MIDDLE-LATENCY RESPONSES TO ASSESS OBJECTIVE THRESHOLDS IN PATIENTS WITH NOISE-INDUCED HEARING LOSSES AND MENIERES-DISEASE

In this study 23 patients with noise-induced hearing loss (NIHL) refer red for medico-legal evaluation and a group of 18 cases with Meniere's disease were evaluated audiologically using the middle-latency respon se (MLR). Cross-correlation functions were used to assess precisely ML R thresholds in low and middle frequencies. Cross-correlation data obt ained from suprathreshold levels to below threshold levels were compar ed with the normal limits of parameters found at threshold levels, all owing us to determine the true MLR threshold. Our results revealed tha t this MLR threshold and visual detection thresholds were different in 18% of both the NIHL and Meniere's disease groups. In this population the true MLR threshold was greater by 5 dB. These findings demonstrat ed that cross-correlation functions can enhance the sensitivity of the definition of the MLR threshold. True MLR threshold were compared wit h subjective pure-tone audiometric (PTA) thresholds at the same freque ncies (0.5, 1 and 2 kHz). The true MLR threshold and PTA threshold wer e in agreement within 10 dB in 91% of the NIHL group and all of the Me niere's disease group. The PTA threshold was greater by 15 dB or more in the remaining NIHL group. If a criterion of 15 dB discrepancy indic ates non-organic hearing loss, it can be inferred that 9% of an NIHL p opulation referred for medico-legal evaluation is exaggerating subject ive audiometric thresholds.