Differential changes of magnocellular and parvocellular visual function in early- and late-onset strabismic amblyopia

PURPOSE: Studies in nonhuman primates show that monocular visual deprivation starting at different ages has different effects on cells in the parvocellular and magnocellular laminae of the lateral geniculate nucleus. The present study used color and luminance contrast sensitivity (CS) measurements to look for differences in parvocellular- and magnocellular-related visual function in human subjects with strabismic amblyopia. METHODS: Fifteen subjects with early- and 14 with late-onset strabismic amblyopia and similar ranges of visual acuity were studied, together with 15 subjects with normal vision. Contrast sensitivities were measured to an equiluminant (L-M cone-modulated) grating with slow onset and an achromatic (L+M cone-modulated) 0.8-cpd grating with rapid onset using an adaptive METHOD: RESULTS: Luminance and color CS were lower in the amblyopic eyes than in the fellow eyes of all amblyopes. For luminance CS, this was due both to an increase in sensitivity of the fellow eye and to a reduction in sensitivity in the amblyopic eye. Color CS was greatly reduced in the amblyopic and fellow eyes of subjects with strabismic amblyopia of early- and late onset compared with subjects with normal vision. The reduction in color CS compared with luminance CS was significantly greater in eyes with late- rather than early-onset amblyopia. CONCLUSIONS: Parvocellular and magnocellular function are differentially affected in the amblyopic and fellow eyes of subjects with strabismic amblyopia. The difference is more marked in late-onset amblyopia than in early-onset amblyopia.     

Acuity of amblyopic children for small field gratings and recognition stimuli

Tests of grating acuity often underestimate amblyopia and underrefer esotropic infants with a fixation preference. To evaluate the effect of the large grating field used in preferential looking (PL) procedures, an eight-alternative, small field (about 1 degrees) grating acuity test was devised. Gratings contained at least eight cycles. Thirty-seven strabismic and/or anisometropic amblyopes, ages 3-13 yr, were tested. In most amblyopic eyes, grating acuities were better than recognition acuities; the difference was reduced, however, in the small field test compared to the large field (6 degrees test (means, 1 oct vs. 1.6 oct; paired-t = 5.5, P less than .001). Nevertheless, the same relation between grating and recognition acuities occurred for small as for large fields: an increased discrepancy between grating and recognition acuities accompanied poorer acuity. This larger discrepancy is attributed to increased probability summation of amblyopic eyes for low spatial frequencies. For preschool children who can be tested by both procedures, the eight-alternative grating acuity test may be preferable to operant PL because it is more easily administered and materials are simpler.     

The contrast sensitivity function and childhood amblyopia

We measured contrast sensitivity function and visual acuity in both eyes of strabismic and anisometropic amblyopic patients. There was a linear relationship between contrast sensitivity function and visual acuity in the amblyopic eye. As visual acuity decreased, the contrast sensitivity function decreased along the contrast sensitivity axis, and peak sensitivity shifted to lower spatial frequencies. After patching therapy, when visual acuity reached 20/20 in each eye, suggesting that the amblyopia was cured, there continued to be statistically significant difference in the contrast sensitivity functions between the eyes. The contrast sensitivity function from the previously amblyopic eye was depressed compared to the nonamblyopic eye. A comparison between patients with strabismic and anisometropic amblyopia showed that, when matched for visual acuity, the contrast sensitivity functions were similar for both the nonamblyopic and amblyopic eyes. However, a large difference was found between the amblyopic and nonamblyopic eyes of each group.     

Effects of luminance on the visual acuity of strabismic and anisometropic amblyopes and optically blurred normals

Evidence is accumulating to suggest fundamental differences between strabismic and anisometropic amblyopia. We explored differences in these amblyopes by assessing the effects of luminance on optotype acuity, using targets that ranged from photopic to low mesopic levels. Our results show that reducing luminance has less of a detrimental effect on the acuity of strabismic than anisometropic amblyopic eyes. Furthermore, the effect of decreasing luminance on the acuity of optically blurred normal eyes mimicked the effect we found for anisometropic amblyopic eyes. These results are consistent with the hypothesis that the fundamental deficit in anisometropic amblyopia is impaired resolution while in strabismic amblyopia the fundamental deficit is impaired spatial directionalization.     

Thickness of the retinal nerve fiber layer in amblyopic and normal eyes: a scanning laser polarimetry study.

PURPOSE: To compare scanning laser polarimeter (GDx, Laser Diagnostic Technologies, San Diego, Calif) measurements of the peripapillar retinal nerve fiber layer in amblyopic and normal eyes. METHODS: Scanning laser polarimetry was performed on 21 patients with unilateral strabismic amblyopia who had an absence of neurologic diseases or glaucoma and a minimum age of 7 years. A mean retardation map was calculated from separate scans or was considered to be the best scan obtained for each eye. Polarimetric indices were analyzed comparing amblyopic and contralateral normal eyes. RESULTS: The mean age was 15 +/- 9 years (7-35 years) and the male:female ratio was 13:8. There were 6 right and 15 left amblyopic eyes, with the amblyopic group having a mean visual acuity of 0.3 +/- 0.1. The mean (+/- SD) indices did not differ significantly between normal and amblyopic eyes, except the number that summates information from the individual parameters, which was higher in normal (20.71 +/- 11.98) than in amblyopic (15.14 +/- 6.81) eyes, P =.02. CONCLUSION: There was no statistical difference in thickness of the nerve fiber layer between amblyopic and normal eyes. A previous study found similar results in adults with strabismic amblyopia.     

Optic Disc Area and Retinal Area in Amblyopia

Purpose: To compare retinal area to optic disc rim area ratios of hyperopic normal, strabismic eyes with equal acuity, amblyopic, and fellow eyes. Methods: Neuroretinal rim areas of 293 amblyopic and fellow eyes, and 77 non-amblyopic hyperopic right eyes, and 84 non-amblyopic strabismic right eyes were measured by magnification corrected retinal photography and planimetry. Retinal area estimates were based on axial lengths. Inclusion criteria: All subjects had bilateral hyperopia. Patients with glaucoma or known optic nerve atrophy were excluded. The normal and strabismic groups had equal visual acuity in each eye better than 20/40. The amblyopic group had acuity worse than 20/40 in one eye uncorrectable with lenses and without gross anatomic defects. Results: The amblyopic group included 137 with strabismus and 89 with anisometropia exceeding 1.5 diopters. There were highly significant differences between the ratio of retinal area to optic disc rim area of the amblyopic and of normal eyes (unpaired t-test, p = 8.6 × 10^-6), the amblyopic and strabismic right eyes (unpairedt?test, p = 4.22 × 10^-8) as well between the fellow and amblyopic eyes (pairedt?test, p = 2.13 × 10^-5). The difference between the normal and strabismic eyes without amblyopia was not significant (p = 0.82). There was a 20 percent increase in the retinal receptor areas of hyperopic amblyopic eyes as compared to hyperopic eyes without amblyopia despite reduced retinal areas in the amblyopic eyes. Dysplastic and/or asymmetric optic discs were present in 163 of 293 (56 percent) amblyopic patients, 47 of 84 (56 percent) strabismic, and 10 of 77 (13 percent) normal patients. Conclusions: The increase in the receptor area may be an explanation for diminished acuity and impaired visual function in amblyopic eyes.     

Visually evoked response (VER) in amblyopia: A diagnostic and prognostic indicator?

A study comprising 15 cases of anisometropic amblyopia and 15 cases of strabismic amblyopia was undertaken to analyse the alterations in the visually evoked response (VER) in amblyopia. Ten normal cases were taken to standardize the technique. A Nicolet Clinical Averager CA₁₀₀ was used to record the VER. A full ophthalmic check up including cycloplegic refraction, visual acuity recording and fixation pattern by visuscope was carried out in all the cases.

In normal cases the amplitude and implicit time of VER showed no statistically significant difference between the two eyes. However, a wide inter-subject variability was observed. the maximum amplitude was seen with a check size of 60′. In amblyopic eyes (both strabismic and anisometropic) there was a significant reduction in the amplitude and a significant increase in the implicit time when compared to the normal fellow eye. A significant amplitude difference of more than 5 μV, as also a significant increase of implicit time (more than 10 msec) was noticed only when the visual acuity was below 6/18. Recording of VER in amblyopic eyes with visual acuity less than 6/60 mostly resulted in no discernible pattern. A non-recordable pattern on VER in amblyopic eyes signifies a poor visual acuity and possibly suggests a bad prognosis for the treatment of amblyopia.     

Defective processing of motion-defined form in the fellow eye of patients with unilateral amblyopia.

The following three measurements were made on a group of 20 pediatric and 5 adult patients with unilateral amblyopia: (1) speed threshold for recognizing motion-defined dotted letters; (2) recognition acuity for isolated solid letters of 4% contrast; and (3) Snellen line acuity for high-contrast letters. Normal limits were established with a group of 30 pediatric and 10 adult control subjects. The main finding was that, in amblyopic children, a high percentage (83%, 15 of 18) of fellow eyes showed a degraded ability to recognize motion-defined letters, even though Snellen acuity and 4% letter acuity were normal for age. The fellow eyes of all nine patients with strabismic amblyopia showed this pattern of loss, as did four of six fellow eyes of patients with anisometropic amblyopia and two of three fellow eyes of patients with anisometropic plus strabismic amblyopia. Only two clinically unaffected eyes were normal for motion-defined letters. These eyes belonged to patients with anisometropic amblyopia. Eighteen of the 19 previously amblyopic eyes tested were abnormal for motion-defined letters even though Snellen acuity was within normal limits for 6 of these eyes. In adults, only one of five fellow eyes failed the motion-defined letter test. It was concluded that the degradation of form perception associated with amblyopia can be different for luminance-defined and motion-defined form and that defective processing of motion-defined form is common in the fellow eyes of children with unilateral amblyopia.     

Pupillary function in human amblyopia

Quantitative measurements of pupillary function (response amplitude and latency) were made for normal eyes and For normal and fellow amblyopic eyes of groups of strabismic and anisometropic amblyopes. Stimuli consisted of luminance modulation of a large, evenly lit area (pupil light reflex) as well as contrast modulation of sinusoidal gratings (pupil grating response) of fixed, space-averaged luminance. Measurements were made of the direct and the consensual reflex under monocular stimulation. A comparison of the amplitude of the pupil light reflex as a function of luminance modulation showed no significant differences between normal and fellow amblyopic eyes for both the strabismic and anisometropic groups of amblyopes studied. A similar comparison of the associated response latencies showed significant difference between normal and fellow amblyopic eyes for both groups. In general, reductions in response amplitude and latency of (he pupil grating response were found in individuals from each group when comparing the good and the affected eyes, although the observed group differences were only significant in the strabismic group. Interestingly, statistically significant reductions in both amplitude and latency for both the pupil light reflex and the pupil grating response were found between the eyes of normal observers and the so-called normal eyes of amblyopes in both groups studied. These results suggest that the type of pupillary deficit in amblyopia is a complicated one, depending not only on the type of amblyopia (strabismic or anisometropic) and the type of stimulus employed (light or pattern), but also on the parameter assessed (amplitude or latency) and whether the amblyopic result is referenced to its fellow normal eye or to the normal eye of a non-amblyopic observer- Since the pupil response to light flux changes is not mediated exclusively via the retinal projection to the midbrain and may also involve the activity of central visual pathways, the results obtained in this study cannot be used to provide definitive evidence for the site of abnormality in amblyopia.     

屈光不正性、屈光参差性及斜视性弱视的位置辨别比较

目的比较屈光不正性、屈光参差性及斜视性弱视患者的位置辨别功能差异以及单眼弱视双眼间位置辨别差异。方法选取在广西视光中心就诊的139例(237眼)弱视患者,按弱视类型分为屈光不正性弱视组40例(80眼)、屈光参差性弱视组61例(82眼)、斜视性弱视组38例(75眼),检测3组弱视位置辨别功能并进行比较分析。结果 3组弱视位置辨别功能检测结果均集中在2～4级之间,中位数均为3级,3组位置辨别功能结果比较,差异无统计学意义(Hc=0.530,P>0.05)。单眼弱视双眼位置辨别功能比较,差异无统计学意义(MH=1.837,P>0.05)。单眼弱视眼与双眼弱视眼位置辨别功能比较,差异无统计学意义(Z=-0.588,P>0.05)。结论屈光不正性、屈光参差性及斜视性弱视儿童位置辨别功能不存在差异,单眼弱视双眼位置辨别功能无差异,单眼弱视眼与双眼弱视眼位置辨别功能相似。