INFLUENCE OF A REDUCTION OF RELATIVE MIDD LE-EAR PRESSURE ON HEARING THRESHOLDS (AIR AND BONE-CONDUCTION) OF HEALTHY-SUBJECTS

Underpressure in the tympanic cavity causes increased impedance of the middle ear. Gelle was the first to describe increased bone conduction levels following alteration of ear canal pressure in healthy ears. Up to now, no investigation which quantitatively describes the elevation of the hearing threshold induced by various levels of underpressure i n the middle ear has been published. In a pressure chamber, we induced relative underpressure in the middle ears of 15 adults with normal he aring, We measured hearing thresholds and calculated medium values at four separate levels of underpressure, At an underpressure of 3.3 kPa, air conduction was reduced by a few dB at 500 and 1000 Hz. Alteration s of bone conduction were first seen at 6.6 kPa accompanied by increas ed deterioration of air conduction. Both effects became more obvious a t 10 ItPa; and at a maximum underpressure of 13.3 kPa, a deterioration of air conduction by more than 25 dB was seen at 250, 500, and 1000 H z. Bone conduction deteriorated by more than 10 dB at 500 and 1000 Hz. There was no uniformity in the development of bone conduction thresho ld in the condition of underpressure: Several ears expressed only slig ht changes, but in some ears we saw an increase of bone conduction at the same rate as air conduction. Minor alterations were observed in fr equencies above 1000 Hz. These results may be only partially explained by middle ear effects like the reduction of the ostio-tympanic compon ent of bone conduction caused by increased stiffness of the ossicles. We believe that disturbances of inner ear mechanics play a role in the deterioration of bone conduction levels, too. A model describing path ways of pressure equalization in the labyrinth following underpressure in the middle ear is developed and discussed. Furthermore, our result s indicate new aspects in clinical differential diagnostics of acute l ow-frequency hearing loss.