BS12. Confirmation of previous results of the occlusion effect through auditory steady state response in normal hearing adults

Abstract

The occlusion effect (OE) is a well known phenomenon in audiology clinical practice. It is often defined as the increase in sound pressure in front of the tympanic membrane for low frequencies through 1000 Hz, and the subsequent improvement in hearing sensitivity at these frequencies when a bone conducted signal is presented to the skull and the opening of the ear canal is occluded. Hence, some authors recommend the application of a correction factor to compensate for the OE. It is also well known that behavioral audiometry is not suitable for difficult to test population (i.e., children, malingers, people with some kind of neurodevelopmental disorder), so they need to be evaluated through objective electrophysiological measures. However, only two studies have assessed the OE using auditory steady state responses (ASSRs), so there is no agreement about their use in clinical practice. The aim of this study was to confirm the findings from previous ASSRs studies of the OE for a larger sample of normal hearing adults. Thirty-two normal hearing adults (32 ears) with a mean age of 21 ± 2 years participated in this study. For each participant, one ear was selected at random, and the first measures to be obtained (occluded or unoccluded) were randomly determined. The stimulus comprised a combination of four sinusoidal carrier tones, 500, 1000, 2000, and 4000 Hz, modulated in amplitude (95% depth) at the following rates: 104.2, 107.8, 111.4, and 115 Hz, respectively. It was presented through bone conduction for each participant under two different conditions (occluded and unoccluded ear). For both conditions, bracketing technique was used for determining thresholds with a vertical montage (negative electrode: ipsilateral mastoid; positive: Cz and ground: front). The overall ambient noise was 52 dB sound pressure level. Repeated measures analysis of variances was performed to compare occluded and unoccluded ASSR thresholds and amplitudes at 30 dB hearing level for 500, 1000, 2000, and 4000 Hz. Occlusion caused a significant decrease of bone conducted ASSR thresholds at low frequencies and a significant increase at 4000 Hz. Mean ASSR amplitudes were significantly higher after occlusion at low frequencies. Some subjects; however, showed no OE at frequencies at which it is expected to be present. The electrophysiological correction factors were 12 and 8 dB HL for 500 and 1000 Hz, respectively. Despite the high ambient noise issue, results of previous studies are confirmed here in a larger sample of cases. Our findings indicate that threshold variations at high frequencies need to be considered when conducting bone conduction testing with ear canal occlusion. The fact that the occlusion effect is not present in every individual may have some clinical implications. Our findings also suggest that correction factors recommended for behavioral testing can be applicable when using ASSRs.