Types of Hearing Loss
The Causes and Management of the Two Types of Hearing LossOnly once it has been taken from them are most people likely to truly appreciate the extent to which they depend upon their ability to perceive and interpret sounds. Of these, comparatively few will be aware of the intricate mechanisms that make this sensory experience possible. However, many of those who, as a result of one or both types of hearing loss, have been denied this ability could well have retained it, given a little more knowledge of auditory physiology and an understanding of some of the factors that can lead to its impairment.
Beyond the visible external appendage known as the pinna, the ear is comprised of three distinct sections. The outer ear contains a convoluted channel that terminates in a membrane known as the tympanum or eardrum. This ear canal serves to funnel external sounds to the eardrum, which then vibrates to an extent determined by the volume and frequency of the incoming sounds. These vibrations then agitate the first of three tiny bones or ossicles in the middle ear, transferring the vibrations successively to the other two. The last of the ossicles impacts on a window-like membrane leading to the third chamber, or inner ear.
The mechanism responsible for the transmission of sound in both the outer and middle sections of the ear is mechanical conduction. Any damage or deformities that may occur in either of these can interfere with this action and lead to the type of hearing loss known, appropriately, as conductive deafness.
Often, this is a temporary condition and can arise when one or the other of these regions of the ear experiences a physical blockage. In the outer region, for instance, this could be an excessive build-up of wax or trapped water from showering or swimming, any of which can prevent the eardrum from vibrating. In the middle ear, infections can lead to the accumulation of fluid that dampens the vibrations of the ossicles. This is common in infants, and the condition is known as otitis media or “glue-ear”. Enlarged adenoids can create a similar effect by compressing the Eustachian tube.
In some subjects, infections recur despite repeated treatment, and without surgical intervention, the conductive deafness may become permanent. A rarer cause of permanent conductive insufficiency is otosclerosis, in which hardening around the ossicles interferes with their vibrations. Fortunately, people affected by this type of hearing loss can generally manage their auditory impairment very effectively with a conventional or bone-anchored hearing aid.
An organ located within the third chamber of the ear, and known as the cochlea, serves to convert mechanical vibrations into nerve impulses. These are conveyed by the auditory nerve to specialised centres in the brain that interpret them as specific sounds, such as spoken words or musical notes. Responsible for the conversion of those incoming mechanical vibrations are the specialised hair cells that line the interior of the fluid-filled cochlea. When the third ossicle impacts on the cochlear window, the movement is transferred to the fluid, which in turn agitates the sensitive cilia of the hair cells. It is this movement that results in the conversion to a nerve impulse.
There are a number of factors that may lead to the partial or total destruction of some or all of these sensitive hair cells. Such damage results in varying degrees of the second type of hearing loss described by audiologists as sensorineural. Among the many things that may be responsible for damage to the hair cells are exposure to certain organic solvents such as toluene, xylene, and benzene, while in some subjects, OTC medications such as aspirin and ibuprofen may be the culprits. Ironically, when used to treat infections, including those affecting the ear, some antibiotics, including streptomycin and gentamycin, can also cause sensorineural impairment.
However, of all those factors with the potential to damage the hair cells, repeated and prolonged exposure to loud noise is by far the most common. No longer confined to the workplace where it is still the most prevalent industrial injury, noise-induced hearing loss is now affecting pre-teenage children in significant numbers, due to their love of listening to music at full volume on iPods and smartphones. Manageable with conventional hearing aids in most cases, those with severe to profound sensorineural impairment may require a cochlear implant.
Both types of hearing loss are progressive, and early diagnosis will ensure effective management. If you suspect a problem, consult an Ear Institute audiologist without delay.