Electrocochleography and Auditory Brainstem Electric Responses in Patients with Pontine Angle Tumors

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Abstract
In 45 patients with surgically proven pontine angle tumors, compound action potential (AP) and summating potential (SP) were recorded with transtympanic electrocochleography (ECochG) together with brainstem electric responses (BSER). The aims were to quantify the mechanism by which tumors cause hearing loss and to evaluate the diagnostic potentials of ECochG and BSER for detecting eighth nerve and brainstem tumors. Except for AP latency and narrow band AP waveform, response parameters recorded by ECochG are uncor-related. Four uncorrelated parameters were abnormal in only 10% of the cases, three in 25%, two in 40%, and one in 90%. The BSER criterion was the latency delay between waves I and V and resulted in about 90% detection, improving to 95% when used in combination with the interaural wave V delay criterion. ECochG results provide evidence that, for hearing losses up to 60 dB HL, the origin is cochlear, resembling that caused by Menière's disease. Evidence is presented that the increase in I-V delay in the BSERs is caused by differential action of the tumor upon low and high frequency fibers in the auditory nerve and that desynchronization of the firings of the nerve fibers is of more importance than an increase in neural conduction time. ECochG as the sole test for detection of pontine angle tumors appears to be of limited value. Brainstem response on its own has great merits; however, it should be emphasized that no wave I was detected in about 30% of the cases. The 95% detection score obtained with BSER depends on specifying the latency of wave I. For these cases, we substituted the latency of the AP recorded by ECochG.