DISTORTION-PRODUCT OTOACOUSTIC EMISSIONS AND SELECTIVE SENSORINEURAL LOSS IN IDDM

OBJECTIVE - To provide information about possible subclinical damage o f the cochlear outer hair cells (OHCs) by means of transiently evoked otoacoustic emissions (TEOAEs) and distortion-product otoacoustic emis sions (DPOAEs) in subjects with IDDM. RESEARCH DESIGN AND METHODS - TE OAEs and DPOAEs were recorded in 47 IDDM patients with normal hearing and in age- and sex-matched nondiabetic subjects. Peripheral neuropath y was diagnosed by nerve conduction velocity (NCV) at the peroneal and sural nerves. RESULTS - A subclinical peripheral neuropathy was found in 15 diabetic patients. Mean TEOAE amplitude was found to be signifi cantly reduced in diabetic patients with a reduced NCV (7.6 +/- 3.2 dB ; Scheffe's test: P = 0.03), but not in those without neuropathy (9.5 +/- 4.3 dB), with respect to control subjects (11 +/- 3.1 dB). Neuropa thic patients also showed mean reduced DPOAE amplitude values in the r egion of middle and high frequencies from 1,306 to 5,200 Hz (P < 0.05) , whereas no difference was found at the lowest-frequency amplitudes. A frequency-selective reduction of DPOAEs was also found in non-neurop athic patients (P < 0.05) in the region of higher frequencies at 3,284 , 4,126, and 5,200 Hz compared with control subjects. No correlations were found among duration of diabetes, HbA(1c) values, TEOAEs, and DPO AEs. CONCLUSIONS - Our results suggest that IDDM patients show an earl y abnormality of the micromechanical properties of the OHCs. In IDDM p atients without a subclinical peripheral neuropathy, damage is limited to the higher frequencies and can be detected only by DPOAEs, whereas in IDDM patients with neuropathy, damage also involves the middle ran ge of frequencies and can be detected by TEOAEs and DPOAEs. Therefore, DPOAEs seem to be able to detect the earliest cochlear selective-freq uency dysfunction in IDDM patients without peripheral neuropathy. DPOA Es appear to be of greater clinical interest than TEOAEs; the former s eem to be frequency specific and can be recorded at any chosen frequen cy, including high frequencies.