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Dynamics of Cover Test Eye Movements

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Dynamics of Cover Test Eye Movements 0093-7002/83/6008-0712$02.00/0 Vol. 60, No.8, pp. 712-724 AMERICAN JOURNAL OF OPTOMETRY & PHYSIOLOGICAL OPTICS Printed in U.S.A. Copyright © 1983 AMERICAN ACADEMY OF OPTOMETRY Dynamics of Cover Test Eye Movements ELI PELI- GLEN McCORMACKt The New Eng/and College of Optometry, Boston, Massachusetts ABSTRACT The cover test is the most commonly used The dynamics of unilateral cover test eye technique for the detection and diagnosis of heterophoric and strabismic oculomotor imbal­ movements, never before systematically in­ I vestigated with an objective recording system, ance. ,2 The unilateral cover test, in which one are shown to be more complex than textbook eye is briefly covered and then uncovered, is accounts of them. We administered the cover used principally to reveal the presence of a strab­ test to nine heterophoric subjects by means of ismic deviation. The alternating cover test, in electromechanical occluders. Eye movements which the cover is repeatedly switched from one were recorded using the infrared photoelectric eye to the other, is used to quantify the devia­ technique. Saccadic and vergence move­ tion. This report concerns the dynamics of uni­ ments of the fixating eye were observed in lateral cover test eye movements in normally almost all records when the occluded eye was binocular patients. The standard clinical descriptions of unilat­ uncovered. These movements were found in l 2 esophores and exophores and in both large eral cover test eye movements • assert that and small phoria cases. Such movements when one eye is covered (the "cover phase") it were previously described in other asymmetric will deviate to its heterophoric posture, while vergence tasks and appear to obey Hering's the noncovered eye maintains steady fixation. law of equal innervation. Uncovering the dom­ When the cover is removed (the "uncover inant eye, in cases .qf clear dominancy, re­ phase") the formerly covered eye regains fixa­ sulted in shorter latency and larger amplitude tion, and again it is assumed that the fixating saccades than did uncovering the nondomi­ eye does not move. The required movement is nant eye. These large saccades were fre­ therefore an asymmetric vergence in which the quently of unequal amplitude in each eye. "fixating" eye should remain steady while the Trained subjects appear to use dynamic covered eye performs the entire vergence. Such overshoots to increase this saccadic inequal­ descriptions portray the static position of the ity and thereby attain vergence during sac­ eyes before and after occlusion, and not the cades. Movements after the application of a actual trajectory of the eyes that lead to their cover to one eye, while grossly similar to text­ static position. Thus, Griffin notes that a slight book descriptions of them, are found to con­ "~ick" ~f the fixating eye may be noted in pa­ tain small vergence drifts and refixation (cor­ tients wIth large phorias. recting) saccades in the nonoccluded eye. If the fixating eye of heterophores made no movement during the cover test (i.e., in small heterophorias) then the cover test eye move­ Key W?rds: cover test, asymmetric vergence, ment pattern would be in violation of Hering's dynamic overshoots, heterophoria, unequal law of equal innervation3 and run counter to a saccades number of studies on asymmetric vergence eye 4 movements. ---6 Because the intermittent occlu­ sion of the cover test is a stimulus condition not previously used in asymmetric vergence studies, Presented at the 1981 Annual Meeting of the Amer­ and because there are no reports available on ican Academy of Optometry, Orlando, Florida. the dyn~mics of cover test eye movements using Received AprilS, 19S2; revision received March 29, a recordmg technique sufficient to resolve these 1983. dynamics, we decided that a reassessment of * Electrical Engineer, MSEE, Research Associate, cover test eye movements using a high-speed Student. objective recording technique was needed to re- t Optometrist, Ph.D., Member of Faculty. solve the question of the general conformity of 712 August 1983 Cover Test Eye Movements-Peli & McCormack 713 unilateral cover test eye movements to Hering's phase differs from the accommodation-only con­ law and to the known characteristics of asym­ dition because of the absence of any accommo­ metric vergence eye movements. dative stimulus change in the cover phase. Semmlow and Heerema7 measured, in normal The other extreme of the vergence eye move­ observers, movements of the covered eye after a ment spectrum demands a vergence response unilateral cover. Their method of applying the despite a constant accommodative demand, and cover when the eyes are at the phoria position is known as the "disparity-only" case. Disparity­ of gaze was intended only to demonstrate certain only stimuli may be presented haploscopi­ accommodative/convergence relationships, and cally,4.13,14 or by the interposition of a prism limits the conclusions one may draw from their before one eye (e.g., the 4 t. base-out test). results about the normal dynamics of cover test Hering's law of equal innervation seems to hold eye movements. A few studies of cover test eye best for the disparity-only condition because movement dynamics based on visual observa­ almost every. response is found to contain an 9 tions have been published,s-lO Pickwe1l ,10 no­ irrelevant saccade of large magnitude. Many of ticed, in patients with high heterophoria, move­ these disparity-only movements also show what ment of the noncovered eye during the uncover appears to be a non-Hering's law inequality phase of the test. He found that the fixating eye of "irrelevant" saccade amplitude6 in which the frequently makes a saccade away from the point covered eye. generates the large saccade. (Ken­ of regard which is followed by an approximately yon et a1. 15, 16 have also observed this inequality symmetric vergence movement back to the point in accommodation-only experiments.) These of regard. This first movement of the fixating unequal saccades have the effect of reducing the eye, named the "irrelevant" movement or "flick" fusional vergence response required to meet the by PickwelPo as it did not reflect the demand of disparity demand in the disparity-only case. the stimulus, was assumed to be an expression The vergence demand of the uncover phase of of Hering's law. Pickwell10 noted that irrelevant the unilateral cover test is very similar to the saccades were not reported in earlier studies. asymmetric disparity-only demand. In both Irrelevant movements have been demon­ cases, the accommodative stimulus is held con­ strated in a variety of experiments on asymmet­ stant and the disparity of the target, subtended ric vergence.4-6 The vergence stimuli in some of entirely at the nonaligned eye, is the only verg­ these experiments are aligned on the visual axis ence stimulus. These two vergence stimuli differ of one eye, thus demanding no change in the qualitatively only with respect to the intermit­ position of the aligned eye when vergence de­ tency of binocular vision in the cover test. If the mands are varied. We shall compare the cover absence of binocular vision does not affect the test response to these types of eye movements. subsequent eye movements in the uncover phase Ono and Nakamizo4 studied asymmetric verg­ of the cover test, then we can expect these ence eye movements throughout a spectrum of movements to be similar to those found in the accommodation and convergence conditions. disparity-only condition and that the irrelevant They found movement of the aligned eye in saccade characteristics should be present in almost all trials. The magnitude of this saccadic small phoria cases as well as large phoria cases. movement varied with the different conditions. Both of these expectations were realized. One extreme of the asymmetric vergence spec­ trum is represented by the Johannes Mueller 4 METHODS experiment • .'i,11,12 in which the eye not aligned with the far and near targets is constantly oc­ Nine subjects, aged 24 to 35 years, all had cluded. This case is known as the "accommo­ normal binocular vision and 6/6 (20/20) or bet­ dation only" condition4 as no disparity-driven ter visual acuity while wearing their habitual vergence is possible with one eye occluded. Most distance corrections. One was tested while wear­ of the movement is performed by the covered ing soft contact lenses, two wore spectacles, and eye, which manifests an asymmetric vergence the remaining six subjects were nearly emme-' which appears to violate Hering's law. The re­ tropic. Six subjects were exophoric and three sults show, as noted by Ono and N akamizo, 4 were esophoric. Informed consent was obtained that the "fixating" eye moves toward the oc­ from all subjects before their participation. cluded eye, but its movement is much smaller. Eye movements were recorded using an in­ Thus, some type of irrelevant or Hering-type frared photoelectric technique.17 The electronics movement is maintained even under these mon­ for the photoelectric eye movement monitor ocular conditions. The accommodation-only were assembled in our laboratory. Infrared sen­ condition is similar to the cover phase of the sors and emitters were fitted to an adjustable cover test by virtue of the absence of binocular spectacle frame mounting device (Rehder Devel. visual control over either movement. The cover Co., Castro Valley, CAl. A small bite-board was 714 AM J OPTOM & PHYSIOL OPTICS Vol. 60, No.8 used to stabilize both head movements and the these cases, the experimenter waited (up to 8 viewing distance of 30 em. For calibration pur~ sec in some cases) for the completion of the eye poses the subjects tracked a horizontally moving movement, as determined by on-line monitor­ spot on a cathode ray tube.
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