Deafness tinnitus and vertigo

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CENTRAL CONNECTIONS (contd)

Auditory: From the cochlear nucleus, second order neurons either pass upwards in the lateral lemniscus to the ipsilateral inferior colliculus or decussate in the trapezoid body and pass up in the lateral lemniscus to the contralateral inferior colliculus.

Third order neurons from the inferior colliculus on each side run to the medial geniculate body on both sides.

Fourth order neurons pass through the internal capsule and auditory radiation to the auditory cortex.

The bilateral nature of the connections ensures that a unilateral central lesion will not result in lateralised hearing loss.

Vestibular:

MIDBRAIN

MIDBRAIN

To cortex

-To nuclei for eye muscles

Medial ... longitudinal fasciculus

Vestibular nuclei

MIDBRAIN

Dorsal cochlear nuc.

From cochlea

Medial geniculate body

Inferior colliculus

Lateral lemnisci

1. Directly to cerebellum.

2. Second order neurons arise in the vestibular nucleus and descend in the ipsilateral vestibulospinal tract.

3. Second order neurons project to the oculomotor nuclei (III, IV, VI) through the medial longitudinal fasciculus.

4. Second order neurons project to the cortex (temporal lobe). The pathway is unclear.

5. Second order neurons project to the cerebellum.

Medial geniculate body

Inferior colliculus

Lateral lemnisci

1. Directly to cerebellum.

2. Second order neurons arise in the vestibular nucleus and descend in the ipsilateral vestibulospinal tract.

3. Second order neurons project to the oculomotor nuclei (III, IV, VI) through the medial longitudinal fasciculus.

4. Second order neurons project to the cortex (temporal lobe). The pathway is unclear.

5. Second order neurons project to the cerebellum.

(There is a bilateral feedback loop to the vestibular nuclei Vestibulospinal tract from the cerebellum through the fastigial nuclcus.)

DEAFNESS: Three types of hearing loss arc recognised:

1. Conductive deafness: failure of sound conduction to the cochlea.

2. Sensorineural deafness: failure of action potential production or transmission due to disease of the cochlea, cochlear nerve or cochlear central connections.

Further subdivision into cochlear and retrocochlear deafness helps establish the causative lesion.

3. Pure word or cortical deafness: a bilateral or dominant posterior temporal lobe (auditory cortex) lesion produces a failure to understand spoken language despite preserved hearing.

TINNITUS: a sensation of noise of ringing, buzzing, pulsing, hissing or singing quality.

Tinnitus may be (i) continuous or intermittent, (ii) unilateral or bilateral, (iii) high or low pitch.

As a rule, when hearing loss is accompanied by tinnitus, conductive deafness is associated with low pitch tinnitus - sensorineural deafness is associated with high pitch tinnitus, except Meniere's disease where tinnitus is low pitch. Pulsing tinnitus has a vascular cause.

VERTIGO: an illusion of rotatory movement due to disturbed orientation of the body in space. The sufferer may sense that the environment is moving. Vertigo may result from disease of the labyrinth, vestibular nerve or their central connections.

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