The rebound phenomenon of glycerol-induced changes in the endolymphatic space

Volumetric changes of the scala media were histologically investigated in normal guinea pigs to see whether glycerol-induced volumetric change of the scala media followed a biphasic course similar to the auditory threshold in the glycerol test. Glycerol was administered orally in a 12 ml/kg dose. The volume of the scala media was assessed by examining the cross-sectional area of the scala media in the mid-modiolar sections of the cochlea. Histological study revealed that the time-course of the change in the volume of the scala media after glycerol intake showed biphasic changes. Specifically, the early phase of the glycerol effect is a decrease in endolymph volume. The volume of the scala media significantly decreased by 11.4+/-2.9% 2 h after glycerol intake. Thereafter, the volume began to increase, and reached its peak 6-12 h after intake. In addition, the volume of the scala media significantly increased by 17.6+/-1.1% after 6 h. The present study indicated that the secondary increase in the volume of the scala media following glycerol intake played an important role in the rebound phenomenon in the glycerol test, although the mechanism underlying the hearing loss with the endolymphatic hydrops remains to be elucidated.