Tetramethylammonium for in vivo marking of the cross-sectional area of the scala media in the guinea pig cochlea

A physiologic technique was developed to measure endolymphatic cross-sectional area in vivo using tetramethylammonium (TMA) as a volume marker. The technique was evaluated in guinea pigs as an animal model. In the method, the cochlea was exposed surgically and TMA was injected into endolymph of the second turn at a constant rate by iontophoresis. The concentration of TMA was monitored during and after the injection using ion-selective electrodes. Cross-section estimates derived from the TMA concentration measurements were compared in normal animals and animals in which endolymphatic hydrops had been induced by ablation of the endolymphatic duct and sac 8 weeks earlier. The method demonstrated a mean increase in cross-sectional area of 258% in the hydropic group. Individually measured area values were compared with action potential threshold shifts and the magnitude of the endocochlear potential (EP). Hydropic animals typically showed an increase in threshold to 2 kHz stimuli and a decrease in EP. However, the degree of threshold shift or EP decrease did not correlate well with the degree of hydrops present.