Temporary and permanent threshold shift caused by hearing aid use.

Excessive amplification by hearing aids causes temporary threshold shift (TTS) and permanent threshold shift (PTS). This investigation addressed the question whether it might be possible to predict the eventual amount of PTS caused by excessive amplification from the amount of TTS it causes after a day of hearing aid use. Asymptotic TTS (ATS) to be expected as a result of hearing aid use was predicted for 8 children with sensorineural hearing loss and the predicted ATS was compared with observed permanent deterioration of their thresholds attributed to hearing aid use. There was good agreement between the predicted ATS and observed PTS at 500 to 2000 Hz. It follows that, for prediction of PTS caused by hearing aid use, the mean of the sound levels produced in the ear by the hearing aid is the correct equivalent continuous level (ECL) to use and that the Modified Power Law (MPL) is the appropriate method of adjusting the predictions for sensorineural hearing loss, because these have been shown to be appropriate for prediction of TTS caused by hearing aid use. Predictions of the PTS to be expected for the children that were carried out using the MPL and the mean level as the ECL were in good agreement with the observed PTS at 500 to 2000 Hz, whereas predictions of PTS based on an alternative method of correction for sensorineural hearing loss (the Continuation Hypothesis) were significantly less than the observed amounts. The results of the PTS predictions therefore confirmed the conclusions drawn from the results of the ATS predictions.