Biomedical Engineering Reference
In-Depth Information
hearing loss it appears that everyone around them is mumbling. The voices of women and
children, which are higher, tend to be the most difficult to understand.
At some stage during their lives, most people are exposed to noise levels that can
cause hearing loss. Loud sounds generally damage the sensory hair cells in the inner ear,
with the rate of destruction being proportional to the sound intensity and the duration.
A single extremely loud noise can lead to immediate hearing loss, and though it is often
only temporary, lasting a few hours to a day, it can become permanent if the sound is
repeated often enough. Common sources of noise, as listed in Table 6-1, include loud
music, particularly if listened to through headphones, power tools, machinery, and many
petrol-powered vehicles, particularly chainsaws and lawn mowers. Explosions and gunfire
can also damage hearing.
As mentioned earlier, young children often suffer from some degree of conductive
hearing loss after an ear infection (otitis media) because the infection often leads to a
buildup of fluid in the middle ear. Chronic infections of the middle ear often result in both
conductive and sensorineural hearing loss.
Autoimmune disorders such as rheumatoid arthritis, systemic lupus erythematosus,
and polyarteritis nodosa may also lead to hearing loss. These generally result in fluctuations
in hearing ability that progressively become worse as the immune system attacks cells in
the cochlea.
Drugs, particularly intravenous antibiotics such as vancomycin, can destroy sensory
hair cells and cause permanent hearing loss if given in high doses. Other drugs implicated
include quinine and chemotherapy drugs like cisplatin. Aspirin can also cause reversible
hearing loss.
Sudden deafness is the type that occurs within minutes or over a few hours. It may be
caused by something as trivial as wax accumulation or by head trauma, sudden changes
in pressure (as occurs in aircraft), or internal pressure changes caused by severe straining
(as may occur with weight lifting or blowing one's nose).
Hearing loss that is more severe in one ear than the other may be caused by a tumor.
These are mostly benign but should be removed as they can lead to tinnitus, difficulty with
balance, and facial numbness as well as deafness if left to grow unchecked.
6.4.2 Diagnosis
Audiologists perform hearing tests that can determine the degree of hearing loss over a
specific frequency range. As a first step, a sequence of sounds is generated with random
pitch and amplitude and the patient indicates whether each can be heard. During these
tests, which are conducted one ear at a time, white noise is played into the other ear.
This test determines the frequency response of each ear and is presented relative to the
normal hearing range, as shown in Figure 6-6. In this figure, X shows the response of
the left ear, O shows the response of the right, and S shows the response of both ears
together.
Other tests include speech threshold audiometry in which the patient identifies two-
syllable words at different volumes. The volume above which 50% of the words are
correctly identified is considered to be the threshold. Discrimination tests involve present-
ing pairs of similar one-syllable words. People with conductive hearing loss usually have a
normal discrimination score, albeit at a higher volume, whereas people with sensorineural
hearing loss often have abnormal discrimination at all volumes.
