Sensorineural Hearing Loss
In sensorineural hearing loss the damage lies in the inner ear, the acoustic nerve, or both. Most physicians call this condition “nerve deafness.” In the majority of cases, it is not curable. The cochlea has approximately 30,000 hearing nerve endings (hair cells). Those hair cells in the large end of the cochlea respond to very high-pitched sounds, and those in the small end (and throughout much of the rest of the cochlea) respond to low-pitched sounds. These hair cells, and the nerve that connects them to the brain, are susceptible to damage from a variety of causes.
The word “sensorineural” was introduced to replace the ambiguous terms “perceptive deafness” and “nerve deafness.” It is a more descriptive and more accurate anatomical term. Its dual character suggest that two separate areas may be affected, and, actually, this is the case. The term “sensory” hearing loss is applied when the damage is localized in the inner ear. Common synonyms are “cochlear” or “inner-ear” hearing loss. “Neural” hearing loss is the correct term to use when the damage is in the auditory nerve proper, anywhere between its fibers at the base of the hair cells and the auditory nuclei (relay stations in the brain). This range included the bipolar ganglion of the eighth cranial nerve. Other common names for this type of loss are “nerve deafness” and “retrocochlear hearing loss.” These names are useful if applied appropriately and meaningfully, but too often they are used improperly.
There are various and complex causes of sensorineural hearing loss, but certain features are characteristic and basic to all of them. Because of histories obtained from patients are so diverse, they contribute more insight to the etiology (cause) than into the classification of a hearing loss.
Although at present, it is common practice to group together both sensory and neural components, it has become possible in many cases to attribute a predominant part of the damage, if not all of it, to either the inner ear or the nerve. Because of some success in this area and the likelihood that ongoing research will allow us to differentiate between even more cases of sensory and neural hearing loss, we shall divide the terms and describe the distinctive features of each type. This separation is advisable because the prognosis and treatment of the two kind of hearing impairment differ. For example, in all cases of unilateral sensorineural hearing loss, it is important to distinguish between a sensory and neural hearing impairment, because the neural type may be due to an acoustic neuroma (benign tumor in the ear-brain interface) which could become serious. Those cases in which we cannot identify as either sensory or neural and those cases in which there is damage in both regions we shall classify as sensorineural.
Sensorineural hearing loss is one of the most challenging problems in medicine. A large variety of hearing impairments fall under this category. The prognosis for restoring a sensorineural hearing loss with presently available therapy is poor. However, in a minority of cases specific treatable causes are found, and dramatic improvements are occasionally achieved. Although some spontaneous remissions and hearing improvements with therapy have occurred, particularly in cases involving sensory loss, a great need for further research still exists.