Previous hearing loss and susceptibility to future permanent threshold shifts

The association between prior hearing losses and susceptibility to future permanent threshold shifts was examined by a matched pairs case-control study. Data were obtained from the United States Air Force Hearing Conservation Data Registry, which contains over three million audiometric records. Cases with permanent hearing losses were matched with normal controls for relative noise exposure intensity, years of exposure, and age. Based on analysis with normal or abnormal baseline hearing as the risk factor, cases with abnormal initial hearing were no more likely than controls to develop future permanent threshold shifts (odds ratio = 1.06, 95% confidence interval 0.61-1.85).