External- and middle-ear factors affecting evoked otoacoustic emissions in neonates

Babies without any of the known risk factors for hearing impairment were taken from normal maternity wards and tested daily for 3 days post-partum. Tympanometric and evoked otoacoustic emission data were obtained from 121 babies. Middle-ear pressure and middle-ear compliance values were obtained with a Grason Stadler Middle-Ear Analyser (GSI-33) using a 1000-Hz probe tone. All babies had their evoked emissions measured on the Programmable Otoacoustic Emissions Measurement System (POEMS) equipment. The middle-ear compliance, the proportion of normally shaped tympanograms, the middle-ear pressure and the proportion passing EOAE testing all improved over the 3 days. However, the statistically significant factors affecting the pass rate appeared to be only the middle-ear pressure (for a small number of babies) and the degree of obstruction of the external ear canal. Whilst both these factors play a part in determining emission pass/fail rates they do not fully account for the observed changes in pass rate. There are developmental and other mechanisms which must contribute to the increase in pass rate as the baby matures.