Click-evoked vestibulo-ocular reflex

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.

Diagnostic Implications of Stimulus Polarity Effects on the Auditory Brainstem Response

The purpose of this study was to determine whether clicks presented in rarefaction or condensation modes produce more accurate diaghostic information. Subjects were 20 consecutive patients who were seen at the Mayo Clinic for unilateral acoustic neuromas. The nontumor ear served as a control to minimize intersubject variability in the latencies. A standard audio logic evaluation was followed by an auditory brainstem response (ABR) test for which the stimuli were rarefaction and condensation clicks. Responses were analyzed for the presence of waves I, III, and V; absolute latencies of waves I, III, and V; interpeak intervals I–III, III–V, and I–V; and interaurallatency difference for wave V. The results indicated that measures from both polarities were similar in this set of patients and that neither click polarity provided diagnostic advantages over the other. Recommendations are to collect ABRs to both click polarities individually to obtain the full complement of waves on which to base the diagnostic impression.

Late Waves in the Response Recorded from the Intracranial Portion of the Auditory Nerve in Humans

We showed in previous studies that the click-evoked responses from the exposed eighth nerve in some patients contain quasiperiodic components that appear in the interval between 4 and 16 milliseconds after the click stimulus, and that the phase of these oscillatory components reverses when the click polarity is reversed. When the responses to clicks of opposite polarity are subtracted from each other, these late waves appear as a prolonged quasiperiodic oscillation with a frequency around 700 Hz. These late components appear more frequently in patients with high-frequency hearing losses than in patients with normal hearing. We have attributed these components to prolonged and in-phase oscillations of a large portion of the basilar membrane, possibly caused by the generation of standing waves on the basilar membrane. The results of the present study show that these oscillations correspond closely in both frequency and phase to the late oscillations that are seen in the cross-correlograms of the responses from the exposed eighth nerve to pseudorandom noise. The finding that similar quasiperiodic components can be retrieved from the responses from the exposed eighth nerve to both transient and continuous sounds is taken as an indication that the neural activity that these components represent reflects some general property of the cochlear frequency analyzer. We also found that the speech discrimination is not noticeably different in patients with such waves compared with what it is in patients with similar hearing loss who do not have such waves. This is taken as an indication that spectral analysis in the cochlea is less important in speech discrimination than previously assumed. The importance of timing of auditory nerve activity for speech perception is indicated by the finding that surgical trauma of the intracranial portion of the auditory nerve that only causes a moderate degree of deterioration of the pure tone threshold reduces speech discrimination scores to zero.