dynamic vergence eye movements in strabismus and amblyopia
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Dynamic vergence eye movements instrabismus and amblyopia:
symmetric vergenceRobert V. Kenyan, Kenneth J. Ciujfreda,* and Lawrence Stark
Dynamic vergence eye movements in response to step target displacements along the midlinewere measured by an infrared reflection technique in 11 patients having either intermittentstrabismus, constant-strabismus amblyopia, or amblyopia without strabismus. We found theabsence of normal disparity (fusional) vergence in all patients having strabismus and in somepatients having amblyopia without strabismus. A characteristic response consisting of a bin-ocular accommodative vergence component and an early binocular saccadic component wasused to foveate the target of interest with the dominant eye. Vergence responses in our controlsubjects and patients with the nondominant eye occluded were similar to those recorded in ourpatients during binocular vieioing. These results suggest that disparity information is notutilized by patients, probably as a result of long-term, ongoing suppression in the deviated oramblyopic eye. Accommodative vergence with the aid of an early foveating saccade was theprimary mechanism for tracking targets in three-dimensional space.
Key words: strabismus, amblyopia, vergence, eye movements,Hering's law, accommodation
V,ergence eye movements in patients withstrabismus and amblyopia have been inferredfrom patients' subjective responses to dispa-rate stimuli presented simultaneously to eacheye, rather than objectively recorded andquantified (for a review of early work, seeBurian and von Noorden1). Burian,2 follow-ing Bielschowsky3 and Schlodtmann,4 be-
From the School of Optometry, University of California,Berkeley.
This research was supported by N.I.H. training grantEY00076 (R. V. K. and K. J. C.) and an Auxiliary tothe American Optometric Association Research grant(K. J. C ) .
Submitted for publication July 26, 1978.Reprint requests: Robert V. Kenyon, Man-Vehicle Lab-
oratory, Department of Aeronautics and Astronautics,Room 37-215, Massachusetts Institute of Technology,Cambridge, Mass. 02139.
*Present address: Department of Basic Optometric Sci-ences, State College of Optometry, State University ofNew York, 100 E. 24th St., New York, N. Y. 10010.
lieved that some strabismic patients couldproduce "fusional movements" when dispa-rate stimuli, placed in the peripheral retina ofeach eye, were reported fused. Patients re-ported changes in the localization of two hap-lopic fiduciary marks located in the center ofthe visual field, and this was interpreted asevidence for actual eye rotations. Hallden5
confirmed Burian's finding in patients havinganomalous retinal correspondence (ARC). Hesimilarly defined fusional movements as a re-turn to the no-disparity condition in the cen-tral field following the introduction of a dis-parity. In Hallden's study, changes in sub-jective localization of targets were estimatedby haploscopic techniques, and from this hecalculated changes in vergence angle. Thesefusional movements took several minutes tocomplete, and static measurements of targetlocations were made only about once a min-ute. However, in none of the above studieswere eye movements objectively recorded.
60 0146-0404/80/010060+15$01.50/0 1980 Assoc. for Res. in Vis. and Ophthal., Inc.
Volume 19Number 1 Symmetric vergence in strabismus, amblyopia 61
Mariani and Pasino6 criticized Burian'sstudy and pointed out that other factors couldaccount for his results. They referred toKretschemer,7 who showed that changes inthe angle of anomaly rather than true fusionalmovements could be the mechanism em-ployed to fuse disparate targets. (The angle ofanomaly is measured from the fovea in thenondominant eye to a point in the nondomi-nant eye that corresponds to the same visualdirection as the fovea of the dominant eye.)From their own experiments on fusion instrabismic patients, still without objectiveeye movement recordings, Mariani and Pa-sino6 inferred that most patients changed theangle of anomaly instead of performing truefusional movements, thus confirming Kret-schemer's findings.
In an attempt to reconcile these differ-ences and further extend these observations,we objectively recorded dynamic vergenceeye movements in 11 patients having ei-ther constant-strabismus amblyopia, ambly-opia without strabismus, or intermittentstrabismus, while changing binocular fixationbetween near and far midline targets. Thesetargets produced disparity by changing dis-tance from the patients as is found in normaleveryday binocular viewing. Quantificationof the dynamic aspects of the vergence re-sponse in these patients shows the absence ofnormal disparity vergence movements andthe use of normal accommodative vergenceeye movements and saccades to track targetsin three-dimensional space.
Binocular horizontal eye position was recordedby an infrared reflection method.8- 9 The recordingsystem had an overall bandwidth of 150 Hz, a lin-ear range of at least 7, and a noise level of 6 minarc. The infrared method does not distinguish be-tween eye rotation and eye translation; hence,translational movements, if large enough, couldintroduce artifacts in the eye movement record-ings. However, Krishnan and Stark,10 using amethod that distinguishes between rotation andtranslation, have shown that eye translation duringsaccadic or vergence eye movements is minimal.Furthermore, using data from Fry and Hill," theyestimated that the translational movement of the
eye would only be 0.01 mm or 0.05 for a 10rotation, below the noise level of our recordingsystem.
To ensure that the responses were independentof the target system, two systems were used tostimulate vergence. In each system, targets wereplaced along the subject's midline at 25 and 50 cmaway from the estimated center of rotation of eacheye. These targets were carefully adjusted in bothhorizontal and vertical planes to minimize occur-rence of eye movements resulting from targetmisalignment. In one system, the two targetsconsisted of small Lucite plates with fine crossesetched in the front surfaces subtending visual an-gles of 1.5 and 3.0 for the far and near targets,respectively. The fine lines of the crosses sub-tended angles of 2 and 4 min arc for the near andfar targets, respectively. A miniature bulb (GE222) was installed to provide target illumination ofthe etched cross alone. In the other system,targets were made from strips of exposed film withfine crosses etched into the emulsion, and thetargets were mounted in front of a 2 cm squareholder and back-illuminated by a grain-of-wheatfilament bulb through a light diffuser. The holdersfor the far and near targets subtended angles of2.3 and 4.5, respectively. The fine lines compos-ing the crosses subtended angles of 2 and 4 min arcfor far and near targets, respectively.
The luminance of the targets was measuredwith an S.E.I, photometer under the same lowphotopic illumination conditions used during theexperiments. The luminance for the Lucite platesmeasured 0.5 log ft-lamberts. The contrast forthese targets with the formula (lmax tarn/Lax +Imin) was 0.53. For the second system using theexposed film, the target luminance measured 1.7log ft-lamberts and the contrast was 0.93.
Experimental procedures were adapted to theclinical situation and thus kept as simple and asuniform as possible. Targets were alternately il-luminated in a pseudorandom sequence by theexperimenter. Each subject was instructed tokeep the illuminated target clear and single at alltimes. These procedures were performed (1) withboth eyes viewing and (2) with the nondominanteye covered but still allowing eye movements tobe recorded from both eyes. Calibrations wereperformed before, during, and after each brief ex-perimental run.
Eleven patients having strabismus and/or func-tional amblyopia and two normal control subjectsparticipated in the study. Patients were recruitedfrom the general clinic at the School of Op-tometry, University of California, Berkeley. All
62 Kenyon, Ciuffreda, and StarkInvest. Ophthalmol. Vis. Sci.
Table I. Clinical data of subjects
Subject Age PrescriptionVisualacuity
Constant-strabismus amblyopia:B. S.
LE +2.00RE +2.25LE +3.75RE +0.50LE -1.50RE -1.75LE +0.75RE +0.25
Amblyopia without strabismus:S. H.
LE -0.75RE pi =LE -2.50RE -5.00LE +5.00RE +3.00
= -0.25 x 130
= -0.50 x 165
= -0.50 x 40= -0.50 x 180
= -2.00 x 90-0.50 x 19= -1.25 x 172= -0.75 x 5
Intermittent strabismus without amblyopia:J. L.
LE +0.75RE +0.50LE -5.00RE -4.50
patients:LE -5.00RE -2.50LE -0.75RE -2.75
LE PianoRE PianoLE +8.0 =RE +8.0
- -0.75 x 20
= 0.75 x 165= -0.50 x 8= -0.75 x 90
= 0.25 x 175
1-2* ET LE
18* ET LE
10* ET LE;1* HT LE5-6*ETLE;2* HT LE
20* ET RE;6* HT RE15* XT LE
V2* nasal LE
1* nasal LE
2.5* nasal;2* superior LE2.5^.5* nasal;3-4* superior LE
2* nasal; 2*inferior LE2* Nasal; 2*inferior RE2* temporal LE
Central, steadyLE, RE
NRC100"NRC60"NRC400" -> 60"