Sensorineural Hearing Loss

The Nature and Management of Sensorineural Hearing Loss

To those who enjoy normal auditory function, that there should be a need to classify deafness may come as a surprise. For those less fortunate, however, as part of their audiological examination, in addition to an audiogram to quantify the extent of impairment and the frequencies of sound most affected, they would have also been required to undergo the Rinne and Weber tests. These are necessary in order to confirm whether the hearing loss is of sensorineural, conductive, or mixed origin, and their results help the audiologist to decide on the most effective means with which to manage it.

The ear consists of three adjoining chambers. The outermost of these acts to funnel external sounds via the auditory canal, which then causes the tympanic membrane or eardrum to vibrate, much like the diaphragm in a microphone. Resting against the other side of the eardrum, in the middle compartment, is the first of three tiny bones or ossicles. Known as the malleus or hammer, the first bone conducts the vibrations via the second bone named the incus or anvil, to the third one of the trio, known as the stapes or stirrup. This impinges on the cochlear window – the gateway to the inner ear. Sensorineural hearing loss arises due to damage in this third compartment, whilst anomalies in the outer and/or middle ear result in conductive deafness. When both types of impairment occur simultaneously, the condition is referred to as mixed.

The vibrating cochlear window causes wave-like disturbances in the fluid within, which are then transmitted along its length to the region known as the organ of Conti. This region is lined with specialised cells known as hair cells. Their delicate hair-like projections generate nerve impulses when stimulated by the ripples in the cochlear fluid and these are transmitted by the auditory nerve to the sound centre or auditory cortex in the brain for interpretation. In addition to mixed, conductive, and sensorineural, there is a fourth type of deafness. When the auditory nerve is unable to perform its function, this is referred to as neural hearing loss.

In addition to classifying auditory impairment based on its point of origin, it is further classified according to its severity. On a conventional audiogram, zero decibels is taken as the threshold of human audition. Those whose threshold is between 20 and 40 dB are classified as mildly impaired, between 40 and 75 dB is moderate, and a threshold of between 75 to 90 dB is termed severe and anyone with a threshold above that is said to be profoundly deaf. Most of those with conductive problems together with those whose sensorineural hearing loss is moderate or less can usually gain sufficient benefit when fitted with a conventional hearing aid to resume a normal lifestyle.

For those whose functioning hair cells are sufficiently depleted to result in severe to profound impairment, a device that simply amplifies incoming sounds is unlikely to be of any benefit, so an alternative means by which to manage their deafness will be necessary. Prior to October 1982, their condition would have meant a life of silence. It was then that one Graham Carrick of Melbourne became the first recipient of a cochlear implant and heard his first sounds after 17 years of silence. This remarkable device has since transformed the lives of hundreds of thousands of people with sensorineural hearing loss worldwide. So how does it work?

There is little point in directing vibrations along the conductive pathway formed by the outer and inner ears and the cochlear fluid when the mechanism for converting them into nerve impulses doesn’t work. Instead, the implant is designed to bypass it entirely. An implanted receiver converts the input from an externally-worn microphone, speech processor, and transmitter into a series of modulated electrical impulses applied directly to the auditory nerve via an electrode array. The signal received by the auditory cortex differs from that of a normal nerve impulse, but those with sensorineural hearing loss normally adapt quite quickly to the new sensations, perceiving them as the familiar patterns of speech and other sounds.

Not everyone is a suitable candidate for an implant, however, so a thorough preliminary examination will be conducted to ensure that the implantation is possible and likely to be of benefit. If you have auditory difficulties, be sure to consult an audiologist who will determine its severity and whether it is conductive or sensorineural hearing loss in order to implement the most effective management option.
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