However, due to IHC damage, the whole tuning curve becomes raised, giving a loss of sensitivity across all frequencies (See Figure 6). It is only necessary for the first row of OHCs to be damaged for the elimination of the finely tuned ‘tip’ to occur. This supports the idea that the incidence of OHC damage and thus a loss of sensitivity to quiet sounds, occurs more than IHC loss.When the IHCs or part of the basilar membrane are damaged or destroyed, so that they no longer function as transducers, the result is a ‘dead region’. Dead regions can be defined in terms of the characteristic frequencies of the IHC, related to the specific place along the basilar membrane where the dead region occurs. Assuming that there has been no shift in the characteristic frequencies relating to certain regions of the basilar membrane, due to the damage of OHCs. This often occurs with IHC damage. Dead regions can also be defined by the anatomical place of the non-functioning IHC (such as an “apical dead region”), or by the characteristic frequencies of the IHC adjacent to the dead region. Dead region audiometry
Pure tone audiometry (PTA)
Dead regions affect audiometric results, but perhaps not in the way expected. For example, it may be expected that thresholds would not be obtained at the frequencies within the dead region, but would be obtained at frequencies adjacent to the dead region.
Acoustic neuroma (vestibular schwannoma) – benign neoplasm of Schwann cells affecting the vestibulocochlear nerve
Meningioma – benign tumour of the pia and arachnoid mater
Ménières disease – causes sensorineural hearing loss in the low frequency range (125 Hz to 1000 Hz). Ménières disease is characterized by sudden attacks of vertigo, lasting minutes to hours preceded by tinnitus, aural fullness, and fluctuating hearing loss. It is relatively rare and commonly over diagnosed. Bacterial meningitis e.g. pneumococcal, meningococcal, haemophilus influenzae may damage the cochlea – Hearing loss is one of the most common after-effects of bacterial meningitis. It has been estimated that 30% of bacterial meningitis cases result in mild to profound hearing loss. Children are most at risk: seventy percent of all bacterial meningitis occurs in young children under the age of five. Viral
AIDS and ARC patients frequently experience auditory system anomalies. Mumps(epidemic parotitis) may result in profound sensorineural hearing loss (90 dB or more), unilaterally (one ear) or bilaterally (both ears). Measles may result in auditory nerve damage but more commonly gives a mixed (sensorineural plus conductive) hearing loss, and can be bilaterally. Ramsay Hunt syndrome type II (herpes zoster oticus)
Bacterial
Syphilis is commonly transmitted from pregnant women to their fetuses, and about a third of the infected children will eventually become deaf. Ototoxic and neurotoxic drugs and chemicals
Some over-the-counter as well as prescription drugs and certain industrial chemicals are ototoxic. Exposure to these can result in temporary or permanent hearing loss. Some medications cause irreversible damage to the ear, and are limited in their use for this reason.