Changes in Vestibulo-Ocular Reflex Gain After Surgical Plugging of Superior Semicircular Canal Dehiscence

Abstract

Superior semicircular canal dehiscence (SCD), which is characterized by a "third mobile window" in the inner ear, causes various vestibular and auditory symptoms and signs. Surgical plugging of the superior semicircular canal (SC) can eliminate the symptoms associated with increased perilymph mobility due to the presence of the third window. However, the natural course of vestibular function after surgical plugging remains unknown. Therefore, we explored longitudinal vestibular function after surgery in 11 subjects with SCD who underwent SC plugging using the middle cranial fossa approach. Changes in vestibulo-ocular reflex (VOR) gain in all planes were measured over 1 year with the video head impulse test. We also evaluated surgical outcomes, including changes in symptoms, audiometric results, and electrophysiological tests, to assess whether plugging eliminated third mobile window effects. The mean VOR gain for the plugged SC decreased from 0.81 ± 0.05 before surgery to 0.65 ± 0.08 on examinations performed within 1 week after surgery but normalized thereafter. Four of seven subjects who were able to perform both VOR tests before surgery and immediately after surgery had pathologic values (SC VOR gain < 0.70). Conversely, the mean VOR gain in the other canals remained unchanged over 1 year. The majority of symptoms and signs were absent or markedly decreased at the last follow-up evaluation, and no complications associated with the surgery were reported. Surgical plugging significantly attenuated the air-bone gap, in particular at low frequencies, because of increased bone conduction thresholds and deceased air conduction thresholds. Moreover, surgical plugging significantly increased vestibular-evoked myogenic potential thresholds and decreased the ratio of summating potential to action potential in plugged ears. Postoperative heavily T2-weighted images were available for two subjects and showed complete obliteration of the T2-bright signal intensity in the patent SC lumen in preoperative imaging based on filling defect at the site of plugging. Our results suggest that successful plugging of dehiscent SCs is closely associated with a transient, rather than persistent, disturbance of labyrinthine activity exclusively involved in plugged SCs, which may have clinical implications for timely and individualized vestibular rehabilitation.

Current Trends, Controversies, and Future Directions in the Evaluation and Management of Superior Canal Dehiscence Syndrome

“…It is hypothesized to be related to the surgical trauma. Potential mechanisms include: (1) labyrinthitis, (2) loss of perilymph disturbing labyrinthine function, (3) compression of the membranous labyrinth with displacement of endolymph causing a “hydrops-like” condition, and (4) membranous labyrinth tears allowing ion exchange between the otherwise confined compartments ( 111 , 115 , 116 , 130 , 131 ). Additionally, a reduction of SSC function from occlusion repair may cause acute vestibular impairment.…”

“…Vestibular examination within the first postoperative week will likely show spontaneous and/or post-head-shaking nystagmus (90% of patients), and often as an irritative nystagmus indicating increased excitability (70% of patients), alternatively as a paralytic nystagmus indicating hypofunction (only data on patients with repair by occlusion technique) ( 115 ). VOR testing following surgical repair by occlusion will most often show reduced function of the SSC (4/4 and 4/7 patients with reduced VOR gain) ( 130 , 131 ) and may also show decreased function of the ipsilateral posterior and horizontal semicircular canals ( 116 , 130 ). This is consistent with vestibular impairment in the acute postoperative setting.…”

“…This is consistent with vestibular impairment in the acute postoperative setting. One study suggested that over time (months), VOR gain for the SSC can normalize to preoperative values (11 patients) ( 131 ), whereas other studies show sustained reduction and only partial improvement in SSC function (19, 5 and 10 patients) ( 81 , 130 , 132 ). Reduced SSC function alone can likely not explain cases of prolonged vestibular impairment.…”

Quantitative three-dimensional image analysis of the superior canal after surgical plugging to treat superior semicircular canal dehiscence

“…Hypermobile fluid dynamics in the otic capsule presumably constitute an important mechanism that increases the pressure shunt into the vestibular end organ 5 . Surgical plugging of a dehiscent SC can impede the effect of the third mobile window, thus immediately reducing subjective symptoms and restoring cochleovestibular hyper-responsiveness 2 .…”

“…Moreover, magnetic resonance imaging (MRI) using fast imaging with a steady-state acquisition sequence can reliably exclude a SCD diagnosis, with a negative predictive value of 100% 7 . High-resolution MRI reformatted to the plane of the SC, such as a three-dimensional (3D) multiplanar and/or maximum intensity projection reconstruction, can help with visualizing the signal from residual fluid in the SC following canal plugging 2 . Based on 3D T2-weighted images, a normal status after SC plugging is reportedly achieved in fewer than 70% of affected patients, presumably due to a filling defect involving the superior ampulla or incomplete plugging 8 .…”