Hearing Loss
A hearing impairment or hearing loss is a full or partial decrease in the ability to detect or understand sounds. Caused by a wide range of biological and environmental factors, loss of hearing can happen to any organism that perceives sound.
Sound waves vary in amplitude and in frequency. Amplitude is the sound wave’s peak pressure variation. Frequency is the number of cycles per second of a sinusoidal component of a sound wave. Loss of the ability to detect some frequencies, or to detect low-amplitude sounds, that an organism naturally detects, is a hearing impairment.
Conductive hearing loss occurs when sound is not normally conducted through the outer or middle ear (or both). Since sound can be picked up by a normally sensitive inner ear even if the ear canal, ear drum, and ear ossicles are not working, conductive hearing loss is often only mild and is never worse than a moderate impairment. Hearing thresholds will not rise above 55-60 dB from outer or middle ear problems alone. Generally, with pure conductive hearing loss, the quality of hearing (speech discrimination) is good, as long as the sound is amplified loud enough to be easily heard.
A conductive loss can be caused by any of the following:
Ear canal obstruction
Middle-ear abnormalities
A sensorineural hearing loss is due to insensitivity of the inner ear (the cochlea), or to impairment of function in the auditory nervous system. It can be mild, moderate, or severe, including total deafness.
The great majority of human sensorineural hearing loss is caused by abnormalities in the hair cells of the organ of Corti in the cochlea. There are also very unusual sensorineural hearing impairments that involve the VIIIth cranial nerve (the Vestibulocochlear nerve) or the auditory portions of the brain. In the rarest of these sorts of hearing loss, only the auditory centers of the brain are affected. In this situation, central hearing loss, sounds may be heard at normal thresholds, but the quality of the sound perceived is so poor that speech can not be understood
Measles may result in auditory nerve damage
Meningitis may damage the auditory nerve or the cochlea
Autoimmune disease has only recently been recognized as a potential cause for cochlear damage. Although probably rare, it is possible for autoimmune processes to target the cochlea specifically, without symptoms affecting other organs. Wegener’s granulomatosis is one of the autoimmune conditions that may precipitate hearing loss.
Mumps (Epidemic parotitis) may result in profound sensorineural hearing loss (90 dB or more), unilateral (one ear) or bilateral (both ears).
Presbyacusis is deafness due to loss of perception to high tones, mainly in the elderly. It is considered by some to be a degenerative process, although there has never been a proven link to aging. (See impact of environmental noise exposure above.)
Adenoids that do not disappear by adolescence may continue to grow and may obstruct the Eustachian tube, causing conductive hearing impairment and nasal infections that can spread to the middle ear.
AIDS and ARC patients frequently experience auditory system anomalies.
HIV (and subsequent opportunistic infections) may directly affect the cochlea and central auditory system.
Chlamydia may cause hearing loss in newborns to whom the disease has been passed at birth.
Fetal alcohol syndrome is reported to cause hearing loss in up to 64% of infants born to alcoholic mothers, from the ototoxic effect on the developing fetus plus malnutrition during pregnancy from the excess alcohol intake.
Premature birth results in sensorineural hearing loss approximately 5% of the time.
Syphilis is commonly transmitted from pregnant women to their fetuses, and about a third of the infected children will eventually become deaf.
Otosclerosis is a hardening of the stapes (or stirrup) in the middle ear and causes conductive hearing loss
There can be damage either to the ear itself or to the brain centers that process the aural information conveyed by the ears.
People who sustain head injury are especially vulnerable to hearing loss or tinnitus, either temporary or permanent.
Exposure to very loud noise (90 dB or more, such as jet engines at close range) can cause progressive hearing loss. Exposure to a single event of extremely loud noise (such as explosions) can also cause temporary or permanent hearing loss. A typical source of acoustic trauma is a too-loud music concert
