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Figure 14-1: Serial VOR Responses, at baseline, during ototoxicity &
recovery  Serial
responses on one ototoxic patient (typical) shown with respect to population
data for VOR and visual-VOR (VVOR) responses as a function of frequency and
recovery (compensation). In order to see clearly during head movement, it is
necessary to achieve a gain of unity and zero phase (re: head/eye velocity)
at all frequencies of active or passive head movement. A, The VVOR gain
(left ordinate) and phase (right ordinate) of 10 normal subjects as a
function of frequency (abscissa) indicating unity gain and zero
(compensatory) phase (re: velocity) during passive head rotations with the
subject's eyes open and viewing an earth fixed, full field visual surround.
Rotational stimulus was sum-of-sines 0.01-1.5 Hz. Nystagmus recorded with
EOG. Error bars indicate 2 SD. B, Plot of baseline and serial test results
of VOR response to same rotational stimulus, plotted with respect to a
normal population (shaded).72 Note recovery of responses above
0.1 Hz to low normal range during compensation. Response phases did not
return to normal below about 0.8 Hz. C, Both VVOR and VOR responses of this
patient following compensation. VVOR gain returned to normal; however, a
high-frequency phase lag persists with eyes open (VVOR) but not with eyes
closed (VOR), indicating increase dependence upon vision to stabilize eyes,
with respect to visual surrounds, with loss of vestibular function. D,
Comparison of VOR and optokinetic responses (OKR) from the same patient. The
OKR phase begins to lag above 0.1 Hz (normal). The VOR, and consequently,
the VVOR, phase lags occur at higher frequency, demonstrating OKR
contribution to the VVOR, which is especially important for compensation of
low frequency head movements when vestibular function is compromised. Higher
frequency head movements cannot be compensated by the ototoxic subjects.
This patient=s compensatory
strategy must limit head movements to frequencies below approximately 1 Hz
to avoid oscillopsia. (From Black FO, Perterka RJ, Elardo SE: Vestibular
reflex changes following aminoglycoside induced ototoxicity. Laryngoscope
97:582, 1987; with permission.)
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