Binaural hearing, atresia, and the masking dilemma.

A masking dilemma occurs when energy from a non-test ear crosses over the head to a test ear. In cases of bilateral atresia, obtaining thresholds on the poorer ear is problematic. Near threshold, however, sufficient ear-bone isolation exists to test with validity but not so much above threshold, even for the ultra-high (> 10 kHz) frequencies. This aspect of the bone audiogram should not be overlooked. We suggest two uses of binaural bone conduction hearing to help resolve the dilemma--one an auditory brainstem response variation, the other a high-frequency lateralization procedure. Both are also applicable in unilateral atresia with and without a sensorineural component. The use of an insert earphone for masking in the unobstructed ear will provide adequate interaural attenuation to resolve most but not all dilemmas. That is to say, the maximum isolation of an insert earplug is on the order of 100 dB, so it has a limit. The dilemma extends through the ultra-high frequencies (> 10 kHz) because the intra-aural attenuation is no more than 10 dB. In the cited case of unilateral congenital atresia of the external ear, the intra-aural threshold differences in the high frequencies plus the resolution of the masking dilemma in the low frequencies (without the need for more masking) leads us to conclude that the unmasked thresholds are valid. Masking can be extended above 10 kHz, but this is not generally within the capacity of commercial audiometers, a feature that should be included.