Mixed Hearing Loss

The Nature, Causes and Management of Mixed Hearing Loss

Audition, or the ability to hear, is a physiological function that we seldom stop to consider until that ability is compromised in some way. That said, most people will have experienced a temporary partial impairment on at least one occasion. Understanding the mechanisms involved will provide better insight into the permanent condition that audiologists refer to as mixed hearing loss. The causes of the temporary malfunctions fall into one of two categories.

The eardrum, tympanum, or tympanic membrane responds to the impact of sound waves in much the same way as the diaphragm in a microphone. It vibrates in sympathy and conducts the waves in the form of mechanical vibrations, via the three tiny bones or ossicles in the middle ear, to the inner ear, where they are then converted into the nerve impulses required for interpretation by the brain.

A build-up of earwax that becomes impacted against the tympanum will limit its movement in response to sound waves and this, in turn, reduces the intensity of the conducted signal, and the volume of the perceived sound. A similar effect can occur when pus accumulates in the middle ear due to infection in the condition known as otitis media. Once the wax has been removed or the infection successfully treated, audition returns to normal.

The other common cause of temporary deafness is exposure to loud noises, such as a gunshot. In this case, there is temporary damage to the tiny hair cells lining the fluid-filled organ in the middle ear known as the cochlea. Conducted mechanical vibrations strike the cochlear window, agitating the fluid within. With fewer healthy hair cells, nerve impulses to the brain are reduced. The damaged hairs normally recover within a day or two, and sounds become clear once more.

Mixed hearing loss is defined as a condition in which both the conductive and the sensory functions of the ear are compromised. The condition is a permanent one, as neither form of impairment can be reversed. For each type of component of this condition, there are a number of possible causes.

For instance, where infections tend to recur, the condition can become chronic and eventually result in permanent damage to the ear’s conductive mechanisms. Often, an ENT surgeon can intervene in time to perform procedures, such as a tonsillectomy and adenoidectomy, which can reduce the risk of recurrent infections. In infants, the latter procedure may sometimes be used to eliminate an obstruction with reversal of the symptoms. In other cases, a myringotomy (insertion of grommets) can aerate the middle ear and prevent the build-up of pus. Other causes of inefficient conduction include otosclerosis, in which hardening of the bone surrounding one of the ossicles limits its ability to vibrate, and a malformation of the external ear canal, which occurs at birth and is known as atresia.

Sensorineural inefficiency, the other component of mixed hearing loss, occurs when the hair cells lining the cochlea are damaged beyond their ability to recover. Since the body is unable to produce new hair cells, the resulting auditory impairment is permanent. This occurs naturally as a result of aging, but is rarely a problem during one’s working life. Many more factors can cause this condition prematurely. Of these, prolonged and repeated exposure to loud noise, which results in noise-induced hearing loss (NIHL), is the most common. NIHL is alarmingly frequent among young people who play their mobile music devices at full volume.

Other causes include the secondary effects of certain illnesses, such as mumps, measles, and meningitis; exposure to ototoxic chemicals, such as toluene and xylene; and some drugs, including ibuprofen, aspirin, and the antibiotics, gentamicin and erythromycin.

Where impairment is mild to moderate, a conventional hearing aid can be adequate for the successful management of conductive, sensorineural or mixed hearing loss. In the case of those whose impairment is classified as severe to profound, the selective amplification provided by these devices will often no longer be sufficient, and the sole remaining solution may be a cochlear implant.

These devices collect soundwaves, amplify, and digitise them by means of microphones and a speech processor worn externally, transmitting them to an implanted receiver which distributes the various frequencies via an electrode array inserted into the cochlea. The resulting electrical impulses act similarly to nerve impulses, conveying the sensation of sounds to the brain.

If you suspect hearing loss, there is no time for mixed feelings. Consult a professional at your nearest Ear Institute clinic.
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