Characteristics of the audiometric 4,000 Hz notch (744,553 veterans) and the 3,000, 4,000, and 6,000 Hz notches (539,932 veterans).

INTRODUCTION Air-conduction audiograms with 3,000, 4,000, and/or 6,000 Hz thresholds at higher hearing levels (HLs) than the adjacent 2,000 and 8,000 Hz thresholds are historically referred to as "notched audiograms." Since the initial observations of the notched audiograms, noise exposure has been the designated causal factor [1-4]. * Evidence from more recent animal and human studies has led many investigators to question (1) whether or not noise exposure is the sole contributing factor to the notched audiogram and (2) the pathophysiology of noise-induced hearing loss [5-19]. Additionally, notched audiograms are reportedly associated with individuals who have no marked history of noise exposure; conversely, some individuals with a substantial history of noise exposure exhibit no evidence of a notched audiogram [14,20]. Intuitively, because noise exposure typically occurs in a free or sound field, it is reasonable to expect both ears to be equally affected by noise incidences. This line of reasoning may not, in fact, be the case. Recent observations indicate that it is more common to observe unilateral high-frequency notches than bilateral high-frequency notches [20-23]. Support of this plethora of unilateral high-frequency notches was provided by Nondahl et al. [18, p. 698: Table 1], who evaluated audiometric notches of 2,159 participants in the Epidemiology of Hearing Loss Study (EHLS) at Beaver Dam, Wisconsin [24]. Nondahl et al. examined the high-frequency audiometric notches by using four algorithms [18, p. 698: Table 1], three of which required a notch be present in one or both ears [22,25-26], whereas the fourth required notches be present in both ears [27]. Using these strict definitions, 31.7 [25], 25.9 [22], 47.2 [26], and 11.7 percent [27] of the EHLS participants had notches. When the first three algorithms were adjusted to include a notch in both ears, the EHLS prevalences dropped to 9.1, 4.7, and 24.9 percent, respectively, which are in better agreement with the 11.7 percent prevalence reported for the Hoffman et al. algorithm [27]. The implication from these differing results is that there were more unilateral notches than bilateral notches in this population-based study. A recent study from our laboratory substantiated this observation with data from 3,430 veterans [28]. In that study, notched audiograms at 4,000 Hz were observed in 41.1 percent of the participants in at least one ear, with 15.4 percent having bilateral notches (auris utraque [AU] or "both ears"), 13.8 percent having unilateral left ear (LE) notches, and 11.9 percent having unilateral right ear (RE) notches. (Note: These percentages are based on audiogram pairs that were not reported in the original report.) Thus, of the audiogram pairs with notches at 4,000 Hz, 37.5 percent had bilateral notches and 62.5 percent had unilateral notches. Although the Wilson report had a sizable group of participants [28], previous multisite clinical trials in the Department of Veterans Affairs (VA) have revealed significant site differences based on geographic location. One example is a current VA Rehabilitation Research and Development-funded multisite study designed to look at the efficacy of computerized auditory training with more than 250 participants recruited from the (1) Bay Pines VA Healthcare System, Bay Pines, Florida; (2) James H. Quillen VA Medical Center, Mountain Home, Tennessee; and (3) Portland VA Medical Center, Portland, Oregon. Multiple significant baseline characteristic differences were found among sites. Specifically, participants recruited from Bay Pines were significantly older with lower levels of education and reading ability than the participants recruited from the other two sites. There were no differences in pure-tone averages (PTAs); however, the participants from Bay Pines reported significantly fewer comorbidities and lower hearing handicap as measured by the Hearing Handicap Inventory for the Elderly [29]. …