Sensory Presbycusis
Noise-induced hearing loss. Aging and noise exposure are the key factors implicated. The normal range of human hearing is 20 to 20,000Hz, with an upper limit of 10,000 by about the sixth decade. The range for human speech is 500 to 3000Hz. Exposure to excessive noise results in damage that impedes hearing at all frequencies above 4000Hz. An average daily noise exposure over 20 to 40 years to sound intensity of 90dBA (decibels on the A scale), as in some industrial settings, may result in a permanent pure-tone threshold shift beginning at 3000 to 4000Hz. We experience sounds ranging from 30dB (a soft whisper) to 14dB (a gunshot blast or a jet engine). Conversational speech is approximately 66dB.[7] Occupational noise is the most common source of excessive noise. However, personal leisure activities, such as listening to music and sporting events, are also of importance.
A rapidly increasing prevalence of hearing loss has been identified in women 45 to 64 years of age,[1] which probably represents an increase in noise-induced hearing loss. Gates and colleagues[12] studied a cohort of women in the 1980s who had worked in factories during World War II and found that 40% of the women had an audiometric pure-tone threshold "notch" at 4000 or 6000Hz--the classic audiometric sign of noise-induced hearing loss. The association between the classic audiometric findings of noise-induced hearing loss and prior work in factories is suggested but cannot be proven since a noise-exposure history was not sought and documented in this cohort of women.
Women of all ages have better hearing at frequencies above 2000Hz than do men, with a difference of up to 20dB at 4000Hz.[12,13,14,15,16,17,18,19] Sensory presbycusis is primarily due to outer hair-cell loss within the cochlea. Gates and associates[12] were able to establish the cochlea as the site of hearing loss in the Framingham cohort.[12] The researchers performed tympanometry on 1656 men and women in their sixth to ninth decades to evaluate the presence of stiffening of the middle-ear mechanism, a postulated cofactor in the pathogenesis of presbycusis. They found that middle-ear pressure abnormalities were uncommon, thereby demonstrating that the cochlea was the site of hearing loss. There were no significant differences in middle-ear pressure by age or gender.
Gates and others[12] were also able to show that the differences in high-frequency hearing between men and women in the Framingham cohort were not due to the effects of neural presbycusis. Despite theories, dating back to the 1960s, that age-related vascular changes could be linked to hearing loss,[20,21,22,23,24] the Framingham cohort showed no relationship between the maximum word-recognition score and the 30-year prevalence of any type of cardiovascular disease (CVD) in either gender. The investigators found that the mean percent of correct responses on word-recognition tests were superior for women. However, when they compared word-recognition scores between genders across 5-year age groups in relation to the configuration of the audiometric threshold, it was concluded that the poorer word-recognition scores of men were reflective of the higher prevalence of men with sharply falling high-frequency loss audiograms. In the cohort, screening signs of neural deficits that are characteristic of neural presbycusis (eg, poor discrimination, absent stapedius reflexes, abnormal acoustic reflex decay) were uncommon.
The results of studies examining gender differences in the rates of change in hearing threshold do not indicate a consensus. In 1981 Moller[16] studied 70- to 75-year-olds and found almost no change in pure-tone thresholds of men during 5 years of follow-up, but he noted a 2- to 12-dB decrease of hearing levels in women. Hearing thresholds in women deteriorated considerably throughout the entire range of frequencies, particularly at 4000 and 8000Hz. Gates and colleagues,[12] in contrast, found in their study of pure-tone thresholds that the rate of change with age did not differ by gender. Finally, Pearson and others[19] noted in their longitudinal study that "the rate of decline in hearing sensitivity accelerates with age in both men and women, but men decline more than twice as fast as women at most ages and frequencies." The variability in the rates of hearing loss expressed in these studies emphasizes the lack of knowledge concerning the fundamental process of hearing loss in men and women.
The poorer hearing at higher frequencies observed in men has generally been attributed to greater levels of exposure to occupational and recreational noise. In support of this explanation is the fact that no significant gender differences in high-frequency hearing have been noted in animal studies.[25] Furthermore, in societies free of hazardous noise exposure, the hearing thresholds of elderly women and men are equivalent.[8,26]
Controversy remains about whether there is a gender difference in susceptibility to age-related hearing loss. In populations screened to exclude individuals with significant noise exposure, women have superior hearing in the 2000- to 8000-Hz range, with differences of up to 20dB at 4000Hz.[27,28] In addition, when men and women are exposed to similar levels of noise, men exhibit more severe hearing loss.[29,30]
Thus, while women have typically been found to have superior high-frequency hearing, sociologic changes that expose men and women to the same hazardous noise levels may ultimately blur the gender distinction, and it may be expected that women, too, will maintain less of their high-frequency hearing as they age.
Medscape General Medicine. 1997;1(2) © 1997
Cite this: Hearing Loss: Does Gender Play a Role? - Medscape - Oct 01, 1997.
