Some Observations on the Nature of the Audiometric 4000 Hz Notch: Data from 3430 Veterans

Abstract

Background: Pure-tone, air-conduction audiograms notched at 4000 Hz have long been considered the signature configuration for noise-induced hearing loss even though there is an extensive literature that does not mesh with this simple explanation. There are many reports of notched audiograms from individuals with no history of noise exposure and, conversely, reports of audiograms with no notches from individuals with a history of noise exposure. Recent reports increasingly suggest that unilateral 4000 Hz notches are common. The prevalence of notched audiograms at 4000 Hz is dependent on the definition of the notch and the population under study. Purpose: To examine the prevalence and characteristics of audiograms that are notched at 4000 Hz. Research Design: Retrospective, descriptive. Study Sample: The participants were 3430 veterans evaluated in the Audiology Clinic at the VA Medical Center, Mountain Home, Tennessee. The mean age was 62.3 yr. Data Collection and Analyses: The data were collected in the course of a 60 min, routine audiological evaluation. In addition to pure-tone audiometry, a history, otoscopy, speech audiometry in quiet and in noise, and aural-acoustic immittance measures were included in the clinic protocol but were not evaluated in this report. A notch was defined when the 4000 Hz threshold minus the 2000 Hz threshold and the 4000 Hz threshold minus the 8000 Hz threshold both were ≥10 dB. Results: Overall the mean LE (left ear) thresholds at 2000, 3000, and 4000 Hz were at hearing levels 2–3 dB higher than the hearing levels for the corresponding mean RE (right ear) thresholds; the differences were significant. A notched audiogram was observed in 40.6% of the participants in at least one ear with 15.4% having bilateral notches, 28.8% LE notches, and 27.1% RE notches. Unilateral 4000 Hz notches were almost twice as prevalent as bilateral 4000 Hz notches. Viewed as a function of age, notched audiograms were most common (˜35% of the participants) in the 40 and 50 yr groups with a diminishing prevalence in the 60–80 yr groups. The mean notch depth at 4000 Hz was consistently 20–26 dB across the seven age groups. In comparison to the thresholds of the audiograms that were not notched, the thresholds of the audiograms with 4000 Hz notches (1) at 250–2000 Hz were at hearing levels 2–3 dB lower, (2) at 3000 and 4000 Hz were at hearing levels 8–17 dB higher, and (3) at 8000 Hz were at hearing levels 3–4 dB lower; the threshold differences were significant at all frequencies for both ears. Conclusions: The data suggest that unilateral, 4000 Hz notched audiograms are as common or more common than bilateral notched audiograms and that unilateral notched audiograms are equally common for the LE and RE. The prevalence and characteristics of 4000 Hz notched audiograms in this veteran sample are similar to those observed in the population as a whole.