[Assessment of outer hair cell function recovery by means of the DPOAE threshold]

Distortion product otoacoustic emissions (DPOAE) originate from nonlinear mechanical sound processing in the inner ear, mainly due to normal outer hair cell function. Outer hair cell impairment can be detected by means of DPOAE. With a special primary tone level paradigm which optimises the primary tone level difference over a wide stimulus level range (L1 = 0.4 L2 + 39 dB, L2 = 20 to 65 dB SPL, f2 = 0.5 to 8 kHz, f2/f1 = 1.2), DPOAE can be measured at levels close to the psychophysical hearing threshold. From the DPOAE growth functions, which are constructed from measurements at different stimulus levels, a DPOAE threshold can be defined (Boege et al., 1998), which can be interpreted as a threshold of mechanical processing by the outer hair cells. In this study, DPOAE thresholds were examined in 9 patients with reversible cochlear hearing loss, i.e. sudden deafness or noise trauma. With increasing hearing loss, low primary tone level DPOAE in particular decreased, and thus the DPOAE threshold increased. The DPOAE threshold makes it possible to detect mechanical sensitivity losses in a frequency-specific manner. Thus, loss of mechanical amplification may be differentiated from a non-mechanical cause of hearing loss.