Aerolabyrinth in the perilymphatic fistulae as a cause of sudden deafness: experimental study in the guinea pig

Abstract

Perilymphatic fistula has been believed to cause acute, profound and reversible sensory hearing loss. Over the past five years, exploratory tympanotomy were performed in 27 patients with perilymphatic fistula. In three patients, air bubbles came out of the round window niche in the course of perilymphatic leakage. The perilymphatic fistulae observed in these three patients were attributable to the implosive route. These clinical evidence prompted the experiments using guinea pigs in order to answer the following questions: 1. Can an air bubble enter the perilymphatic space when pressure in the middle ear suddenly rises beyond a certain clitical level? 2. Does an air bubble in the perilymphatic space cause serious but reversible hearing impairment? As a result, experiments showed that: 1. The round window membrane is ruptured when pressure in the middle ear is elevated suddenly even at a level of 400 mmH2O. 2. Air can enter the scala tympani through the rupture. 3. The air bubbles in the perilymphatic space increased the AP (Action Potential) threshold of the auditory nerve immediately. The increase in the threshold was on the order of 10 dB or 50 dB. 4. That change of the AP threshold was reversible after eliminating the air bubbles. These experimental findings might indicate that the air bubbles disturbed the propagation of traveling waves of the perilymph. Thus the present study strongly supports the assumption that air bubbles in the perilymph may cause profound but reversible hearing loss in patients with perilymphatic fistulae.