Prediction of deterioration in hearing due to hearing aid use.

There is a definite risk of overamplification by hearing aids. Guidelines should therefore be established that will minimize the risk of damage to hearing involved in hearing aid use. A mathematical model that can be used for this purpose is derived from equations for predicting noise-induced permanent threshold shift given in International Standard ISO 1999 combined with the Modified Power Law. The model implies that any noise exposure that would cause deterioration of the hearing threshold levels of a person with normal hearing would also be harmful to the hearing of a person with sensorineural hearing impairment. It follows that, in order to ensure that no deterioration occurs in the hearing of a hearing aid user, the output levels from the aid must be such that they would not cause any damage to a person with normal hearing. This constraint can be met for hearing aid users with mild-to-moderate sensorineural hearing loss but cannot be met for users with severe-to-profound loss because it would result in the provision of insufficient gain, particularly at the higher frequencies. If the model is valid, then for this group, some appropriately small amount of hearing damage must be accepted as the cost of the advantages gained from the use of a hearing aid. Verification of the model is essential before the model is used in clinical practice to determine the risk of deterioration in hearing due to hearing aid use.