Regulation of endolymphatic fluid volume.

Direct measurements of the dispersal of markers in endolymph have failed to support previously established hypotheses of endolymph homeostasis, specifically longitudinal flow, radial flow, and dynamic flow theories. Rather, they suggest that in the normal state endolymph is maintained without a significant involvement of volume flow at all. Ions appear to be transported into and out of the endolymphatic space in a similar manner to that for a single cell, with each ion transport process contributing to the electrolyte pool. In abnormal volume states, however, longitudinal volume flow of endolymph may contribute to homeostasis. Procedures that enlarge the endolymphatic space result in endolymph flow toward the base of the cochlea, contributing to the removal of electrolytes and volume. Similarly, procedures that decrease cochlear endolymph volume induce apically directed flow in the cochlea, contributing to the addition of electrolytes and volume to the endolymphatic space. The endolymphatic sac responds to endolymph volume disturbance, showing op posite responses to volume increases and decreases. Although evidence is still limited, the endolymphatic sac appears to act as a "bidirectional overflow" system. While volume disturbances originating from out-of-balance transport processes anywhere in the labyrinth may be corrected by the sac, dysfunction of the sac itself is likely to have a substantial effect on endolymph status.