The vestibular ocular reflex in Alzheimer's disease

(Accepted 27 March 1986)

The vestibular ocular reflex in Alzheimer's disease" VASUDEVAN LAKSHMINARAYANAN', ROBERT P. FRIEDLAND2, ELIZABETH C. MULLER', ELISABETH KOSS2 and LAWRENCE STARK'

Department of Physiological Optics, School of Optometry, University of California, Berkeley, CA 94720; 2Section on Brain Aging and Dementia, LNS, National Institute on Aging, National Institutes of Health, Bethesda, M D 20892, USA

ABSTRACT. The vestibular ocular reflex (VOR) and VOR suppression in both light and darkness were tested in subjects with probable Alzheimer's disease of mild to moderate severity. It is found that mechanisms responsible for VOR and VOR suppression are apparently intact in early stages of this disease. Key words: vestibular ocular reflex ; Alzheimer's disease

INTRODUCTION Oculomotor abnormalities have been noted in Alzheimer's disease (AD) but the extent and pathophysiology of these changes have not been well defined. Impairments in eye tracking (Hutton et al., 1984), perseveration of gaze (Hutton et al., 1979), delayed oculomotor reaction times (Hershey et al., 1983 ; Pirozzolo & Hansch, 1981) and saccadic intrusions (Jones et al., 1983) have been reported. Of particular interest in this regard is the Vestibular Ocular Reflex (VOR) which helps maintain gaze in spite of head movement. Because central cholinergic deficiency is well documented in AD in both cortical and subcortical structures (Davies, 1979; Rosser et al., 1982), and cholinergic neurotransmission is involved in vestibular function (Wamsley et al., 1981), we investigated VOR and VOR sup-

* Reprint requests fo: V. Lakshminarayanan, Department of Physiological Optics, School of Optometry, University of California, Berkeley, C A 94720, USA

Neuro-ophthalmology - 1986, Vol. 6, No. 4, pp. 205-208 0 Aeolus Press Amsterdam 1986

pression in both light and darkness in subjects with probable AD. SUBJECTS AND METHODS Six subjects with a clinical diagnosis of probable AD were studied (Table 1). All subjects conformed to current research criteria for the diagnosis (McKhann et al., 1984), with a progressive generalized loss of intellectual function of gradual onset with especially pronounced memory changes. The following conditions were specifically ruled out in all cases: strabismus, chronic hypertension, evidence of cerebrovascular disease, previous head trauma, epileptic seizures, drug intoxication, severe depression, previous psychiatric illness, alcoholism, family history of other dementing illness, mental retardation or other medical illness affecting cognition. All subjects underwent general medical and neurological examination, laboratory tests of blood and urine, EEG and X-ray com-

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TABLE 1. Data on subjects used in the study Subject

Sex

Age

Severity*

of dementia

cs FE AV RV MM VM

* As

M M M

M F F

58 62 62 62 58 51

moderate mild mild mild to moderate moderate moderate to severe

determined by the Mattis dementia rating scale.

The eye and head movement signals were amplified and recorded simultaneously on a strip chart recorder (Hewlett Packard 7404A, bandwidth of 100 Hz) and. a microcomputer (LSI 11/23). The data was analyzed using special signal processing software (GEDIT).

Experimental protocol and instructions to subjects

Subjects wore the head movement helmet and eye monitor while seated comfortably in a nonrotating chair in a dimly lit room in front of a screen approximately one meter away. The subjects’ eye and head movements were calibrated, when possible, by having them rotate their eyes (or head) between fixation lights at various angular separations (20 and 40 degrees). Eye movement setup When the fixed target, a light emitting diode (LED), was lit, the VOR could be tested by The infrared photoelectric reflection technique instructing the subjects to perform horizontal (Stark et al., 1962) was used to record horizontal head rotations in an oscillatory manner while eye position. The bandwidth of the IR detector fixating the LED target. This procedure was repeated by turning off both the fixation target system was set at 100 Hz. and the dim room lights to test VOR function in darkness, but instructing the subject to maintain Head movement setup fixation on a point one meter away, where the The head movement system has been described LED (now off) used in the previous procedure elsewhere (Nam et al., 1984). Briefly, a flange was positioned. When the target itself is moving with the was bolted as close as possible to the center of rotation on top of a plastic welder’s helmet, head, the VOR is usually suppressed ( a n strapped snugly and securely to the head of the gemeister & Stark, 1983). This VOR suppression subject. One end of a rod, flexible enough to act could be tested by having the subject track a as a universal joint was fixed vertically to the LED target fixed to the head, but positioned in flange while the other end was clamped to the front of the eyes (about 45 cm), while executing movable arm of a precision low torque conduc- head oscillations. Convergence was not a probtive plastic type potentiometer which was con- lem in this procedure. VOR suppression in darknected to a 12V battery. Thus, the resultant elec- ness was measured by turning off the LED tartrical signal was varied directly by torsional get, but instructing the subject to maintain movement of the head. fixation in front of the eyes.

puted tomography of the brain. No medication with effects on eye or head movements were being taken by the subjects at the time of testing. Consent was obtained from all subjects tested following a full explanation of the experimental procedures.

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RESULTS AND DISCUSSION VOR and VOR suppression tests were qualitatively normal in all six subjects when they maintained attention on the task. Figure la shows the VOR (in light) test result for subject VM (the most severely demented patient tested). The classical counterphase eye and head movement records were obtained. It was not possible to calVOR

ibrate the movements on this subject due to attentional drifts and poor cooperation with instructions. However, the results indicate that normal VOR functions were present. VOR test results in darkness for subject MM are shown in Fig. Ib, and once again, the VOR is maintained. Figures lc and Id show VOR suppression test results. The VOR suppression test eye movement records are noisy in that subject compliance VOR SUPPRESSION

Fig. 1. Eye and head movements. Top trace of each pair is eye movement; bottom head movement. Abscissa: time (in secs). Ordinate : displacement (in degrees). Magnitude of head movement i s approximately 20 degrees. a. (upper left): Normal VOR test in light, Subject VM. b. (lower left): VOR test in darkness, Subject MM. c. (upper right): VOR suppression test in light, Subject MM. Note reduced eye movement response. d. (lower right) : VOR suppression test in darkness, Subject MM. Note absence of counterphase eye-head motion and also extra saccades (as well as blinks) in eye movement trace.

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with instructions were not adequate. It should be noted that this (VOR suppression task) is a relatively difficult task and eye movement records for normal control subjects tested (not shown) were also relatively noisy (though of lesser noise magnitude). The VOR gain, defined here as the ratio of eye displacement to head displacement, was computed to be approximately 0.9 and VOR suppression about 0.2. These values were computed for subject FE who was able to give reasonable calibration data. The VOR gain is within normal range and the suppression is adequate. Quantitative measurements in other subjects were precluded since good eye movement calibrations could not be obtained, as mentioned earlier, due to non-compliance with instructions. These findings suggest that the peripheral and

central vestibular mechanisms responsible for VOR and VOR suppression may be intact in mild and moderate stages of AD. It is known that all central cholinergic systems are not equally affected in the disease. While the cholinergic deficit is widespread in the cortex, it is not generalized to all subcortical areas (Rosser et al., 1982). The present results indicate that the cholinergic components of the VOR may be spared in the disease. ACKNOWLEDGEMENTS V. Lakshminarayanan was partly supported by a graduate fellowship from the Regents of the University of California, and by a grant from the National Eye Institute EYO 3674 (to Dr. J. M, Enoch, Dean, School of Optometry, University of California, Berkeley). This work was supported in part by the Medical Research Program of the Veterans Administration.

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