Objectives: To investigate whether vibrational impulse stimuli applied to the skull can be used to evoke the vestibulo-ocular reflex (VOR) and detect vestibular lesions. Methods: Twenty four patients with unilateral vestibular loss (UVD), five with bilateral vestibular loss, two with ocular palsies, and 10 healthy subjects participated. Vibrations of the skull were induced with head taps and with a single period of 160 Hz tone burst on the inion, vertex, and the mastoids while the patients viewed a distant target. Several patients were also examined while viewing a near target, with eccentric gaze and in tilted postures. Responses were recorded by EOG. Results: Responses occurred between 5 ms and 20 ms and seemed to be compensatory to the second phase of the sine wave of vibration impulse and were greatly diminished/absent in patients with bilateral VD and ocular palsies. The patients with UVD had asymmetrical responses in the vertical EOG with stimuli applied on the inion and vertex, with enhancement of the response amplitude on the side of vestibular loss and/or diminution on the healthy side. The asymmetry ratios between the healthy subjects and patients with UVD, and among patients with UVD were statistically significant. Some gaze and positional influences could be demonstrated consistent with otolithic reflexes. Conclusion: If the asymmetric responses to skull vibration in UVD result from passive oscillatory movements of the orbital tissues they may reflect the otolith mediated sustained skew torsion. Conversely, if generated by active eye movements, their likely origin is a phasic VOR. B oth primary otolithic and canal afferents of a monkey can be activated by vibration. The lowest phase locking thresholds have been determined at 270 to 280 dB and median values in the most sensitive frequency range (200-400 Hz) at 220 to 240 dB of gravitational acceleration.1 It is still not clear whether activation of vestibular receptors by vibration has the same mechanical basis as the response to more physiological head movements. Mechanical factors are not the only determinants of response dynamics since vestibular nerve fibres can show a frequency dependent increase in gain greater than that predicted for the mechanics of sensory end organs. The mechanics of the otolithic membrane can be approximated by a damped second-order system with a resonant frequency of the order of 50-500 Hz. Thus, in contrast to the cupula-endolymph system in which the upper frequency limit is set below 60 Hz, the otolithic membrane is much better suited for transmission of bone vibrations in the audio frequency range. Conversely, canal neurones tend to be more irregular than otolith neurones, and hence might be expected to have lower vibration thresholds.
