Bone-conduction hyperacusis induced by superior canal dehiscence in human: the underlying mechanism

Our ability to hear through bone conduction (BC) has long been recognized, but the underlying mechanism is poorly understood. Why certain perturbations affect BC hearing is also unclear. An example is BC hyperacusis (hypersensitive BC hearing)-an unnerving symptom experienced by patients with superior canal dehiscence (SCD). We measured BC-evoked sound pressures in scala vestibuli (P_SV) and scala tympani (P_ST) at the basal cochlea in cadaveric human ears, and estimated hearing by the cochlear input drive (P_DIFF = P_SV - P_ST) before and after creating an SCD. Consistent with clinical audiograms, SCD increased BC-driven P_DIFF below 1 kHz. However, SCD affected the individual scalae pressures in unexpected ways: SCD increased P_SV below 1 kHz, but had little effect on P_ST. These new findings are inconsistent with the inner-ear compression mechanism that some have used to explain BC hyperacusis. We developed a computational BC model based on the inner-ear fluid-inertia mechanism, and the simulated effects of SCD were similar to the experimental findings. This experimental-modeling study suggests that (1) inner-ear fluid inertia is an important mechanism for BC hearing, and (2) SCD facilitates the flow of sound volume velocity through the cochlear partition at low frequencies, resulting in BC hyperacusis.