Investigation of the physiological basis of summating potential changes in endolymphatic hydrops.

An increase in the ratio of the summating potential to the action potential components of the electrocochleogram is known to be a feature of endolymphatic hydrops. We investigated the value of the SP/AP ratio in response to condensation and rarefaction click stimuli delivered separately. In patients with electrophysiological evidence of endolymphatic hydrops there was found to be a significantly greater SP/AP ratio to condensation clicks than rarefaction clicks. This finding supports the hypothesis that the increased SP/AP ratio in hydrops is due to mechanical asymmetry of the basilar membrane.