Disorders of Acoustic Function
Sensorineural deafness can be secondary to diseases of the cochlea, auditory nerve or brain stem. The cochlea may be congenitally aplastic, or it could be damaged by maternal rubella. Mumps or meningitis may cause nerve deafness from childhood. Drugs like streptomycin, gentamicin, quinine and salicylates could damage the cochlear hair cells and produce deafness.
The auditory nerve could be involved by tumors of the cerebellopontine angle or by basal meningitis. Deafness could also result by demyelinating lesions in the brain stem. Middle ear deafness is usually secondary to wax, chronic otitis media, otosclerosis and fracture of temporal bone.
Vestibular dysfunction usually presents with vertigo especially if the lesion is acute. Meniere's disease is a disorder of labrynthine function characterized by recurrent attacks of vertigo associated with tinnitus and deafness. It begins in the fifth decade. These attacks last for minutes to an hour. Nystagmus is present during the acute attack with a slow phase towards the affected ear. The attacks usually occur in clusters with progressive hearing loss. The pathologic change consists of dilatation of the endo-lymphatic system with destruction of the cochlear hair cells. Rest in bed and antihistaminics during the acute attack are useful. In intractable situations, surgical destruction of the labyrinth allays the symptoms.
Caloric test: Each ear is syringed with warm water (44 degrees) and cold water (30 degrees) with the patient resting supine and head flexed to 30 degrees, till a jerky nystagmus sets in. The show phase is due to local stimulation and the fast phase is mediated through cortical connections. Irrigation with cold water produces nystagmus with fast component to the opposite side. The nystagmus persists for 90-140 sec. Syringing with warm water produces nystagmus with the fast component to the same side. The time for onset of nystagmus and its duration are recorded. When there is perforation of the tympanic membrane, water should not be used for the test, instead jets of air at different temperatures can be used.
Significance of the caloric tests: The caloric tests depend upon the integrity of the vestibular apparatus, its connections and the higher cortical influences. This test is used in assessing the integrity of these structures. In addition, since the fast phase of the nystagmus is mediated through the cortical coma, loss of the fast phase indicates progressive loss of cortical function.
Tinnitus: This is a sensation of noise caused by abnormal excitation of the acoustic pathoway in the absence of any external stimuli. TiInnitus may take several forms such as tuning, roaring, whistling, bussing etc. It is felt most during sleep when the environment is silent. It may interfere with normal hearing. Common causes include lesions of the nose and middle ear such as Eustachian catarrh, Otitis media or occlusion of the external ausitory media or occlusion of the external auditory meatus by wax. Several drugs like salicylates, quinine, and streptomycin may produce tinnitus. Neurological disorders like acoustic neuroma, meningitis, and vascular lesions of the lateral aspects of the pons may lead to tinnitus. Tinnitus may also occur as an aura of temporal lobe epilepsy. Acute vestibular neuronitis could occur at any age and it is characterized clinically by paroxysmal and usually a single attack of vertigo without tinnitus of deafness.
For more pictorial information of brain and nervous system pathologies, click on this link: Pictorial Brain Anomalies.
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