Paper in Neurology
- Aw ST, Todd MJ, Aw GE, Magnussen JS, Curthoys
IS, Halmagyi GM (2006) Click-evoked vestibulo-ocular reflex:
stimulus response properties in superior canal dehiscence. Neurology,
66 (7): 1079-87.
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
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[degrees], and the magnitude was less than
3[degrees]/second; in SCD, the amplitude was up to 60 times
larger at 0.66[degrees], and its magnitude was between 5 and
92[degrees]/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.