Transmission of cerebrospinal fluid pressure via the cochlear aqueduct and endolymphatic sac

The concept of perilymphatic and endolymphatic pressure balance is generally linked to the theory that the endolymphatic sac transmits cerebrospinal fluid (CSF) pressure changes to the endolymph to equalize CSF pressure changes transmitted to the perilymph via the cochlear aqueduct. This theory, and the significance of other mechanisms of CSF pressure influence on the labyrinth, were evaluated experimentally. Continuous measurements of perilymphatic, CSF, venous, and arterial pressures were performed on cats with the cochlear aqueduct patent or obstructed and the inferior cochlear vein intact or occluded. Intracranial pressure changes were induced by subarachnoid infusion of artificial CSF in live and dead animals. With the cochlear aqueduct patent, CSF pressure changes were transmitted to the perilymph without any significant dampening or time lag. With the cochlear aqueduct obstructed, CSF pressure changes induced significantly lower and delayed changes in perilymphatic pressure. Similar results were obtained whether the animals were alive or dead and the cochlear vein intact or blocked. This indicated a passive mechanism not induced by changes in labyrinthine fluid production or blood flow. Long-standing, stable elevation of CSF pressure with the cochlear aqueduct blocked induced a slowly increasing perilymphatic pressure, always stabilizing at a pressure rise significantly less than that of CSF. The results do not suggest any major pressure transfer via perineural or perivascular routes. The endolymphatic sac is postulated to mediate a reduced and delayed transfer of increased intracranial pressure to the labyrinth.