Address for correspondence: Dr. David S. Zee, Dept. of Neurology, Johns Hopkins Hospital, Pathology Building, Rm 2-210, 600 Wolfe St., Baltimore, MD 21287. Voice: (410) 955-3319; fax: (410) 614-1746.
dzee{at}dizzy.med.jhu.edu
The gain and symmetry of vestibulo-ocular reflexes for high-frequency,
high-acceleration movements of the head are altered following
unilateral vestibular lesions. These changes have been well
characterized for rotational head movements (thrusts), and provide
reliable markers of dysfunction in individual semicircular canals.
Alterations in the vestibulo-ocular reflex (VOR) evoked by lateral,
whole-body translations have also been observed. In an approach
directed at the development of a bedside test of otolith function,
we have recorded (scleral search coil) the VOR evoked by brief,
high-acceleration lateral translations (heaves). We delivered
these stimuli manually and also developed a "head sled" device
that minimizes any rotational contaminating component of the
stimulus. Our geometrical analysis of the stimuli enables us
to take into account the translational and rotational components
of the movement, and to calculate an ideal response required
for stabilization of images on the fovea at different fixation
distances. We observed a tracking response (visually assisted
VOR) that was close to ideal for image stabilization when these
methods were used to analyze responses to slow, low-amplitude
lateral translations of the head. When applied to rapid, high-acceleration
(0.5
g) translations, the VOR was found to be less than compensatory
in subjects with normal vestibular function. In a patient with
unilateral vestibular hypofunction following intratympanic gentamicin
injections, both the rotational and the translational VOR were
asymmetric. Responses for translations toward the treated side
had lower gain than those for translations toward the normal
side. These findings provide a basis for further development
of this technique as a clinical test and as a method for quantitative
evaluation of otolith function.
