High-frequency pseudo-hypofunction of the superior semicircular canal dehiscence syndrome: a vHIT – oVEMP dissociation explained by hydrodynamic energy loss

Background Superior semicircular canal dehiscence syndrome (SCDS) produces a wide spectrum of vestibular and auditory abnormalities due to the presence of a "third mobile window." While enhanced otolith-mediated responses are well recognized, the behavior of high-frequency semicircular canal function remains debated. Many patients show reduced superior canal vestibulo-oculomotor reflex (VOR) gains despite preserved or exaggerated utricular responses.ObjectiveTo investigate the relationship between dynamic semicircular canal responses assessed by video Head Impulse Test (vHIT) and otolith responses assessed by oVEMPs in people with superior semicircular canal dehiscence.MethodsFifty-three patients with SCDS underwent full vestibular testing including vHIT of all six semicircular canals, cervical and ocular vestibular-evoked myogenic potentials (cVEMPs, oVEMPs) to 500-Hz BCV, and high-frequency oVEMPs to 4,000-Hz stimuli.ResultsSuperior semicircular canals showed the largest VOR gain reductions, with the left anterior (LA) canal most affected (mean gain 0.86 +/- 0.18). Bilateral SCDS exhibited significantly lower gains than unilateral SCDS, with mean LA gain falling below the functional threshold (0.68 +/- 0.14). In contrast, all patients demonstrated enhanced utricular responsiveness, including increased 500-Hz oVEMP amplitudes and consistent 4-kHz N10 responses.ConclusionOur findings confirm the presence of high-frequency oVEMP hyperresponsiveness, yet with a VOR pseudo-hypofunction due to a loss of high-frequency mechanical energy through the dehiscence, which diminishes the utricular vortex and cupular deformation. This frequency-dependent hydrodynamic dissociation provides a comprehensive mechanistic framework for interpreting vestibular test batteries in SCDS. Combined assessment with vHIT and high-frequency oVEMPs offers a powerful, physiologically grounded diagnostic approach.