[Early acoustic evoked potentials in patients with "small" vestibular schwannomas]

The prognostic potential of click-evoked early acoustic evoked potentials (EAEP) for vestibular schwannoma (VS) diagnostics is considered insufficient for tumors small than approximately <1.5 cm in diameter. However, up until now, the derivations at higher sound pressure levels have mainly been used for EAEP evaluation. The aim of this study was to examine whether registered EAEP are more sensitive to pathological changes at low sound pressure levels and with smaller tumors.

Comparative examinations (pair-matched) were conducted with 34 patients. An appropriate patient with an identical hearing level threshold - either a purely cochlear hearing deficiency or a normal hearing level - was matched to each tumor patient. EAEP were registered at 80, 60, and 40 dB HL. The characteristic parameters where assessed visually and evaluated statistically, both in respect to patient groups and in bilateral comparisons. Furthermore, the relationship between the level-dependent wave V latency differences in bilateral comparison and tumor size were evaluated.

Low-level cochlear hearing deficiencies did not cause a change in wave V latency, such that the pathological side differences at 40 dB HL in patients with VS can be considered a "purely" retrocochlear hearing deficiency. The sensitivity of EAEP registered at 40 dB HL decreases only with tumors under about <5 mm in diameter. Combining the values of wave V latency and interaural wave V latency difference effectively reduces false-positive findings at 40 dB HL to zero.

Beyond the suprathreshold derivations, potential registrations at 40 dB HL can also be diagnostically useful to differentiate cochlear from retrocochlear lesions. In the case of small VS and good hearing levels, these registrations may also be used for intraoperative monitoring to allow better evaluation of level-dependent changes in the auditory nerve.