS. Jerk nystagmus (the usual nystagmus of neurological disease) has a fast and a slow component
to the rhythmic movement. It is seen in vestibular, brain stem, cerebellar and (very rarely) cortical lesions. The direction of the nystagmus is named after the fast component, which can be thought of as a reflex attempt to correct the slower component. Considerable difficulties exist when attempts are made to use the direction of jerk nystagmus alone as a localizing sign, although it is both a common and valuable indication of abnormality. The following are useful starting points:

Meniere’s disease

Drugs (e.g. gentamicin, anticonvulsant intoxication)
Toxins (e.g. ethyl alcohol)
Vestibular neuronitis’
Multiple sclerosis
Migraine
Acute cerebellar lesions
Cerebellopontine angle lesions
Partial seizures (temporal lobe focus)
Brain stem ischaemia or infarction
Benign positional vertigo

HORIZONTAL OR ROTARY NYSTAGMUS may be either of peripheral (middle ear) or central (brain stem and its connections) origin. In peripheral lesions it is usually transient (minutes or hours); in central lesions it is long-lasting (weeks, months or more).

ERTICAL NYSTAGMUS is caused only by central lesions. DOWN-BEAT NYSTAGMUS, a rarity, is caused by lesions around the foramen magnum.

Investigation of vestibulocochlear nerve lesions

AUDIOMETRY is of value in distinguishing sensorineural deafness from conductive deafness. CALORIC TESTS are used to assess function of the labyrinth. These record the evoked nystagmus when first ice cold, then warm, water is run into the external meatus. Decreased or absent nystagmus indicates ipsilaterallabyrinth, eighth nerve or brain stem involvement. In the normal caloric test:

ICE COLD WATER IN THE LEFT EAR causes nystagmus with the fast movement to the right WARM WATER IN THE LEFT EAR causes nystagmus with the fast movement to the left The right ear would give opposite responses.

AUDITORY EVOKED POTENTIALS record the response from a repetitive ‘click’ stimulus. The level of the lesion may be detected by abnormalities in the response.

Lesions of the eighth nerve and its connections

Lesions at five levels can be recognized by the associated signs.

Cortex 

Vertigo sometimes occurs as an aura in a partial seizure of temporal lobe origin. Deafness is very rare in cortical lesions.

Pons 

Vertigo is common with demyelinating or vascular lesions of the brain stem that involve the vestibular nuclei and their connections. A sixth or seventh nerve lesion, an internuclear ophthalmoplegia or contralateral hemiparesis
help localization. Nystagmus is frequently present. Deafness is very rare in pontine lesions.

Cerebellopontlne angle

Perceptive deafness occurs. Sixth, seventh and fifth nerve lesions develop, followed by cerebellar signs (ipsilateral) and later ‘pyramidal’ signs (contralateral). Nystagmus is usually present.

Causes include acoustic neuroma, meningioma and secondary neoplasm.

Petrous temporal bone

A seventh nerve lesion may be present. Causes include trauma, middle ear infection and Paget’s disease of bone.

End-organ disease
The main causes are:

  • Meniere’s disease
  • Drugs (e.g. gentamicin)
  • Noise
  • Middle-ear infection
  • Intrauterine rubella
  • Congenital syphilis
  • Mumps
  • ‘Vestibular neuronitis

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