Effect of perilymphatic air perfusion on cochlear potentials

Perilymphatic fistula is now widely recognized to cause acute profound hearing loss. It is still controversial, however, which mechanism it is that causes the reversible hearing loss. Recently, it has been suggested by two groups of researchers that the intrusion of air bubbles into the perilymphatic space (a condition called pneumolabyrinth or aerolabyrinth) through the ruptured labyrinthine window(s) may be one of the causes. In order to examine the mechanism underlying the hearing loss associated with pneumolabyrinth, the perilymphatic space of the guinea pig cochlea was perfused with air and cochlear potentials were recorded. Although perfusion of the scala tympani with air at a rate as high as 200 microliter/min caused an immediate and drastic decrease of the cochlear microphonics (CM) and the compound action potential (AP), it had little effect on the endocochlear dc potential (EP) during perfusion for 20 min. A decline in EP was seen in half the ears, but only when the duration of perfusion exceeded 30 min. These results show that the EP has an amazing resistance to air trapped in the scala tympani of the cochlea and that the initial decrease of hearing acuity after the elimination of perilymph from the scala tympani (or introduction of air into the scala tympani) is probably due to interference in CM and AP generation mechanisms rather than to strial dysfunction.