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Relating Binocular and Monocular Vision in Strabismic and Anisometropic Amblyopia

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CLINICAL SCIENCES Relating Binocular and Monocular Vision in Strabismic and Anisometropic Amblyopia Ritwick Agrawal, MS, MD; Ian P. Conner, PhD; J. V. Odom, PhD; Terry L. Schwartz, MD; Janine D. Mendola, PhD Objectives: To examine deficits in monocular and bin- ing greater deficits in Vernier acuity. Both amblyopic ocular vision in adults with amblyopia and to test the fol- groups were then characterized according to the degree lowing 2 hypotheses: (1) Regardless of clinical subtype, of residual stereoacuity and binocular motion integra- the degree of impairment in binocular integration pre- tion ability, and 67% of subjects with strabismus com- dicts the pattern of monocular acuity deficits. (2) Sub- pared with 29% of subjects with anisometropia were clas- jects who lack binocular integration exhibit the most se- sified as having “nonbinocular” vision according to our vere interocular suppression. criterion. For this nonbinocular group, Vernier acuity is most impaired. In addition, the nonbinocular group Methods: Seven subjects with anisometropia, 6 sub- showed the most dichoptic contrast masking of the am- jects with strabismus, and 7 control subjects were tested. blyopic eye and the least dichoptic contrast masking of Monocular tests included Snellen acuity, grating acuity, the fellow eye. Vernier acuity, and contrast sensitivity. Binocular tests included Titmus stereo test, binocular motion integra- Conclusion: The degree of residual binocularity and in- tion, and dichoptic contrast masking. terocular suppression predicts monocular acuity and may be a significant etiological mechanism of vision loss. Results: As expected, both groups showed deficits in monocular acuity, with subjects with strabismus show- Arch Ophthalmol. 2006;124:844-850 MBLYOPIA IS A DEVELOP- universally reduced and is often absent in mental visual disorder both types of amblyopia. In subjects with characterized by abnor- anisometropia, central vision stereoacu- mal form vision and bin- ity is normal at low spatial frequencies, ocular functions. Past re- subnormal at intermediate spatial frequen- search has differentiated the performance cies, and unmeasurable at higher spatial A 10 of subjects with strabismic and anisome- frequencies. In subjects with strabis- tropic subtypes of amblyopia.1-6 Often the mus, this loss is more profound at all spa- motivation is to better understand the tial frequencies (probably as a result of a pathogenesis of amblyopia. An impor- history of diplopia early in life that is tant source of evidence is monocular po- avoided through competitive suppres- sition acuity tasks. For example, Vernier sion of the visual inputs from one eye). Bin- acuity loss is more severe in subjects with ocular motion integration is another bin- strabismus compared with that in sub- ocular function. Arguments have been jects with anisometropia. In subjects with made that the motion pathway is rela- strabismus, the deficit is larger than that tively spared in amblyopia,11-13 so that re- predicted by linearly scaling their grating sidual binocular integration ability may ex- acuity,5 indicating that additional defi- ist when tested via motion sensitivity, at cits are involved. Different models at- least in the anisometropic subtype.14 Fi- tribute this acuity loss according to the size, nally, interocular inhibition can be mea- shape, or density of receptive fields in the sured via the dichoptic contrast masking Author Affiliations: visual cortex, or to spatial disarray in their that occurs when a suprathreshold stimu- Departments of Neurobiology topographical location.2,7-9 In general, these lus is presented to one eye, because the (in- and Anatomy (Drs Agrawal, models suggest that abnormal “noise” crement) contrast threshold is elevated for Conner, and Mendola), arises in the visual cortex, probably in areas the other eye.15 In subjects with ambly- Ophthalmology (Drs Odom, beyond the primary visual cortex. opia, masking is usually present, al- Schwartz, and Mendola), and Another source of evidence is deficits though it may be abnormal in strength.16-18 Radiology (Dr Mendola) and 14 Center for Advanced Imaging in binocular vision. Binocular integra- Investigators in a recent large study of (Dr Mendola), School of tion of the image formed in each eye re- 427 subjects with amblyopia and 68 con- Medicine, West Virginia sults in several distinct visual abilities. trol subjects came to a provocative conclu- University, Morgantown. Depth perception via stereopsis is almost sion regarding the relationship between (REPRINTED) ARCH OPHTHALMOL / VOL 124, JUNE 2006 WWW.ARCHOPHTHALMOL.COM 844 ©2006 American Medical Association. All rights reserved. Downloaded From: https://jamanetwork.com/ on 10/01/2021 Table 1. Characteristics of Clinical Groups Interocular Difference Snellen Acuity Deviation Refraction* in Spherical Age at Age at Eye Patching Subject in Prism Equivalent of Diagnosis, Correction Treatment, Binocular Stereoacuity, No. OS ODDiopter OS OD Refractive Error y Surgery, y mo Vision arc seconds Control 1 20/20 20/20 −0.25 ϩ 0.50 00 0.50 40 ϫ154° 2 20/16 20/20 −2.25 ϩ 0.50 −3.75 ϩ 0.75 1.25 40 ϫ179° ϫ178° 3 20/20 20/20 00 00 0 40 4 20/16 20/16 −0.25 ϩ 0.25 00 0.25 40 ϫ39° 5 20/16 20/20 00 00 0 40 6 20/20 20/16 0.50 0.50 0 40 7 20/20 20/25 −1.75 ϩ 1.75 −1.50 ϩ 1.25 1.0 80 ϫ166° ϫ 175° Anisometropia 1 20/63 20/20 −0.75 ϩ 0.50 1.75 ϩ 2.50 3.5 9 2 Yes 800 ϫ32° ϫ145° 2 20/16 20/32 3.50 ϩ 0.50 −1.00 ϩ 0.25 2.7 9 1 Yes 100 ϫ116° ϫ109° 3 20/63 20/16 0.00 ϩ 0.25 2.00 ϩ 0.75 2.25 8 1 Yes 800 ϫ37° ϫ101° 4 20/32 20/20 −0.25 ϩ 0.25 2.00 ϩ 0.75 2.0 5 24 Yes 800 ϫ71° ϫ101° 5 20/25 20/50 2.75 ϩ 0.50 1.00 ϩ 0.50 1.75 5 12 Yes 400 ϫ11° ϫ64° 6 20/25 20/20 3.25 ϩ 1.00 2.00 ϩ 0.75 1.5 10 2 No 800 ϫ27° ϫ124° 7 20/40 20/160 −12.00 ϩ 2.0 −11.75 ϩ 1.5 0.5 9 12 No 7000† ϫ132° ϫ032° Strabismus‡ 1 20/40 20/20 4 2.25 ϩ 1.25 2.50 ϩ 3.50 1.5 2 2 60 No 800 ϫ88° ϫ95° 2 20/160 20/20 12 1.00 ϩ 0.50 1.75 ϩ 1.00 1.0 4 6 24 No 3500 ϫ118° ϫ71° 3 20/16 20/32 6 2.50 ϩ 0.50 2.50 ϩ 3.50 0.75 3 0.5, 10, 18 24 No 7000† ϫ44° ϫ98° 4 20/25 20/16 6 00 00 0 0.5 ? No 7000† 5 20/25 20/50 8 0.25 ϩ 0.50 0.25 ϩ 0.50 0 10 0 Yes 200 ϫ162° ϫ2° 6 20/25 20/25 0 −1.00 ϩ 1.00 −100 ϩ 0.50 0.25 5 9, 19 1 Yes 200 ϫ171° ϫ15° *00 Indicates no refractive error. †A score of 7000 arc seconds was assigned to subjects who scored incorrect on every trial. ‡All esotropic except for exotropic strabismus in subject 6. monocular and binocular deficits. In this study, McKee and visual resolution. In addition, we introduced a test not in- colleagues proposed that the severity of binocularity loss cluded before, dichoptic contrast masking, so that the de- may sometimes better predict the nature of the physiologi- gree of interocular inhibition could be directly assessed. cal changes in amblyopia than the clinical etiology. In other words, the degree of binocularity may at least partially pre- METHODS dict abnormality on monocular tasks. They compared the Vernier acuity of a binocular group with that of a matched “nonbinocular” group and found a significant difference SUBJECTS (PϽ.001) between them. Similar ideas have been pro- posed in animal models of induced amblyopia.19 This study examined 20 subjects, including 7 control sub- The present study attempts to replicate the conclu- jects, 7 subjects with anisometropia, and 6 subjects with stra- 14 bismus (Table 1); subject groups were matched for mean age sions of the study by McKee et al in a hypothesis-driven (25.1, 28.0, and 26.3 years, respectively) and mean years of edu- manner. We determined whether the results of their study cation (13.7, 14.9, and 13.5 years, respectively). Subjects pro- could be replicated using a more typical moderate num- vided informed consent conforming to the Declaration of Hel- ber of subjects, for whom it is not possible to match sub- sinki (West Virginia University Institutional Review Board jects with binocular and nonbinocular vision according to Protocol 14788). Any subject with known or suspected neu- (REPRINTED) ARCH OPHTHALMOL / VOL 124, JUNE 2006 WWW.ARCHOPHTHALMOL.COM 845 ©2006 American Medical Association. All rights reserved. Downloaded From: https://jamanetwork.com/ on 10/01/2021 rological conditions was excluded from the study. All subjects MONOCULAR TESTS completed an ophthalmologic examination to confirm diagno- sis. For control subjects, dominant and nondominant assign- Snellen Acuity ments were given to the eyes based on acuity measures. Most subjects with amblyopia had a history of patch treatment dur- This test was performed at the West Virginia University Eye ing childhood (ie, a patch was worn to cover the dominant [fel- Center as part of a clinical examination. A standard chart (Lom- low] eye). One subject with exotropic strabismus showed equal bart Instrument, Norfolk, Va) was used. visual acuity in both eyes at the time of testing. Diagnosis of anisometropic amblyopia was made on the ba- Grating Acuity sis of (1) an interocular refractive difference of hyperopia of at least 1.00 diopter (D), astigmatism of at least 1.00 D, or myopia For each trial, the subject saw 2 temporally sequenced screens; of at least –2.50 D or (2) a history of anisometropia but no stra- 1 contained a vertical sinusoidal grating (80% contrast, sub- bismus by clinical alternate cover testing and no history of stra- tending 8° of visual angle).
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