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From Neurology (S.T.A., M.J.T., G.E.A., G.M.H.) and Radiology (J.S.M.), Royal Prince Alfred Hospital, Sydney; and School of Psychology, University of Sydney (I.S.C.), Australia.
Address correspondence and reprint requests to Dr. Swee T. Aw, Neurology Department, Royal Prince Alfred Hospital, Camperdown NSW 2050, Sydney, Australia; e-mail: sweea{at}icn.usyd.edu.au
Background: An enlarged, low-threshold click-evoked vestibulo-ocular reflex (VOR) can be averaged from the vertical electro-oculogram in a superior canal dehiscence (SCD), a temporal bone defect between the superior semicircular canal and middle cranial fossa.
Objective: To determine the origin and quantitative stimulus–response properties of the click-evoked VOR.
Methods: Three-dimensional, binocular eye movements evoked by air-conducted 100-microsecond clicks (110 dB normal hearing level, 145 dB sound pressure level, 2 Hz) were measured with dual-search coils in 11 healthy subjects and 19 patients with SCD confirmed by CT imaging. Thresholds were established by decrementing loudness from 110 dB to 70 dB in 10-dB steps. Eye rotation axis of click-evoked VOR computed by vector analysis was referenced to known semicircular canal planes. Response characteristics were investigated with regard to enhancement using trains of three to seven clicks with 1-millisecond interclick intervals, visual fixation, head orientation, click polarity, and stimulation frequency (2 to 15 Hz).
Results: In subjects and SCD patients, click-evoked VOR comprised upward, contraversive-torsional eye rotations with onset latency of approximately 9 milliseconds. Its eye rotation axis aligned with the superior canal axis, suggesting activation of superior canal receptors. In subjects, the amplitude was less than 0.01°, and the magnitude was less than 3°/second; in SCD, the amplitude was up to 60 times larger at 0.66°, and its magnitude was between 5 and 92°/second, with a threshold 10 to 40 dB below normal (110 dB). The click-evoked VOR magnitude was enhanced approximately 2.5 times with trains of five clicks but was unaffected by head orientation, visual fixation, click polarity, and stimulation frequency up to 10 Hz; it was also present on the surface electro-oculogram.
Conclusion: In superior canal dehiscence, clicks evoked a high-magnitude, low-threshold, 9-millisecond-latency vestibulo-ocular reflex that aligns with the superior canal, suggesting superior canal receptor hypersensitivity to sound.
Additional material related to this article can be found on the Neurology Web site. Go to www.neurology.org and scroll down the Table of Contents for the April 11 issue to find the title link for this article.
This work was supported by National Health and Medical Research Council (Project Grant ID: 293801), Brain Foundation, The Clive and Vera Ramaciotti Foundations, Neurology Trustees Royal Prince Alfred Hospital, Australia and Royal Australian and New Zealand College of Radiologists.
Disclosure: The authors report no conflicts of interest.
Received July 28, 2005. Accepted in final form December 22, 2005.
This article has been cited by other articles:
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  S M Rosengren, S T Aw, G M Halmagyi, N P M. Todd, and J G Colebatch Ocular vestibular evoked myogenic potentials in superior canal dehiscence J. Neurol. Neurosurg. Psychiatry, May 1, 2008; 79(5): 559 - 568. [Abstract] [Full Text] [PDF]
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 M. S. Welgampola, O. A. Myrie, L. B. Minor, and J. P. Carey Vestibular-evoked myogenic potential thresholds normalize on plugging superior canal dehiscence Neurology, February 5, 2008; 70(6): 464 - 472. [Abstract] [Full Text] [PDF]
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