Fixational eye movements in amblyopia and strabismus

Horizontal eye position was monitored using a photoelectric method during monocular and binocular fixation in four patients having amblyopia without strabismus, thirteen patients having constant strabismus with amblyopia, and five patients having intermittent strabismus. Four abnormalities of fixation were found: increased drift, saccadic intrusions, manifest nystagmus, and latent nystagmus. Increased drift was related to the presence of amblyopia, while saccadic intrusions and nystagmus were related to the presence of strabismus. Understanding dynamic aspects of oculomotor control can provide insight into clinical assessment of fixation in amblyopia and strabismus.     

Abnormal saccadic substitution during small-amplitude pursuit tracking in amblyopic eyes.

Small-amplitude, low-velocity, predictable triangular tracking was tested in patients having amblyopia without strabismus, intermittent strabismus, or constant strabismus amblyopia by means of a photoelectric eye-movement recording technique. In the majority of amblyopic patients, abnormal saccadic substitution was found; that is, abnormally large saccades rather than small-amplitude smooth movements were used by the amblyopic eye to follow a spot stimulus that moved horizontally with low to high frequencies. Pursuit for the same range of stimuli was normal for binocular tracking and for monocular tracking with the dominant eye, pointing to a sensory rather than motor basis for the defect. This abnormal saccadic substitution response appeared to be related to the presence of amblyopia rather than strabismus. Several possible mechanisms responsible for causing this unusual response are discussed, including impairment of direction sense over small central regions of the amblyopic eye.     

Normal pattern electroretinograms in amblyopia

Checkerboard reversal stimuli were used to evoke transient pattern electroretinograms (P-ERGs) from the eyes of 14 patients with amblyopia and 14 normal subjects. In the control group, and in normal eyes of patients, pattern electroretinograms were obtained with monocular central fixation. Amblyopic eyes were examined by monocular and binocular fixation, and the fixation point was shifted horizontally and/or vertically until the P-ERG reached its maximal amplitude. After adjusting visual fixation, there were no significant differences in amplitude between the normal and the amblyopic eyes. In addition, the differences between both eyes were compared with the right-left eye variability of the 14 normal subjects. In the amblyopic eyes, no P-ERG abnormality was observed. These results do not support previous reports of reduced P-ERG amplitudes and are in agreement with recent findings obtained under steady-state conditions.     

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。     

Eye position stability in amblyopia and in normal binocular vision

Purpose. We investigated whether the sensory impairments of amblyopia are associated with a decrease in eye position stability (PS). Methods. The positions of both eyes were recorded simultaneously in three viewing conditions: binocular, monocular fellow eye viewing (right eye for controls), and monocular amblyopic eye viewing (left eye for controls). For monocular conditions, movements of the covered eye were also recorded (open-loop testing). Bivariate contour ellipses (BCEAs), representing the region over which eye positions were found 68.2% of the time, were calculated and normalized by log transformation. Results. For controls, there were no differences between eyes. Binocular PS (log(10)BCEA = -0.88) was better than monocular PS (log(10)BCEA = -0.59) indicating binocular summation, and the PS of the viewing eye was better than that of the covered eye (log(10)BCEA = -0.33). For patients, the amblyopic eye exhibited a significant decrease in PS during amblyopic eye (log(10)BCEA = -0.20), fellow eye (log(10)BCEA = 0.0004), and binocular (log(10)BCEA = -0.44) viewing. The PS of the fellow eye depended on viewing condition: it was comparable to controls during binocular (log(10)BCEA = -0.77) and fellow eye viewing (log(10)BCEA = -0.52), but it decreased during amblyopic eye viewing (log(10)BCEA = 0.08). Patients exhibited binocular summation during fellow eye viewing, but not during amblyopic eye viewing. Decrease in PS in patients was mainly due to slow eye drifts. Conclusions. Deficits in spatiotemporal vision in amblyopia are associated with poor PS. PS of amblyopic and fellow eyes is differentially affected depending on viewing condition.     

Binocular interactions in normal and anomalous binocular vision

Binocular interactions for grating patterns were investigated in humans with normal binocular vision and in humans with abnormal binocular visual experience due to strabismus and/or amblyopia via 1) comparison of monocular and binocular contrast thresholds; 2) interocular transfer of the threshold elevation aftereffect; and 3) dichoptic masking. Whereas the normal observers showed improved binocular over monocular contrast sensitivity (i.e., binocular summation) and substantial interocular transfer of the threshold elevation aftereffect, the abnormal observers showed an absence of binocular summation and no significant interocular transfer. The dichoptic masking experiments showed that a suprathreshold masking grating presented to one eye elevated the contrast threshold for gratings presented to the fellow eye, within a narrow range of spatial frequencies (about 1 octave wide at half height) and orientations, centered about the spatial frequency and orientation of the mask. The magnitude and bandwidth of this masking effect was similar in subjects with normal and abnormal binocular vision, occurring even when the masking grating was presented to the amblyopic eye. These effects depend upon the contrast of the masking grating. In individuals with normal binocular vision, a grating with subthreshold contrast presented to one eye reduces the contrast threshold for detection of gratings of similar spatial frequency and orientation presented to the fellow eye. No such subthreshold summation is evident in the amblyopic observers. We conclude that while strabismus and/or amblyopia disrupted the normal excitatory interactions between the two eyes, cortical inhibitory binocular connections were not disrupted.     

Facilitation of vertical vergence by horizontal saccades, found in a patient with dissociated vertical deviation

The authors examined vertical vergence in a 15-year-old girl with dissociated vertical deviation, a 6d` convergent strabismus, no binocular vision, latent nystagmus, and a minimal left amblyopia. Eye movements were recorded during 4s-periods of (1) both eyes open, alternated with 4s-periods of (2a) right eye covered, (2b) left eye covered or (2c) both eyes closed. The patient preferred fixation with the right eye; when this eye was covered, the left eye took over fixation with an almost completely monocular, downward saccade (a horizontal saccade occurred at the same time). The right eye only made a very small saccade, and then started to drift upward with an exponentially decreasing velocity. Enright has shown that fast convergence or divergence may occur when a horizontal saccade is performed at the same time. Similarly, the authors suspected that, in their patient, vertical vergence was facilitated by the horizontal saccade that resulted, in this case of horizontal strabismus, from alternation of fixation. To examine whether this was the case, they compensated for the horizontal strabismus by placing a 30 prism-dioptre base-out prism in front of the left eye (the patient had no binocular vision). With the prism in place, the horizontal saccade that occurred when the right eye was covered was smaller, and only little vertical vergence occurred together with the horizontal saccade. It seems possible that vertical vergence is also facilitated by horizontal saccades.     

Multifocal VEP Difference between Early- and Late-onset Strabismus Amblyopia

Purpose: To measure and compare the multifocal visual evoked potentials (mfVEP) difference between early- and late-onset strabismus amblyopia. Methods: Eleven patients with early- and 11 with late-onset strabismus amblyopia with similar range of visual acuity and 11 normal control subjects were recorded with mfVEP. Results: There was no significant difference in latency or amplitude between amblyopic and fellow eyes for the early-onset amblyopic group, whereas in the late-onset amblyopic group, latencies were significantly prolonged and amplitudes were attenuated in the central region of visual field in the amblyopic eye. The responses in the central region of visual field in both amblyopic and fellow eyes of the early-onset amblyopic group were of longer latency and smaller amplitude than normal. In the late-onset group, the responses from amblyopic eye were of increased latency and decreased amplitude in the central visual field compared with normal, but either latency or amplitude from fellow eye did not differ from normal significantly. Conclusions: There is significant difference in the mfVEP characteristics of patients with early- and late-onset strabismus amblyopia.     

儿童弱视与屈光不正

本文名分析了3099例2.5～14岁儿童的屈光状态.资料表明:非斜视者的屈光不正随着年龄增长,远视的发生和程度逐渐减少而近视的发生和程度逐渐增加。双眼屈光不正性弱视者,中、量度弱视的高、中度远视和近视比轻度弱视者多.单跟屈光不正性弱视者,高、中度远视和高度近视比非弱视眼的多.双眼内斜视性弱视的不同年龄和不同程度弱视的屈光分布无显著差异.单眼斜视性弱视的屈光分布与非弱视眼比,无显著差异.本文结果揭示屈光不正性弱视与高度屈光不正有关;眼位偏斜可能为斜视性弱视的主要原因.     

Profile of amblyopia in a hospital referral practice

OBJECTIVE: Evaluation of the clinical profile and distribution of different sub-types of amblyopia in a referral eye hospital in India. METHODS: This was a prospective hospital-based observational study, evaluating the clinical profile of patients with amblyopia presenting to a referral strabismology practice. The examination included assessment of the visual acuity, the refractive status, the fixation pattern, the grade of binocularity, and evaluation of the associated strabismus, if any before treatment was started. RESULTS: The average age of the patients at presentation was 7.97?6.18 years with 81 out of 683 patients (11.8%) presenting above the age of 20 years. The best-corrected visual acuity (BCVA) was less than 6/60 in the amblyopic eye in 121 out of 733 eyes (16.5%). Strabismic amblyopia was the most common sub-type of amblyopia seen (274/733 eyes, 37.38%). Though patients with anisometropic amblyopia presented at a later age (average of 10.03+/-6.92 years), they had better visual acuity, binocular functions, and centricity of fixation at all ages, relative to other sub-types of amblyopia. The BCVA did not show any co-relation with the age of presentation [co-relation co-efficient (CF) of 0.074], refractive status of the amblyopic eye (CF of 0.078), the type of amblyopia (CF of 0.196), or the type of strabismus present (CF of 0.079). However, a very significant co-relation was seen between the BCVA and the fixation pattern of the amblyopic eye (CF of 0.817). CONCLUSIONS: Lack of knowledge and awareness about amblyopia and its appropriate timely management has been the main cause for the late presentations and significant visual impairment associated with the condition.     

Experience-expectant development of contour integration mechanisms in human visual cortex

Extended contours are a common feature of natural images. Most previous studies have considered contour integration as a two-dimensional process of linking like-oriented elements along their common orientation axis. Yet contours exist in a three-dimensional world, and one might therefore ask about the relationship between contour integration and binocular vision. Using an event-related potential assay of contour integration, we demonstrate that patients with strabismic amblyopia show a relative insensitivity to Gabor-defined contours in their dominant eyes, all of which had normal acuity. These deficits were not seen in the dominant eyes of patients with anisometropic amblyopia without strabismus, but were present in the amblyopic eyes of patients with either strabismus or anisometropia. Deficits were also found in both eyes of strabismus patients who had normal visual acuity in each eye, but who had strongly reduced or absent stereopsis. These results suggest that the maturation of contour detection mechanisms depends at least in part on the presence of normal binocular interaction during a developmental critical period.     

Listing's law in strabismus and amblyopia: a preliminary report

We investigated whether Listing's law applies in patients with diminished or no stereopsis due to strabismus or amblyopia. Eye movements of normal subjects and patients with strabismus and/or amblyopia were recorded during monocular and binocular fixation; from these data the shape and relative orientation of displacement planes were calculated. In normal subjects, monocular or binocular fixation did not influence the thickness and relative orientation of displacement planes. No differences were found between normals and the patient with amblyopia due to anisometropia. In one patient with strabismus but without amblyopia, the orientation of displacement planes depended on the fixation conditions; a coupling between horizontal vergence effort and plane orientation was observed. Patients with amblyopia and strabismus showed abnormally shaped and/or abnormally orientated displacement planes, which depended on the fixation conditions. Differences between both eyes in the shape of the planes were also observed. These results show that normal Listing behavior can be present in subjects with diminished stereopsis. They also show that normal stereopsis does not necessarily mean normal Listing behavior, suggesting that Listing's law is mainly a result of motor strategy.     

Listing's law in strabismus and amblyopia: a preliminary report

We investigated whether Listing's law applies in patients with diminished or no stereopsis due to strabismus or amblyopia. Eye movements of normal subjects and patients with strabismus and/or amblyopia were recorded during monocular and binocular fixation; from these data the shape and relative orientation of displacement planes were calculated. In normal subjects, monocular or binocular fixation did not influence the thickness and relative orientation of displacement planes. No differences were found between normals and the patient with amblyopia due to anisometropia. In one patient with strabismus but without amblyopia, the orientation of displacement planes depended on the fixation conditions; a coupling between horizontal vergence effort and plane orientation was observed. Patients with amblyopia and strabismus showed abnormally shaped and/or abnormally orientated displacement planes, which depended on the fixation conditions. Differences between both eyes in the shape of the planes were also observed. These results show that normal Listing behavior can be present in subjects with diminished stereopsis. They also show that normal stereopsis does not necessarily mean normal Listing behavior, suggesting that Listing's law is mainly a result of motor strategy.     

转换注视眼治疗分离垂直性偏斜的疗效观察

目的 探讨转换注视眼方法治疗单眼分离垂直性偏斜(DVD)或双眼不对称性DVD患者上斜视的效果。方法 收集弱视眼矫正视力≥0．5的儿童DVD患者23例(9例为单眼,14例为双眼),采用戴镜后遮盖和光学方法使原注视眼视力低于现注视眼视力2或3行(国际标准视力表),以达到转换注视眼的目的。观察治疗前、后患者的视力、眼位及视网膜对应情况。结果 采用转换注视眼方法治疗后,原注视眼视力无明显下降,现注视眼视力提高2行者10例;19例患者眼位达到外观美容效果,4例双眼上斜视度数相差不明显(≤10^△)患者原注视眼出现上飘现象;3例近距离注视具有正常视网膜对应、远距离注视表现为高位眼抑制的患者恢复正常视网膜对应。结论 转换注视眼方法可有效矫正DVD患者的上斜视症状,尤其对于单眼DVD和双眼不对称性DVD一侧眼为上斜视、对侧眼为隐性上斜视者效果较好。本方法适用于不愿接受手术治疗、且合并轻中度弱视和屈光不正的儿童DVD患者。     

Binocular coordination of saccades in children with strabismus before and after surgery

PURPOSE: To examine the quality of binocular coordination of saccades in children with various types of strabismus and the effect of strabismus surgery. METHODS: Eight subjects were tested (5-15 years old): five with convergent strabismus, three with divergent strabismus. A standard saccade paradigm was used to elicit horizontal saccades to target LEDs (5 degrees to 15 degrees ). Saccades from both eyes were recorded simultaneously with the photograph-electric Skalar IRIS device (Delft, The Netherlands). This task was run before and about 3 weeks after strabismus surgery. RESULTS: Before surgery, the difference in the amplitude of the saccade between the left eye and the right eye was larger (15% of the saccade size) than in normal children of similar age. After strabismus surgery for all subjects the squint angle was reduced, and the amplitude of the disconjugacy of saccades decreased significantly, dropping to normal values (6%). As in normal children, postsaccadic eye drift (both its conjugate and its disconjugate components) was small in amplitude. The difference compared with normal subjects was that disconjugate drift did not restore the disconjugacy of the saccade itself (e.g., in normal subjects drift is convergent when saccade disconjugacy is divergent and vice versa). Rather, disconjugate drift tended to drive the eyes toward static eye misalignment (e.g., the drift was mostly convergent for convergent strabismics and divergent for divergent strabismics). Surgery had no significant effect on either component of the drift. CONCLUSIONS: The improvement of the binocular coordination of the saccades could be due, at least partially, to central adaptive mechanisms rendered possible by surgical realignment of the eyes. Separate mechanisms control the binocular coordination of saccades and the alignment of the eyes during the postsaccadic fixation period.     

Clinical suppression and amblyopia

In individuals with abnormal binocular vision, such as strabismics and anisometropes, it is common for all or part of one eye's view to be suppressed so binocular confusion and diplopia are eliminated. We examined the relation between the depth of suppression (the amount by which the monocular contrast increment threshold for an eye was elevated by stimulation in the contralateral eye) and the degree of amblyopia (difference in monocular contrast thresholds for the two eyes). There was a significant negative correlation between suppression and amblyopia, so that clinical suppressors with no amblyopia exhibited deep suppression (ie, large threshold elevation) while observers with amblyopia exhibited weaker or no suppression. This negative correlation was found when the two eyes viewed orthogonally oriented contours as well as identically oriented contours. These results suggest that when an eye is amblyopic there is no longer a need for strong suppression of that eye by the contralateral eye.     

Differences in the neural basis of human amblyopia: the distribution of the anomaly across the visual field

Spatio-temporal contrast sensitivities to horizontally-oriented Gaussian-weighted patches of sinusoidal grating stimuli were determined across the nasal and temporal visual fields of strabismic and non-strabismic, anisometropic amblyopes. The visual field distribution of the amblyopic anomaly differs in strabismic and non-strabismic, anisometropic eyes. In strabismus the peripheral region of one or both hemifields is spared; in non-strabismic, anisometropic cases the loss is evenly distributed across the binocular visual field but is not present in the monocular temporal field. These findings suggest that the non-strabismic forms of amblyopia in humans result from binocular competitive imbalance in early life. The strabismic results pose two problems for the present competitive model of amblyopia: in strabismus, amblyopia is mainly limited to central vision and shows an asymmetric distribution.     

基本治愈弱视眼视觉诱发电位的波幅及时值分析

目的 :探讨基本治愈的单眼弱视患儿的弱视眼与对侧优势眼视觉诱发电位 (P VEP)波幅及潜伏期时值的差异 ,提出评价弱视基本治愈的客观检测标准。方法 :对不同弱视程度 (轻度、中度、重度 )和不同病因 (内斜视、外斜视、上斜视、屈光参差 )的 39例基本治愈的单眼弱视患儿进行双眼P VEP检查 ,记录其波幅及时值 ,以对侧优势眼作为对照组 ,对波幅及时值进行统计学分析。结果 :按弱视程度分类的弱视组P VEP波幅及时值与对照组间 (优势眼 )的差异均有显著性 ,即弱视组振幅降低 ,潜伏期时值延长 ;按弱视病因分类的弱视组P VEP波幅及时值与对照组间差异亦有显著性。结论 :P VEP可作为判断弱视治愈的标准之一 ,对于单眼弱视的患儿可将弱视眼与优势眼进行对比 ,在一定程度上判断其视功能的恢复程度。     

A directional impairment of eye movement control in strabismus amblyopia.

Saccadic and pursuit tracking movements were elicited to determine the ability of the amblyopic eye to sense and respond to position and motion of the retinal image. Amblyopic eyes were found to initiate saccades as rapidly as normal eyes (200 to 300 msec.), however, the amblyopic eye's saccades were reduced in amplitude, highly variable, and required refinement by subsequent saccades, particularly in response to nasalward displacements of the retinal image. Pursuit responses of amblyopic eyes to both constant and sinusoidal velocity targets contained brief periods of abnormally slow following movements interrupted by position-corrective saccades. As with the saccadic response, the amblyopic eye's pursuit movements were more accurate for temporal than for nasal retinal image motion. Abnormal saccadic and pursuit eye movements in amblyopia result from reduced monocular position and velocity sensitivity probably associated with binocular suppression scotomas normally found in amblyopia.     

Monocular geometry is selectively distorted in the central visual field of strabismic amblyopes

Strabismic amblyopia is associated with a distorted perception of visual space. The aim of our study was to investigate the monocular space perception of strabismic observers at several locations in the central and peripheral visual field. We tested nine observers with strabismic and/or anisometropic amblyopia, two strabismic subjects with alternating fixation and two normal control subjects. The task was to align a light stimulus with two vertically arranged reference marks. Testing conditions included three separations of the references along the vertical meridian (10 degrees, 20 degrees and 40 degrees) as well as several presentation sites of the vertical references in the nasal and temporal peripheral visual field (5 degrees, 10 degrees and 20 degrees from fixation). Performance with the amblyopic eye was clearly impaired as compared to the nonamblyopic eye. For alignment along the vertical meridian, all amblyopic eyes showed increased uncertainty in their position judgements. Most of the squinting eyes of amblyopes also displayed a systematic lateral displacement of the test stimulus in relation to the reference marks, in the most extreme case up to almost 7 degrees. Usually, larger errors were found with wider separations of the reference marks. In the peripheral field, the differences between the amblyopic and the nonamblyopic eye diminished or disappeared. Thus, monocular geometry appears to be selectively impaired in the central visual field of the deviated eye of strabismic amblyopes. These spatial distortions might be related to the different states of binocular correspondence in the central vs. peripheral visual field, shown by some strabismic amblyopes.