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What the Numbers Mean: An Epidemiological Perspective on Hearing
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Hearing loss is a common problem caused by noise, aging, disease, and heredity. Hearing is a complex sense involving both the ear's ability to detect sounds in the environment and the brain's ability to interpret the sounds of speech. Major determinants of the impact of hearing loss on members of a population include:
- Degree of the hearing loss
- Configuration or pattern of hearing loss across frequencies
- Laterality/bilaterality of hearing loss (one or both ears affected)
- The area(s) of abnormality in the auditory system—such as the middle ear, inner ear, auditory neural pathways, or brain
- Speech recognition ability
- History of exposures to loud noise and environmental or pharmacologic toxicants to hearing
Measuring Hearing Loss
Mild losses may not be noticed and even moderate losses may not cause a problem for people with excellent perceptual abilities and good coping skills. Hearing loss may be identified by the person involved (called"self-report"), by friends and family, and by hearing testing. Formal audiometric testing is the gold standard for diagnosing hearing loss and monitoring treatment. Testing may be done at any age.
For estimates of the prevalence of hearing loss, all of these measures are of value and each provides insight into the burden of hearing loss on society. Self-report of hearing loss and the report of friends and family are important because they are relatively simple to determine and they provide a global assessment of the impact of the problem on the individual.
Formal Audiometric (Hearing) Tests
Formal audiometric testing, on the other hand, provides precise information displayed by frequency and hearing level. A convenient summary of the audiogram for each ear is the pure-tone average (PTA) of thresholds measured at specific frequencies. Thresholds are measured on a decibel (dB) hearing loss (HL) scale referenced to audiometric"zero" (the average hearing level at each frequency for normal young adults). One traditional PTA measure is the speech frequency average of thresholds at 500, 1000, and 2000 hertz (Hz). However, the frequencies to include in the PTA vary; for example, a high frequency such as 3000 Hz is included with the low frequency (500 Hz) and middle frequencies (1000 and 2000 Hz) in some formulations of the PTA. The most common PTA definition found in epidemiological, or population-based, studies is the four-frequency average of 500, 1000, 2000, and 4000 Hz.
As the PTA increases, the hearing ability decreases. Normal hearing for speech is observed in adults with PTAs of 25 dB HL or less. At a PTA of around 40 dB HL in both ears, most people are considered functionally impaired and could benefit from amplification. Severe to profound losses are present when PTAs are greater than 70 dB HL. At this level, hearing aids provide limited benefit and cochlear implants may be considered.
An accurate assessment of hearing includes:
- Laterality (one or both ears affected)
- Degree and pattern of threshold loss across frequencies
- Best ability to understand speech—either with hearing aids or loud speaking levels
Other factors to determine include:
- The rapidity of the onset or progression of hearing loss (people often adapt better to slowly progressive losses than to sudden losses)
- Associated symptoms such as tinnitus (ringing, roaring, or buzzing in the ears or head), hyperacusis (intolerance to normal environmental sounds), and recruitment (when loud sounds are suddenly uncomfortable)
- Treatment options, such as surgery, hearing aids, cochlear implants, aural rehabilitation, speech reading, or assistive listening devices
Implications of Hearing Loss
Hearing loss implications may be estimated in terms of societal burden, effect on the person, and treatment needs. To estimate the societal burden of hearing loss, age-specific rates of self (or family) report are appropriate. For estimating the impact of hearing loss on the person, a PTA more than 25 dB HL generally requires adaptive listening strategies, such as sitting closer to the source of sound. Active treatment, such as hearing aids, is frequently recommended at PTAs greater than 40 dB HL in both ears. The term "deaf" is generally applied to people with profound bilateral loss (PTAs greater than 90 dB HL). Modern cochlear implants are often helpful to children and adults with severe to profound losses (PTAs greater than 70 dB HL) of relatively short duration (less than 10 years). As a result, duration of hearing loss has become another important factor in describing hearing loss.
George A. Gates, M.D.
Virginia Merrill Bloedel Hearing Research Center
University of Washington
Howard Hoffman, M.S.
Chief, Epidemiology and Data Systems
National Institute on Deafness and Other Communication Disorders