Input/output curves to tone bursts and clicks in extratympanic and transtympanic electrocochleography

The aim of this study is to explore the applicability of two specific methods in extratympanic (ET) electrocochleography that have rarely been used there, but are more commonly applied in transtympanic (TT) approaches. These two methods are the use of tone burst stimulation in addition to clicks, and the analysis of amplitude input/output curves as a measure of cochlear recruitment due to a pathological spread of excitation.

Simultaneous extratympanic and transtympanic recordings were made in 30 patients with various types and degrees of cochlear hearing loss. ET recordings were also made in a group of normal subjects. Stimuli were tone bursts at octave frequencies from 500 to 8000 Hz and clicks, at intensity levels of about 90 dB down to response threshold in 10 dB steps. First, a general comparison is made of ET versus TT response properties, then the initial slopes of I/O curves in both sets of data are evaluated.

ET responses are reduced in amplitude with respect to TT responses by a factor of 0.43 on average. Within each subject this factor is independent of stimulus type or level, but it has a substantial intersubject variation. ET and TT latencies are identical. From the normal data a criterion for abnormality of the slope of the I/O curve is derived in a similar way as available for TT data. Using this criterion ET I/O curves appear to be significantly steeper than normal in recruiting ears, albeit with a lesser sensitivity than for the associated TT data.

Extratympanically 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 useful extension of current ET methods as a contribution to assessing a pathological spread of excitation in the cochlea.