Types of Hearing Loss Detail
Patricia E. Connelly, PhD, CCC-A, FAAA, New Jersey Medical School, NEWARK, NJ
A comprehensive audiologic evaluation must be completed in order to determine the type and severity of hearing loss and to make appropriate recommendations for each patient. Pure tone and speech audiometry as well as the immittance test battery must be completed, in addition to any additional assessments necessary for an exhaustive profile of the hearing system. A balance test called electronystagmography (ENG) might also be needed if dizziness or imbalance is also a complaint. Some patients who are bothered by tinnitus only might have a complete tinnitus evaluation. Finally, the audiologic data provide the clinical foundation for recommendations regarding hearing aids and other assistive devices.
In general terms, there are two types of hearing loss, conductive and sensorineural. A combination of both is also seen as a mixed hearing loss. Each is discussed below.
Conductive Hearing Loss
Conductive hearing loss is caused by any condition or disease that impedes the conveyance of sound in its mechanical form through the middle ear cavity to the inner ear. A conductive hearing loss can be the result of a blockage in the external ear canal or can be caused by any disorder that unfavorably effects the middle ear's ability to transmit the mechanical energy to the stapes footplate. This results in the reduction of one of the physical attributes of sound called intensity (loudness), so the energy reaching the inner ear is lower or less intense than that in the original stimulus. Therefore, more energy is needed for the individual with a conductive hearing loss to hear sound, but once it's loud enough and the mechanical impediment is overcome, that ear works in a normal way. Generally, the cause of conductive hearing loss can be identified and treated resulting in a complete or partial improvement in hearing. Following the completion of medical treatment for cause of the conductive hearing loss, hearing aids are effective in correcting the remaining hearing loss.
The audiometric profile that indicates a conductive hearing loss is the presence of air-bone gaps (better hearing by bone conduction than by air conduction), excellent word recognition at a comfortable listening level, and evidence of a middle ear dysfunction on immittance. For situations where a blockage is noted in the external ear canal, hearing testing is deferred until the canal is cleared.
Sensorineural Hearing Loss
The second type of hearing loss is called sensorineural hearing loss. This word can be divided into its two components - sensory and neural - to allow us more clarity in specifying the type of hearing loss. The comprehensive audiometric assessment and supplemental tests can yield the information needed to differentiate between a sensory and a neural hearing loss, although they can co-exist in the same ear. Neural hearing loss is another name for retrocochlear hearing loss.
Sensorineural hearing loss results from inner ear or auditory nerve dysfunction. The sensory component may be from damage to the organ of Corti or an inability of the hair cells to stimulate the nerves of hearing or a metabolic problem in the fluids of the inner ear. The neural or retrocochlear component can be the result of severe damage to the organ of Corti that causes the nerves of hearing to degenerate or it can be an inability of the hearing nerves themselves to convey neurochemical information through the central auditory pathways.
The reason for sensorineural hearing loss sometimes cannot be determined, it does not typically respond favorably to medical treatment, and it is typically described as an irreversible, permanent condition. Like conductive hearing loss, sensorineural hearing loss reduces the intensity of sound, but it might also introduce an element of distortion into what is heard resulting in sounds being unclear even when they are loud enough. Once any medically treatable conditions have been ruled out, the treatment for sensorineural hearing loss is amplification through hearing aids.
Mixed Hearing Loss
A mixed hearing loss can be thought of as a sensorineural hearing loss with a conductive component overlaying all or part of the audiometric range tested. So, in addition to some irreversible hearing loss caused by an inner ear or auditory nerve disorder, there is also a dysfunction of the middle ear mechanism that makes the hearing worse than the sensorineural loss alone. The conductive component may be amenable to medical treatment and reversal of the associated hearing loss, but the sensorineural component will most likely be permanent. Hearing aids can be beneficial for persons with a mixed hearing loss, but caution must be exercised by the hearing care professional and patient if the conductive component is due to an active ear infection.