INPUT-OUTPUT CURVES TO TONE BURSTS AND CLICKS IN EXTRATYMPANIC AND TRANSTYMPANIC ELECTROCOCHLEOGRAPHY

Objective: The aim of this study is to explore the applicability of tw o specific methods in extratympanic (ET) electrocochleography that hav e rarely been used there, but are more commonly applied in transtympan ic (TT) approaches. These two methods are the use of tone burst stimul ation in addition to clicks, and the analysis of amplitude input/outpu t curves as a measure of cochlear recruitment due to a pathological sp read of excitation. Design: Simultaneous extratympanic and transtympan ic recordings were made in 30 patients with various types and degrees of cochlear hearing loss. ET recordings were also made in a group of n ormal subjects. Stimuli were tone bursts at octave frequencies from 50 0 to 8000 Hz and clicks, at intensity levels of about 90 dB down to re sponse threshold in 10 dB steps. First, a general comparison is made o f ET versus TT response properties, then the initial slopes of I/O cur ves in both sets of data are evaluated. Results: ET responses are redu ced in amplitude with respect to TT responses by a factor of 0.43 on a verage. Within each subject this factor is independent of stimulus typ e or level, but it has a substantial intersubject variation. ET and TT latencies are identical. From the normal data a criterion for abnorma lity of the slope of the I/O curve is derived in a similar way as avai lable for TT data. Using this criterion ET I/O curves appear to be sig nificantly steeper than normal in recruiting ears, albeit with a lesse r sensitivity than for the associated TT data. Conclusions: Extratympa nically recorded responses to tone burst stimuli provide basically the same information as transtympanic recordings. However, for a reliable interpretation of responses to low-intensity stimuli a larger amount of signal averaging is required. The analysis of I/O curves is a usefu l extension of current ET methods as a contribution to assessing a pat hological spread of excitation in the cochlea.