[Evoked potentials of the brain stem in normal and pathologic conditions: experience with 692 adult subjects]

The main purpose of the study of evoked potentials has always been to objectively evaluate hearing thresholds in uncooperative subjects and it has become increasingly important in recent years with the introduction of auditory brainstem responses (ABR). Reading the ABR is relatively simple in terms of "audiometry" since it is based on the absence, or presence, of a marker peak wave V. Besides the strictly audiometric uses, study of the bioelectric brainstem complex even makes it possible to obtain important information regarding retrocochlear dysfunction. However, reading the response is not so simple for the otoneurologist since it is not based only on the observation of the absence or presence in order to compare the normal values obtained in homogeneous groups. For the above reason auditory brainstem response testing was performed on 692 subjects in order to define the normal values as well as any characteristic patterns for various audiological pathologies. For all subjects auditory brainstem responses were recorded mono-aurally with clicks of alternating polarity, 11 and 31 pps, at an intensity of 120 dB SPLpe. Use was made of ipsilateral stimulus derivations. Only for the right ear was absolute wave V latency measured at 100 and at 80 dB SPLpe. The amplifier of the bioelectric signal was set at a sensitivity of 20 microvolts with passing filters of 100 and 2000 Hz. Absolute latency, conduction time, symmetry indexes (IT I-V and ITV) and the shift in latency at the two stimulation cadences were statistically processed for homogeneous groups. Thus standards for normality were obtained. The results have shown that the following biological variables are to be taken into consideration: sex, age for males and type of deafness. The indexes most reliable in identifying a retrocochlear dysfunction proved to be: morphological completeness, central I-V conduction times and the two symmetry indexes IT I-V and ITV which are considered complementary. Again in terms of identifying a retrocochlear dysfunction, and in case of an incomplete response, the test can be interpreted thanks to the so-called minor responses which only take into account the absolute wave V latency. However, wave V latency behaviour is rather similar to what has been found in conductive hearing-loss. Finally, in cochlear deafness it was found that response did not vary with a variation in degree of deafness since the results produced were similar to those of normal hearing subjects except in those cases where there was a delay in the appearance of wave I.