effect of tympanic membrane perforation on real-ear to coupler difference acoustic transform function

ABSTRACT

Real-Ear-to Coupler Difference [RECD] is the difference in decibels, as a function of frequency, between the SPL at a specified measurement point in the ear canal and the SPL in a 2cc coupler, for a specified input signal. We investigated the effects of tympanic membrane perforation on the RECD. RECD was obtained using an insertion gain analyzer for 24 adult patients with dry tympanic membrane perforation [group 1] [31 ears] and 20 healthy adult control subjects [group 2] [40 ears]. Cases showed moderate conductive hearing loss in their mean hearing threshold level. There was no statistically significant difference between both anterior and posterior locations as regards the degree of hearing loss or as regards the RECD. There was a statistically significant difference of RECD at frequencies 250Hz, 500Hz and 750Hz, between both groups. The RECD was 3 to 6dB lower in group 1 than in group 2. There was a statistically significant difference on comparing RECD at frequencies 250Hz, 500Hz, 750Hz and l000Hz in right ears, and at frequencies 250Hz and 500Hz and 750Hz in the left ears of both groups. In group 1, there was no statistically significant difference of RECD values at different frequencies between right and left sided perforations and there was no statistically significant difference of RECD values at different frequencies between both genders except at 1500Hz. There was no statistically significant correlation between the RECD and the age of the patients or their air-bone gap. RECD in patients with tympanic membrane perforation is lower than normal at frequencies [250Hz-750Hz]. We recommend that this discrepancy should be compensated for if average normal RECD are used in the preselection of target and gain to avoid under-amplification at lower frequencies. However, the large inter-subject variation strongly suggests the need for individual RECD measurements whenever possible in fitting aids for patients having tympanic membrane perforations