Читать книгу Engineering Acoustics - Malcolm J. Crocker - Страница 172

There are several causes of sensory‐neural hearing loss and all are associated with disorders of the inner ear, the auditory nerve fibers, the auditory cortex in the brain, or combinations of all three. Unlike conductive deafness, sensory‐neural deafness is often most severe at higher frequencies. Background noise can thus mask the consonants in speech and make it unintelligible. Although a hearing aid brings some relief, it is perhaps not so useful as with conductive deafness. Very profound sensory‐neural hearing loss may be restored only by using a cochlear implant.

Congenital deafness is often a cause of sensory‐neural deafness. Here genes producing imperfect hearing mechanisms are inherited. During the first three months of pregnancy, if a mother contracts certain viral diseases such as mumps, influenza, or German measles, these can impair the development of the hearing mechanism in the fetus. During birth itself, brain damage can affect the hearing mechanism. In early childhood, diseases such as meningitis can damage the inner ear or auditory nerve, producing deafness. Perforations in the eardrum can also be caused. However, the eardrum has a remarkable ability to heal itself, even if ruptured by intense sound.

All these forms of sensory‐neural hearing loss mentioned are presently medically untreatable. One exception is Ménière's disease, in which the inner ear becomes distended. Drugs can sometimes improve this condition.

Another reported cause of sensory‐neural deafness is drug‐induced hearing loss which is called ototoxicity. There are many approved drugs that can cause ototoxicity through direct effect on cochlear biology and they include anti‐cancer chemotherapy drugs, some kinds of antibiotics, quinine, and loop diuretics used to treat hypertension [49].

These drugs can cause either temporary or permanent hearing loss and vertigo and tinnitus. Recent studies have suggested that certain industrial chemicals may also potentiate the effect of noise exposure [50, 51]. In this case, damage to the cochlea is the direct result of chemical injury to the fragile hair cells within the organ of Corti.