An expanding mechanical role for the tiny mammalian endolymphatic duct: 2. An explanation for the beaching of whales–failure of osmotic pressure counterbalancing mechanism in the cochlea causing acute vertigo attack

Abstract

Endolymphatic Hydrops (EH) describes a swelling of the endolymphatic duct (ED). It is a key correlate of human Ménière’s disease featuring incapacitating giddy attacks. Direct measurements by micropuncture to be presented support the hypothesis that swelling occurs because water flows through aquaporins into the ED under an osmotic gradient to stabilize basilar membrane position against deep dive pressures. Patients having vestibular attacks often display nystagmus if not also perilymphatic fistulae or dehiscences. As shown by Ketten et al. (2004), stranded whales also exhibit nystagmus and also intracochlear bleeding. Here, EH is modelled as a normal rate-limited osmotic mechanism which likely evolved to cope with synoptic variation. It has been extensively adapted by beaked whales to allow diving to feed on squid at great depths—where they still need to hear. Their more-robust EDs normally cope with very slow equalization against megapascal pressures. An unexpected undersea pressure transient may trigger perilymph-endolymph membrane rupture and consequent vertigo attack, well documented in humans. Due to acoustic spread, whole pods may be variously affected. Humans need to remain stationary while waiting hours for their attack to subside. Marine mammals with limited oxygen stores may panic and surface too quickly, accounting for life-threatening decompression sickness.