Hearing impairment is a disability affecting about 1 in 10 North Americans. Hearing impairment results from a structural abnormality (such as a hole in the eardrum) that may or may not produce a functional disability (such as diminished hearing).
Hearing loss can be conductive (due to faulty transmission of sound waves) or sensorineural (faulty sound reception by nerve cells), or both. Common causes of conductive hearing loss are wax blocking the ear, a perforated eardrum, or fluid in the ears. Common reasons for sensorineural deafness are noise exposure, age-related changes, and ototoxic drugs (that damage hearing). Hearing loss can be:
- mild (a loss up to 40 dB) - with trouble in hearing ordinary conversation
- moderate (40-60 dB) - where voices must be raised to be heard
- severe (over 60 dB loss) - where people must shout to be heard.
According to the World Health Organization, the term "deaf" should only be applied to individuals with hearing impairment so severe that they cannot benefit from sound amplification or hearing aid assistance. The most common cause of sensorineural deafness is aging which produces presbycusis - literally, "old hearing." Those with presbycusis often complain not only of hearing loss - usually in both ears - but also of associated tinnitus or ringing in the ears, and sometimes dizziness. It takes only a slight loss of hearing to make life difficult because although conversation is audible at low frequencies (deeper voices), it is not as easy to hear higher pitched voices. Typically, with hearing loss, the ability to hear high sounds goes first so that there is trouble hearing birds or women's voices, followed by the loss of low-tone reception. The elderly may have trouble hearing the phone ring or distinguishing consonants. The problem is particularly acute when there is a lot of background noise, as on a bus, at the dinner table, or when standing next to an open window facing traffic. Hearing impairment is measured by the amount of level of loss in what are called decibels (dB) hearing level (HL). Decibels are like degrees of a thermometer. As temperature increases, so do the number of degrees. As the volume of sound increases, so do the number of decibels. Normal conversation is usually between 45 to 55 dB. A baby crying hovers around 60 dB and downtown traffic can blister the ear at 90 dB. If you can hear sounds between 0 and 25 dB HL most of the time, your hearing is normal or near normal and you probably do not need a hearing aid, although it may enhance your abilities in some situations. If you only hear sounds above 25 dB HL, your hearing loss may be mild, moderate, severe, or profound. Hearing aids are designed in part to compensate for the level of loss.
The causes of hearing loss may be congenital (present at birth) - genetic, use of ototoxic drugs during pregnancy, prenatal rubella in expectant mothers, infections during pregnancy, perinatal anoxia (fetal oxygen lack), or Rh blood disease. Or, the cause may be acquired hearing loss - noise exposure, presbycusis, infections that affect the middle ear and inner ear such as mumps, measles and influenza, middle ear infections, ototoxicity (drugs that harm the inner ear), head injuries, benign tumors of the hearing nerve (acoustic neuroma), and cancer (rare).
Appropriately chosen, properly fitted, and regularly checked, hearing aids can greatly improve the quality of life for hearing-impaired persons. They are prescribed according to the type and severity of hearing loss, how well someone can manipulate the aid, and the condition of the ear canal. They work by amplifying sound and are most effective in quiet areas - as for one to one conversations and small group interaction. Some devices for the hard of hearing are designed for a particular situation. For example, for watching TV a specific amplifier improves the sound signal and mutes background noise. Most hearing aids currently sold are behind the ear (BTE), in the ear (ITE), or in the ear canal (ITC).