November 1999
Volume 40, Issue 12
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Eye Movements, Strabismus, Amblyopia and Neuro-ophthalmology  |   November 1999
Functional Visual Loss in Amblyopia and the Effect of Occlusion Therapy
Author Affiliations
  • Anita J. Simmers
    From the Visual Perception Unit, Department of Psychology, University of Essex, Wivenhoe Park, Colchester, CO4 3SQ; the
  • Lyle S. Gray
    Department of Vision Sciences, Glasgow Caledonian University, Cowcaddens Road, Glasgow, G4 0BA; and the
  • Paul V. McGraw
    Department of Optometry, University of Bradford, Richmond Road, Bradford, West Yorkshire, BD7 1DP.
  • Barry Winn
    Department of Optometry, University of Bradford, Richmond Road, Bradford, West Yorkshire, BD7 1DP.
Investigative Ophthalmology & Visual Science November 1999, Vol.40, 2859-2871. doi:
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      Anita J. Simmers, Lyle S. Gray, Paul V. McGraw, Barry Winn; Functional Visual Loss in Amblyopia and the Effect of Occlusion Therapy. Invest. Ophthalmol. Vis. Sci. 1999;40(12):2859-2871.

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Abstract

purpose. The aim of this study was to define the nature of functional visual loss in amblyopia and to identify those subjects whose amblyopia is chiefly due to one or more of the following deficits: abnormal contour interaction, abnormal eye movements, abnormal contrast perception, or positional uncertainty.

methods. Fifty amblyopic children with a mean age of 5.6 ± 1.3 years were referred from diverse sources. In addition to routine orthoptic and optometric evaluation the principal visual deficits in the amblyopic eye of each subject were identified using the following measures of visual acuity: high contrast linear, single optotype, repeat letter and low contrast linear, plus Vernier and displacement thresholds. These measures were repeated as the children underwent a prescribed occlusion therapy regime, after parental consent.

results. All amblyopic subjects demonstrated a functional loss in each of the tests used, and occlusion therapy appeared to improve all aspects of the amblyopia. High contrast visual acuity was not always the primary deficit in visual function, and when amblyopic subjects were divided according to their primary visual loss, this visual function was found to show the greatest improvement with treatment.

conclusions. These results suggest that to successfully identify the primary visual deficit and monitor the success of occlusion therapy it is necessary to assess other aspects of visual function in amblyopia.

Previous investigations into amblyopia treatment often contain methodological difficulties, such as small subject numbers or inadequate measurement of sensory function and motor function, 1 2 3 4 5 which prevents accurate conclusions being drawn. Although there have been extensive psychophysical and electrophysical investigations (see Ciuffreda et al. 1991) 6 into both human and animal models of amblyopia, there is still no conclusive evidence as to why some amblyopic subjects respond to treatment and others do not. 1 2 3 4 5  
The associated conditions thought to be responsible for the amblyopia (anisometropia, strabismus, refractive error, or deprivation) are normally used to classify the condition. The use of the apparent causal agent in classifying amblyopia is convenient and leads to immediate corrective action but may not be of value in assessing the multifaceted loss in visual function. The current underlying hypothesis in the treatment of amblyopia is that therapy treats the whole condition, assuming all aspects of visual function are equally reduced, and that the success of amblyopia therapy is most often defined in terms of improvement in high contrast visual acuity, which only measures one aspect of visual function. 7 8 Active therapies for amblyopia treatment, have been used over the years by clinicians and are designed to stimulate a specific visual function, either accommodation, eye movements and fixation pattern, or form recognition in an attempt to eliminate what is thought to be the underlying cause of reduced vision. 9 A full review of these training procedures is not in the scope of this article; for more detailed treatment of this area other sources should be consulted. 6 10  
If the success of amblyopia therapy is to be evaluated it is essential to quantify and monitor the loss in visual function with a range of sensory tests and not by the use of high contrast visual acuity alone. The purpose of this study was to try and define the nature of functional visual loss in amblyopia and to identify those children whose amblyopia is chiefly due to one or more of the following deficits: abnormal contour interaction, 11 12 13 which can be assessed using the difference between linear and single optotype acuity 14 15 16 ; abnormal contrast perception, 12 17 which can be assessed using the difference between a high and low contrast linear acuity measure 14 15 16 ; abnormal eye movements, 18 19 which can be assessed using the difference between a high contrast and repeat letter format acuity measure 14 15 16 ; and positional uncertainty, 20 21 which is assessed by hyperacuity measurements. 
Amblyopic subjects can be assigned to these groups using a battery of simple clinical and psychophysical tests of visual function, 14 15 16 the following tests were included in the study: LogMAR Crowded Acuity Test (CAT), Single Letter Acuity Cards (SLA), Low Contrast Glasgow Acuity Cards (LC), Regan Repeat Letter Acuity Cards (RRL), and Alignment and Displacement Thresholds (AT, OMDT). 
These tests will divide the amblyopic subjects into the four functionally based categories outlined above and quantify the different aspects of functional visual loss in the major amblyopic groupings. 
The aims of these investigations were as follows: to determine the functional characteristics of amblyopia, to ascertain the pattern of recovery of each characteristic during therapy, and to relate each characteristic and its recovery pattern to the classic diagnostic categories. 
Methods
Subjects
Referral of children suitable to participate in the study was invited from various sources (school screening, General Practitioners, optometrists, and health visitors) in and around the greater Glasgow area. All experimental procedures conformed to the recommendations of the Declaration of Helsinki. The study was conducted jointly in the Eye Clinic at Glasgow Caledonian University and the pediatric clinic at Glasgow Eye Infirmary. Subsequently, 50 amblyopic children were referred from diverse sources to provide a prescreened subject base. Amblyopia for the purpose of this study was defined as a corrected acuity of 6/9 or poorer in the amblyopic eye. The mean age of the sample was 5.6 ± 1.3 years. Many of these subjects had not received treatment previously and provided an opportunity to investigate the condition before and during therapy. 
Clinical diagnosis revealed an amblyopic population that consisted of 16 strabismic children, 20 anisometropic children, and 14 children with a combination of both anisometropic and strabismic amblyopia. Anisometropia was defined as an interocular difference of greater than 1.00 diopter sphere or 1.0 diopters of cylinder. Clinical details of all subjects can be found in Table 1
Procedure
In addition to routine orthoptic and optometric evaluation, the tests outlined above were performed to identify the principal visual deficit/s in the amblyopic eye of each subject. For a full description of these tests, see Simmers and Gray 1999. 16  
All letter charts were based on a six-alternative forced choice procedure. The logarithmic scale used in all charts provides a constant change in resolution between each acuity level of 0.1 log units. In addition the use of a logarithmic scale allows the facility to extend the scale in either direction, by increasing or reducing the distance. A modified logMAR scoring system 22 was used, with four letters presented at each acuity level. The system designates 6/6 a score of 1 and 6/60 a score of 0 with visual acuities of less than 6/60 carrying a negative sign (1 − logMAR). Because the angular size of the letters changed by a factor of 0.1 log unit, and because all the letters were equally legible, each letter was scored individually (0.025 log units). The end point was defined when no letters could be identified at a particular acuity level, with the subject being encouraged to guess until this point was reached. This equated the scoring systems between all acuity cards. 
The order of presentation of the test charts and hyperacuity measures was assigned randomly, and all acuity tests were externally illuminated to 300 lux. The same test room was used for each visit. Younger children performed the acuity tests as letter matching tests using an appropriate key card, in older children verbal responses were pursued. Test session duration was approximately 40 minutes. 
From the results of these investigations and after a full refraction, each subject was prescribed occlusion therapy based on the management plan outlined below. 
Type of Amblyopia Present.
Ametropic and meridional amblyopias are usually bilateral, therefore early and accurate correction of refractive error is very important. Alternate occlusion was prescribed if residual amblyopia was present. 
Strabismic or anisometropic amblyopia, correction of any refractive error, and then occlusion prescribed according to degree of amblyopia and state of binocular single vision (BSV). 
Degree of Amblyopia.
High contrast linear acuity 6/18 or worse was defined as full-time (6 hours per day), high contrast linear acuity 6/12 or better as part-time (3–4 hours per day). 
Presence of BSV.
If BSV was constant or intermittent then occlusion was always part-time. If microtropia was present then part-time occlusion was also deemed necessary. 
Baseline data collection were taken with the subjects wearing their full correction but before any occlusion therapy was prescribed. Each subject then received a full orthoptic examination, including all five acuity tests, once a month during the course of treatment prescribed. 
Measurement of compliance with the occlusion regime prescribed was undertaken using a diary system, in which the parents logged the number of hours’ occlusion achieved each day and the visual tasks undertaken. 
Results
For ease of interpretation the results of this study have been divided into pre- and posttherapy. 
Pretherapy
The mean values for initial visual acuity scores with the different acuity cards are summarized below in Table 2 . A two-factor ANOVA was carried out for the factors of the amblyopic subject group (anisometrope versus strabismic versus anisometrope + strabismic) and acuity card (CAT versus SLA versus LC versus RRL). The mean visual acuities obtained using the various acuity cards were found to be significantly different (P = 0.003). No significant difference was found in the visual acuity scores between the amblyopic subject groups (P = 0.99), and no significant interaction was revealed between subject group and acuity card (P = 0.1). 
Ten children failed to complete either hyperacuity measure due to a lack of comprehension or a short attention span. No significant difference was obtained between the vertical and horizontal alignment thresholds (P = 0.82), therefore the mean of these two measurements was taken as the threshold value. The remaining children obtained an alignment threshold (AT) of 234.37 ± 83.92 seconds of arc and a mean oscillatory motion displacement threshold (OMDT) of 100.62 ± 26.44 seconds of arc. Statistical analysis revealed a significant difference between AT and OMDT measures (P = 0.0001). 
Regression analysis revealed a significant correlation between high contrast visual (CAT) acuity and both hyperacuity measurements (AT: Fig. 1 ; r 2 = 0.368; P = < 0.01; and OMDT: Fig. 2 ; r 2 = 0.296; P < 0.05). Measurements of AT (P = 0.0013) and OMDT (P = 0.003) thresholds in amblyopic subjects with strabismus (strabismic only and strabismus associated with anisometropia) yielded significantly higher values than those without (i.e., anisometropic amblyopes). 
As previously stated, classification of amblyopia with regard to the associated amblyogenic factor revealed 20 anisometropes, 16 strabismic amblyopes, and 14 anisometropic + strabismic amblyopes. These children may alternatively be classified with regard to any functional visual loss as determined by the clinical and psychophysical tests outlined above. The primary functional visual deficit was established by dividing the pretherapy score with the normal mean value for the test. The primary functional deficit was then identified as the test with the lowest score. Using this criteria the amblyopic subjects could be subdivided into the following groups illustrated in Table 3 (each individual amblyopic subject was assigned to one group). 
The mean visual acuity and hyperacuity scores when identifying the amblyopic subjects with respect to the primary visual deficit are summarized in Table 4 . This provides extra information with regard to the specific visual deficits present. For instance, group B demonstrated the greatest difference between CAT and LC, therefore indicating a low contrast acuity visual deficit (the difference between CAT and LC is indicative of the difference between points being sampled on the contrast sensitivity function). The mean difference in scores of 0.394 log units (nearly 4 lines better with CAT), a mean difference outside previously set 95% confidence limits (±0.175 log units) for normality. 14 15  
Group D demonstrated the greatest difference between CAT and RRL, therefore indicating increased fixation instability (revealed by a higher score with RRL in comparison with CAT). The mean difference in scores was 0.212 log units (more than 2 lines better with RRL), a mean difference outside previously set 95% confidence limits (±0.175 log units) for normality. 14 15  
Group E demonstrated the most significant difference between CAT and SLA indicative of increased levels of contour interaction (revealed by a higher score with SLA than with CAT). The mean difference in scores was 0.3 log units (3 lines better with SLA), a mean difference outside previously set 95% confidence limits (±0.150 log units) for normality. The magnitude of this crowding effect can also be measured by means of the crowding ratio. 13 14 Group E had a mean ratio of 2.3, again this value is outside previously set 95% confidence limits for normality. 14 15  
Posttherapy
So that the relative improvements in visual acuity thresholds and hyperacuities could be compared, the absolute change in score (in log units) relative to the initial pretherapy value was calculated for each visit. 
Due to cancellations and non-attendance, not every child was seen at identical intervals, therefore visits were categorized with visit 1 representing baseline measurements; visit 2, 4 to 8 weeks after initial visit; visit 3, 10 to 12 weeks; and so on. 
The mean posttherapy visual acuity and hyperacuity scores for the amblyopic subjects with respect to the primary visual deficit are summarized in Table 4
When the amblyopic subjects are grouped according to primary visual loss, it is this visual function that shows the greatest improvement in response to treatment. It is interesting then to compare the mean changes posttherapy in those groups with a specific visual deficit. For instance, group D demonstrated increased fixation instability. The mean difference in scores posttherapy reduced to 0.060 log units (2 or 3 letters better with RRL) due to an increase in high contrast linear acuity (CAT), a mean difference now within the previously set 95% confidence limits (±0.175 log units) for normality. 14 15  
Group E demonstrated increased levels of contour interaction. The mean difference in scores posttherapy reduced to 0.136 log units (1.5 lines better with SLA), a mean difference now within the previously set 95% confidence limits (±0.150 log units) for normality. 14 The mean crowding ratio also reduced to 1.2, a value now within previously set 95% confidence limits for normality. 14 15  
Group B demonstrated significantly reduced low contrast acuity pretherapy; although this visual function improved, the changes were concurrent with high contrast acuity. Therefore, the mean difference in scores posttherapy remained unchanged (0.385 log units), a value still outside previously set 95% confidence limits (±0.175 log units) for normality. 14 15  
Figure 2 shows the cumulative improvement for all subjects (absolute change in log units) during the course of therapy. Occlusion therapy appeared to improve all aspects of visual function, with these results demonstrating concurrent changes in both the acuity card measurements and hyperacuities. High and low contrast linear acuity appeared to have the greatest overall relative improvement in performance. The greatest improvement in visual function appeared to occur with the first 4 to 6 visits, although in the cumulative data high contrast acuity appeared to suddenly surge after visit 6. 
Figure 3 illustrates the relative improvement in performance in all measurements during the course of occlusion therapy individually in anisometropic (Fig. 3A) , strabismic (Fig. 3B) , and anisometropic + strabismic amblyopes (Fig. 3C) . Again all measurements show improvement. 
In the anisometropic subject group, high and low contrast linear acuity showed a significant improvement (0.300–0.350 log units). In this subject group both high and low contrast acuity showed improvements as late as visits 7 and 9. Hyperacuity thresholds demonstrated the smallest degree of improvement. 
In the strabismic subject group all visual functions show a steady and significant improvement, with low contrast linear acuity showing the greatest degree of improvement (just under 0.300 log units). Again, the majority of visual function improvement levels off after visit 4 to 6. Compared with the anisometropes, AT and OMDT improved to a greater extent in strabismic amblyopes. 
In the anisometropic + strabismic subject group all visual functions showed a steady and significant improvement, with high contrast linear acuity showing the greatest degree of improvement (around 0.250 log units). The majority of visual function improvement levels off after visit 4 to 6. Again, interestingly, compared with the pure anisometropes, AT and OMDT improved to a greater extent in this group. 
Figure 4 indicates the rate of improvement with occlusion therapy in relation to the primary functional visual deficit in the amblyopic subjects, as described in Table 4 . In all cases the primary visual deficit demonstrated the most prominent rate of improvement in performance. 
Figure 4A shows that the primary visual deficit in this group was revealed by a significantly reduced CAT score; with occlusion therapy it was this deficit (high contrast linear acuity) that showed the greatest degree of improvement (nearly 0.300 log units). 
The primary visual deficit in the group illustrated in Figure 4B was revealed by a significantly reduced LC score; with occlusion therapy it was this deficit (low contrast linear acuity) that showed the greatest degree of improvement (0.300 log units). However because of the concurrent changes in high contrast acuity, to a similar extent this group maintained a low contrast deficit (mean difference 0.387 log units), a value still outside previously set 95% confidence limits (±0.175 log units) for normality. 14  
The primary visual deficit shown in Figure 4C was revealed by significantly reduced hyperacuity thresholds; and with occlusion therapy, it was this deficit that showed the greatest degree of improvement (0.150–0.300 log units). RRL and SLA failed to demonstrate a substantial improvement (<0.100 log units). Again, the majority of visual function improvement levels off after visit 4 to 6, with the exception of the hyperacuity measures in a single amblyopic subject (which continue to show improvement up to visit 9). 
The group shown in Figure 4D demonstrated an increased fixation instability (revealed by a higher score with RRL than in CAT). The mean difference in scores reduced from 0.212 log units pretherapy to 0.060 log units posttherapy, with a significant increase in CAT score demonstrated (0.275 log units). Interestingly, this group of amblyopic subjects also demonstrated a deficit with hyperacuity measures which also improved significantly posttherapy. Again, it was the primary visual deficit in this group which showed the greatest degree of improvement. 
Group E shown in Figure 4E demonstrated increased levels of contour interaction (revealed by a higher score with SLA than with CAT). The mean difference in scores reduced from 0.300 log units pretherapy to 0.136 log units posttherapy, with a significant increase in CAT score demonstrated (0.443 log units). The mean crowding ratio also reduced from 2.3 to 1.2. However, CAT and SLA visual thresholds are still below previously set 95% confidence limits for normality. 14 Visual function improvement is very slow for most visual functions; however, in this subject group high contrast acuity surges ahead at about visit 7 to 9, which is very similar to the plot for anisometropic amblyopes in Figure 3 (anisometropia was present in 72% of the amblyopic subjects in group E). 
Linear regression analysis revealed a significant correlation between the initial pretherapy score and the absolute change in visual function with acuity card measures, the lower the initial value the greater the improvement (Fig. 5 A; P < 0.01); however, no correlation was evident with either hyperacuity measure (Fig. 5B ; P > 0.05). 
Linear regression analysis revealed no significant correlation between the absolute change in visual function during the course of occlusion therapy and the age of the amblyopic subject at the beginning of treatment with either acuity card measures (Fig. 6 A; P > 0.05) or hyperacuity measures (Fig. 6B ; P > 0.05). 
Demonstrable binocular vision was evident in 34 children, with 28 of these children achieving stereopsis. The mean stereoacuity values were 654.6 ± 853.4 seconds of arc with the TNO stereotest and 338.7 ± 351.02 seconds of arc with the Frisby stereotest; these values improved to 501.8 ± 740.4 seconds of arc (TNO) and 195.5 ± 158.8 seconds of arc (Frisby) during occlusion therapy. A significant difference between thresholds obtained using TNO and Frisby was evident pretherapy (P = 0.001) and posttherapy (P = 0.03); the higher values with the TNO test may be due to its more dissociative design. No amblyopic subjects regained binocularity through the course of occlusion therapy. 
Treatment diaries were completed and returned in 35 cases. Part-time occlusion hours appeared easily administrated, although those children who required full-time occlusion often divided the prescribed time during the course of the day. As may be expected, there was a significant difference (P = 0.001) between part-time hours (mean, 3.8 ± 0.5 hours) administered and full-time hours (mean, 7.9 ± 1.3 hours) of occlusion administered. The most common activities while wearing the patch were school, computer games, and television. In children who wore the patch at school (part-time or full-time) the vision tended to improve quicker, consequently producing a shorter duration of therapy. 
No significant relationship could be found between the absolute improvement in visual function and the amount of occlusion therapy administered in either visual acuity thresholds (Fig. 7 A; P > 0.05) or hyperacuity measures (Fig. 7B P > 0.05). 
Discussion
In this study we have examined periodically various parameters of visual function in a group of young amblyopic subjects throughout the course of occlusion therapy. 
The results of this study suggest strongly that other aspects of visual function should be monitored during occlusion therapy. We provide evidence that many aspects of visual function are adversely affected by the presence of amblyopia; all the subjects in this study demonstrated a functional loss in each of the tests used. Initial pretherapy values can be used to identify the visual deficit that is most impaired. Interestingly, when the amblyopic subjects were divided according to this primary visual loss, it was this visual function that was found to have the greatest improvement during occlusion therapy (Fig. 4) . Thus, an amblyopic patient could show little or no improvement on a traditional Snellen visual acuity test, although changes are occurring in other aspects of visual function (e.g., low contrast perception, fixation steadiness, motion detection). Often these visual functions are not affected equally, that is, a reduction in low contrast may be greater than the deficit with traditional high contrast letter charts. Our results also reveal a significant correlation between the initial pretherapy scores and the absolute improvement in visual function, with the greater deficit leading to the more substantial degree of improvement (Fig. 6)
There is no consensus in the literature as to the correct treatment or the factors influencing the treatment of amblyopia. 6 23 24 25 We could find no relationship between the hours of occlusion prescribed (part-time or full-time) and the absolute improvement in visual function (Fig. 7) , in those children who completed the treatment diaries. Our results show that it is the time course of treatment and not necessarily the duration (hours per day) of occlusion that is a key factor. 
The greatest improvement in visual function(s) occurred within the first 4 to 5 visits (16–24 weeks), although certain subjects did show changes over a longer time interval. In clinical practice, occlusion therapy is often prescribed over many months and sometimes years; the results of our study question the need to continue occlusion beyond this time scale. 
In practice, many clinicians feel that treatment after 6 to 8 years of age is ineffective, and in many cases treatment would remain uninitiated. Nevertheless, several reports in the literature now exist that detail the successful treatment of amblyopia in older children and adults. 5 16 26 27 28 In the present study we could find no relationship between age and degree of visual function improvement (Fig. 6) , compliance with treatment rather than the patient’s age appears to be a key factor. The clinical pertinence of the treatment of older subjects has been questioned, however, 29 with amblyopic subjects reportedly still maintaining fixation with the dominant eye and eventually demonstrating a regression of visual function. The risk of insuperable diplopia in strabismic amblyopia is also a factor that deters treatment in an older age-group. If suppression is overcome, subjects may find it impossible to resuppress or learn to ignore the diplopia at this age, and if fusion is absent it will be impossible to re-join the diplopia 
The results of this study provide further evidence for the multifarious reduction in visual performance that occurs in amblyopia. Success must be assessed not simply by a change in high contrast visual acuity but by examining a number of parameters that relate to an individual’s visual capacity. 
Occlusion therapy appeared to improve all aspects of the amblyopia, with a significant difference between pre- and posttherapy results (Table 5) . However, to establish the most effective form of treatment in amblyopia, it would be interesting to compare in isolation and in combination the effects of occlusion therapy and specific active therapies targeted at an individual’s primary functional visual deficit. If individual therapies for the different functional subcategories could be found, then these individuals could be identified and subsequently targeted with a specific therapy. 
It is, therefore, important to appreciate this diverse reduction in visual performance that occurs in amblyopic eyes and the need to monitor other aspects of visual function during occlusion therapy to allow for the optimum management of the patient. 
 
Table 1.
 
Clinical Details of All Subjects
Table 1.
 
Clinical Details of All Subjects
Subject Age, y Spectacle Prescription Ocular Alignment Previous Occlusion
JL 9.2 RE plano/+3.50× 90 R SOT Image not available
LE plano/+3.00× 80 10Δ
SW 9.1 RE+3.50 DS Straight Image not available
LE+0.75/+0.50× 90
SA 10 Nil R SOT Image not available
12Δ
ST 6.4 Nil L SOT Image not available
12Δ
GA 8 RE+2.50/+1.50× 175 Straight Image not available
LE plano
ND 8 RE plano Straight Image not available
LE−2.00/+5.00× 65
JC 8 RE+2.00/+1.00× 30 Straight Image not available
LE+3.00/+1.00× 145
GS 5.3 Nil R SOT Image not available
10Δ
AK 6.7 RE+2.00 DS Straight Image not available
LE+3.00 DS
NB 4.6 Nil R SOT Image not available
14Δ
FD 6.0 RE+0.50 DS L SOT Image not available
LE+3.00 DS 10Δ
AH 6.0 RE+2.00/−2.00× 180 R SOT Image not available
LE+0.50 DS 16Δ
WD 6.1 Nil R SOT Image not available
12Δ
SWt 6.0 Nil L SOT Image not available
16Δ
ST 5.8 Nil L XOT Image not available
12Δ
KA 6.0 RE+2.00/+1.00× 30 L SOT Image not available
LE+4.00/+1.00× 145 12Δ
DC 5.8 Nil R SOT Image not available
18Δ
LK 5.3 RE+2.00 DS Straight Image not available
LE+4.00 DS
LC 6.8 RE+2.00 DS L SOT Image not available
LE +5.00/+1.00× 145 20Δ
MC 5.2 RE+2.00/+1.00× 30 Straight Image not available
LE+0.50 DS
LC 5.2 RE+4.00 DS Straight Image not available
LE+3.00 DS
CW 6.2 RE+2.50/+0.50× 90 Straight Image not available
LE+1.25/+0.50× 90
PD 6.0 RE+4.00 DS Straight Image not available
LE+3.00 DS
TA 4.7 RE+2.25 DS Straight Image not available
LE+0.50 DS
DM 4.2 RE+1.50/+1.00× 180 Straight Image not available
LE+3.00 DS
HG 4.2 RE+0.50 DS LE SOT Image not available
LE+4.00 DS 20Δ
BN 4.6 RE +7.50/−0.50× 10 LE SOT Image not available
LE+9.00/−0.50× 170 12Δ
HM 6.0 RE+2.75/+1.00× 40 Straight Image not available
LE+4.00/+0.50× 135
AJ 5.9 RE+5.75/+0.50× 80 Straight Image not available
LE+4.25 DS
CS 5.9 RE+2.00/+1.00× 30 LE SOT Image not available
LE+3.00/+1.00× 145 20Δ
LMc 6.5 Nil LE SOT Image not available
25Δ
Subject Age, y Spectacle Prescription Ocular Alignment Previous Occlusion
WL 5.2 RE+0.25 DS Straight Image not available
LE+0.75/+0.75× 90
BA 4.2 Nil RE SOT Image not available
30Δ
CM 4.1 RE+2.00 DS Straight Image not available
LE+0.25/+1.00× 90
GL 5.0 RE+0.25/+0.75× 90 RE SOT Image not available
LE−0.75/+0.75× 80 10Δ
MJ 4.0 RE+2.00 DS LE SOT Image not available
LE+3.00 DS 12Δ
NN 4.0 RE+1.75/+0.75× 180 RE SOT Image not available
LE+0.50/+0.25× 180
SD 4.9 RE +2.00/+1.00× 30 LE XOT Image not available
LE+2.50/+3.00× 145
JMc 4.5 Nil LE SOT Image not available
25Δ
TC 5.5 Nil LE SOT Image not available
10Δ
WG 6.0 RE plano Straight Image not available
LE−2.00/+5.00× 65
WR 4.4 RE +2.25/+1.50× 175 Straight Image not available
LE plano
CJ 4.3 RE+1.25/+0.75× 115 RE SOT Image not available
LE+0.75 DS 10Δ
TS 5.2 Nil LE SOT Image not available
10Δ
PI 6.2 RE+3.5 DS Straight Image not available
LE+0.75/+0.50× 90
BS 4.3 Nil RE SOT Image not available
18Δ
FJ 4.0 RE+1.00/+1.00× 30 LE SOT Image not available
LE+3.00/+2.00× 145 20Δ
LD 4.1 RE+5.00/+1.00× 30 Straight Image not available
LE+6.00/+1.00× 145
GR 4.2 Nil RE SOT Image not available
25Δ
MK 4.2 RE+2.50/−0.75× 25 RE SOT Image not available
LE plano 12Δ
Table 2.
 
Mean Pretherapy Visual Acuity Thresholds
Table 2.
 
Mean Pretherapy Visual Acuity Thresholds
Acuity Card Anisometropic Amblyopes (n = 20) Strabismic Amblyopes (n = 16) Anisometropic, and Strabismic Amblyopes (n = 14) All Amblyopes (n = 50)
CAT 0.485 ± 0.2 0.511 ± 0.2 0.487 ± 0.2 0.499 ± 0.2
SLA 0.708 ± 0.3 0.679 ± 0.3 0.692 ± 0.1 0.663 ± 0.2
LC 0.428 ± 0.2 0.386 ± 0.2 0.350 ± 0.2 0.322 ± 0.2
RRL 0.603 ± 0.3 0.557 ± 0.2 0.575 ± 0.3 0.567 ± 0.3
Figure 1.
 
Comparison of mean alignment threshold (A) and mean oscillatory displacement threshold (B) plotted against mean high contrast visual acuity (CAT) pretherapy.
Figure 1.
 
Comparison of mean alignment threshold (A) and mean oscillatory displacement threshold (B) plotted against mean high contrast visual acuity (CAT) pretherapy.
Figure 2.
 
Comparison of improvements in visual function for all amblyopic subjects during occlusion therapy. The absolute change in score (log units ± SD) is relative to the initial pretherapy value and was calculated for each visit. The number of subjects may vary between visits.
Figure 2.
 
Comparison of improvements in visual function for all amblyopic subjects during occlusion therapy. The absolute change in score (log units ± SD) is relative to the initial pretherapy value and was calculated for each visit. The number of subjects may vary between visits.
Table 3.
 
Identification of the Amblyopes with Primary Functional Visual Deficit as Determined by the Tests Outlined
Table 3.
 
Identification of the Amblyopes with Primary Functional Visual Deficit as Determined by the Tests Outlined
Primary Visual Deficit Determined by Number of Amblyopic Subjects
Group A CAT 12
Reduced High Contrast Visual Acuity (4 A, 5 S, 3 S+ A)
Group B LC 10
Reduced Low Contrast Visual Acuity (4 A, 2 S, 4 S+ A)
Group C OMDT 13
Reduced Hyperacuity Thresholds AT (6 A, 4 S, 3 S+ A)
Group D RRL > CAT 8
Increased Fixation Instability (3 A, 3 S, 2 S+ A)
Group E SLA–CAT 7
Increased Contour Interaction (3 A, 2 S, 2 S+ A)
Table 4.
 
Mean Visual Function Thresholds Pretherapy and Posttherapy (Final Visit) + Standard Deviation (Modified LogMAR) and Hyperacuity Thresholds + Standard Deviation (Seconds of Arc), for All Amblyopic Subjects Grouped with Respect to the Primary Visual Deficit (see Table 3 )
Table 4.
 
Mean Visual Function Thresholds Pretherapy and Posttherapy (Final Visit) + Standard Deviation (Modified LogMAR) and Hyperacuity Thresholds + Standard Deviation (Seconds of Arc), for All Amblyopic Subjects Grouped with Respect to the Primary Visual Deficit (see Table 3 )
Test of Visual Function Primary Visual Deficit
A (n = 12) B (n = 10) C(n = 13) D(n = 8) E(n = 7)
CAT 0.522 ± 0.193 0.490 ± 0.096 0.648 ± 0.138 0.447 ± 0.119 0.232 ± 0.079
0.89± 0.07 0.782 ± 0.169 0.783 ± 0.137 0.835 ± 0.113 0.731 ± 0.116 0.675 ± 0.102
SLA 0.710 ± 0.201 0.518 ± 0.093 0.777 ± 0.162 0.703 ± 0.099 0.532 ± 0.168
1.02± 0.08 0.873 ± 0.137 0.830 ± 0.122 0.873 ± 0.125 0.891 ± 1.120 0.811 ± 0.117
LC 0.408 ± 0.175 0.096 ± 0.158 0.369 ± 0.139 0.412 ± 0.171 0.153 ± 0.123
0.62± 0.09 0.562 ± 0.134 0.397 ± 0.098 0.625 ± 0.091 0.575 ± 0.127 0.479 ± 0.094
RRL 0.619 ± 0.216 0.400 ± 0.234 0.719 ± 0.150 0.659 ± 0.180 0.329 ± 0.074
0.96± 0.07 0.829 ± 0.173 0.792 ± 0.147 0.814 ± 0.117 0.791 ± 0.125 0.714 ± 0.117
OMDT 100.63 ± 21.60 102.12 ± 44.01 124.14 ± 25.33 97.32 ± 21.19 86.29 ± 22.19
59.69± 8.59 64.72 ± 21.31 72.01 ± 48.42 67.59 ± 25.33 61.78 ± 18.15 74.25 ± 25.59
AT 211.85 ± 78.44 241.26 ± 70.84 268.92 ± 95.73 226.68 ± 45.08 236.09 ± 94.77
69.18± 19.38 137.08 ± 73.15 176.09 ± 68.48 183.55 ± 46.73 116.12 ± 51.75 180.14 ± 95.61
Figure 3.
 
Comparison of improvements in visual function for anisometropic amblyopes (A); strabismic amblyopes (B), and anisometropic and strabismic amblyopes (C) during occlusion therapy. The absolute change in score (log units ± SD) is relative to the initial pretherapy value and was calculated for each visit. The number of subjects may vary between visits.
Figure 3.
 
Comparison of improvements in visual function for anisometropic amblyopes (A); strabismic amblyopes (B), and anisometropic and strabismic amblyopes (C) during occlusion therapy. The absolute change in score (log units ± SD) is relative to the initial pretherapy value and was calculated for each visit. The number of subjects may vary between visits.
Figure 4.
 
Comparison of improvements in visual function in relation to the primary functional visual deficit in the amblyopic subjects Reduced High Contrast Visual Acuity (A); Reduced Low Contrast Visual Acuity (B); Reduced Hyperacuity Thresholds (C); Increased Fixation Instability and Reduced Hyperacuity Thresholds (D); and Increased Contour Interaction (E) during occlusion therapy. The absolute change in score (log units ± SD) is relative to the initial pretherapy value and was calculated for each visit. The number of subjects may vary between visits.
Figure 4.
 
Comparison of improvements in visual function in relation to the primary functional visual deficit in the amblyopic subjects Reduced High Contrast Visual Acuity (A); Reduced Low Contrast Visual Acuity (B); Reduced Hyperacuity Thresholds (C); Increased Fixation Instability and Reduced Hyperacuity Thresholds (D); and Increased Contour Interaction (E) during occlusion therapy. The absolute change in score (log units ± SD) is relative to the initial pretherapy value and was calculated for each visit. The number of subjects may vary between visits.
Figure 5.
 
Regression analysis illustrating the relationship between the initial pretherapy value and the absolute change in visual function with acuity card measures (A; in log units) and hyperacuity measures (B; in seconds of arc).
Figure 5.
 
Regression analysis illustrating the relationship between the initial pretherapy value and the absolute change in visual function with acuity card measures (A; in log units) and hyperacuity measures (B; in seconds of arc).
Figure 6.
 
Regression analysis illustrating the relationship between the absolute change in visual function and the age of the amblyopic patient with acuity card measures (A; in log units) and hyperacuity measures (B; in seconds of arc).
Figure 6.
 
Regression analysis illustrating the relationship between the absolute change in visual function and the age of the amblyopic patient with acuity card measures (A; in log units) and hyperacuity measures (B; in seconds of arc).
Figure 7.
 
Regression analysis illustrating the relationship between the absolute change in visual function and the duration of daily occlusion with acuity card measures (A; in log units) and hyperacuity measures (B; in seconds of arc).
Figure 7.
 
Regression analysis illustrating the relationship between the absolute change in visual function and the duration of daily occlusion with acuity card measures (A; in log units) and hyperacuity measures (B; in seconds of arc).
Table 5.
 
Comparison of Pre- and Posttherapy findings
Table 5.
 
Comparison of Pre- and Posttherapy findings
Test Pretherapy Posttherapy P
CAT 0.499 ± 0.193 0.736 ± 0.140 0.0001
SLA 0.663 ± 0.182 0.873 ± 0.124 0.0001
LC 0.322 ± 0.206 0.564 ± 0.120 0.0001
RRL 0.567 ± 0.231 0.795 ± 0.140 0.0001
OMDT 100.62 ± 26.44 69.79 ± 14.72 0.0001
AT 234.37 ± 83.92 166.54 ± 28.69 0.0001
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Wick B, Schor CM. A comparison of the Snellen chart and the S-chart for visual acuity assessment in amblyopi. J Am Optom Asso. 1984;55:359–361.
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Figure 1.
 
Comparison of mean alignment threshold (A) and mean oscillatory displacement threshold (B) plotted against mean high contrast visual acuity (CAT) pretherapy.
Figure 1.
 
Comparison of mean alignment threshold (A) and mean oscillatory displacement threshold (B) plotted against mean high contrast visual acuity (CAT) pretherapy.
Figure 2.
 
Comparison of improvements in visual function for all amblyopic subjects during occlusion therapy. The absolute change in score (log units ± SD) is relative to the initial pretherapy value and was calculated for each visit. The number of subjects may vary between visits.
Figure 2.
 
Comparison of improvements in visual function for all amblyopic subjects during occlusion therapy. The absolute change in score (log units ± SD) is relative to the initial pretherapy value and was calculated for each visit. The number of subjects may vary between visits.
Figure 3.
 
Comparison of improvements in visual function for anisometropic amblyopes (A); strabismic amblyopes (B), and anisometropic and strabismic amblyopes (C) during occlusion therapy. The absolute change in score (log units ± SD) is relative to the initial pretherapy value and was calculated for each visit. The number of subjects may vary between visits.
Figure 3.
 
Comparison of improvements in visual function for anisometropic amblyopes (A); strabismic amblyopes (B), and anisometropic and strabismic amblyopes (C) during occlusion therapy. The absolute change in score (log units ± SD) is relative to the initial pretherapy value and was calculated for each visit. The number of subjects may vary between visits.
Figure 4.
 
Comparison of improvements in visual function in relation to the primary functional visual deficit in the amblyopic subjects Reduced High Contrast Visual Acuity (A); Reduced Low Contrast Visual Acuity (B); Reduced Hyperacuity Thresholds (C); Increased Fixation Instability and Reduced Hyperacuity Thresholds (D); and Increased Contour Interaction (E) during occlusion therapy. The absolute change in score (log units ± SD) is relative to the initial pretherapy value and was calculated for each visit. The number of subjects may vary between visits.
Figure 4.
 
Comparison of improvements in visual function in relation to the primary functional visual deficit in the amblyopic subjects Reduced High Contrast Visual Acuity (A); Reduced Low Contrast Visual Acuity (B); Reduced Hyperacuity Thresholds (C); Increased Fixation Instability and Reduced Hyperacuity Thresholds (D); and Increased Contour Interaction (E) during occlusion therapy. The absolute change in score (log units ± SD) is relative to the initial pretherapy value and was calculated for each visit. The number of subjects may vary between visits.
Figure 5.
 
Regression analysis illustrating the relationship between the initial pretherapy value and the absolute change in visual function with acuity card measures (A; in log units) and hyperacuity measures (B; in seconds of arc).
Figure 5.
 
Regression analysis illustrating the relationship between the initial pretherapy value and the absolute change in visual function with acuity card measures (A; in log units) and hyperacuity measures (B; in seconds of arc).
Figure 6.
 
Regression analysis illustrating the relationship between the absolute change in visual function and the age of the amblyopic patient with acuity card measures (A; in log units) and hyperacuity measures (B; in seconds of arc).
Figure 6.
 
Regression analysis illustrating the relationship between the absolute change in visual function and the age of the amblyopic patient with acuity card measures (A; in log units) and hyperacuity measures (B; in seconds of arc).
Figure 7.
 
Regression analysis illustrating the relationship between the absolute change in visual function and the duration of daily occlusion with acuity card measures (A; in log units) and hyperacuity measures (B; in seconds of arc).
Figure 7.
 
Regression analysis illustrating the relationship between the absolute change in visual function and the duration of daily occlusion with acuity card measures (A; in log units) and hyperacuity measures (B; in seconds of arc).
Table 1.
 
Clinical Details of All Subjects
Table 1.
 
Clinical Details of All Subjects
Subject Age, y Spectacle Prescription Ocular Alignment Previous Occlusion
JL 9.2 RE plano/+3.50× 90 R SOT Image not available
LE plano/+3.00× 80 10Δ
SW 9.1 RE+3.50 DS Straight Image not available
LE+0.75/+0.50× 90
SA 10 Nil R SOT Image not available
12Δ
ST 6.4 Nil L SOT Image not available
12Δ
GA 8 RE+2.50/+1.50× 175 Straight Image not available
LE plano
ND 8 RE plano Straight Image not available
LE−2.00/+5.00× 65
JC 8 RE+2.00/+1.00× 30 Straight Image not available
LE+3.00/+1.00× 145
GS 5.3 Nil R SOT Image not available
10Δ
AK 6.7 RE+2.00 DS Straight Image not available
LE+3.00 DS
NB 4.6 Nil R SOT Image not available
14Δ
FD 6.0 RE+0.50 DS L SOT Image not available
LE+3.00 DS 10Δ
AH 6.0 RE+2.00/−2.00× 180 R SOT Image not available
LE+0.50 DS 16Δ
WD 6.1 Nil R SOT Image not available
12Δ
SWt 6.0 Nil L SOT Image not available
16Δ
ST 5.8 Nil L XOT Image not available
12Δ
KA 6.0 RE+2.00/+1.00× 30 L SOT Image not available
LE+4.00/+1.00× 145 12Δ
DC 5.8 Nil R SOT Image not available
18Δ
LK 5.3 RE+2.00 DS Straight Image not available
LE+4.00 DS
LC 6.8 RE+2.00 DS L SOT Image not available
LE +5.00/+1.00× 145 20Δ
MC 5.2 RE+2.00/+1.00× 30 Straight Image not available
LE+0.50 DS
LC 5.2 RE+4.00 DS Straight Image not available
LE+3.00 DS
CW 6.2 RE+2.50/+0.50× 90 Straight Image not available
LE+1.25/+0.50× 90
PD 6.0 RE+4.00 DS Straight Image not available
LE+3.00 DS
TA 4.7 RE+2.25 DS Straight Image not available
LE+0.50 DS
DM 4.2 RE+1.50/+1.00× 180 Straight Image not available
LE+3.00 DS
HG 4.2 RE+0.50 DS LE SOT Image not available
LE+4.00 DS 20Δ
BN 4.6 RE +7.50/−0.50× 10 LE SOT Image not available
LE+9.00/−0.50× 170 12Δ
HM 6.0 RE+2.75/+1.00× 40 Straight Image not available
LE+4.00/+0.50× 135
AJ 5.9 RE+5.75/+0.50× 80 Straight Image not available
LE+4.25 DS
CS 5.9 RE+2.00/+1.00× 30 LE SOT Image not available
LE+3.00/+1.00× 145 20Δ
LMc 6.5 Nil LE SOT Image not available
25Δ
Subject Age, y Spectacle Prescription Ocular Alignment Previous Occlusion
WL 5.2 RE+0.25 DS Straight Image not available
LE+0.75/+0.75× 90
BA 4.2 Nil RE SOT Image not available
30Δ
CM 4.1 RE+2.00 DS Straight Image not available
LE+0.25/+1.00× 90
GL 5.0 RE+0.25/+0.75× 90 RE SOT Image not available
LE−0.75/+0.75× 80 10Δ
MJ 4.0 RE+2.00 DS LE SOT Image not available
LE+3.00 DS 12Δ
NN 4.0 RE+1.75/+0.75× 180 RE SOT Image not available
LE+0.50/+0.25× 180
SD 4.9 RE +2.00/+1.00× 30 LE XOT Image not available
LE+2.50/+3.00× 145
JMc 4.5 Nil LE SOT Image not available
25Δ
TC 5.5 Nil LE SOT Image not available
10Δ
WG 6.0 RE plano Straight Image not available
LE−2.00/+5.00× 65
WR 4.4 RE +2.25/+1.50× 175 Straight Image not available
LE plano
CJ 4.3 RE+1.25/+0.75× 115 RE SOT Image not available
LE+0.75 DS 10Δ
TS 5.2 Nil LE SOT Image not available
10Δ
PI 6.2 RE+3.5 DS Straight Image not available
LE+0.75/+0.50× 90
BS 4.3 Nil RE SOT Image not available
18Δ
FJ 4.0 RE+1.00/+1.00× 30 LE SOT Image not available
LE+3.00/+2.00× 145 20Δ
LD 4.1 RE+5.00/+1.00× 30 Straight Image not available
LE+6.00/+1.00× 145
GR 4.2 Nil RE SOT Image not available
25Δ
MK 4.2 RE+2.50/−0.75× 25 RE SOT Image not available
LE plano 12Δ
Table 2.
 
Mean Pretherapy Visual Acuity Thresholds
Table 2.
 
Mean Pretherapy Visual Acuity Thresholds
Acuity Card Anisometropic Amblyopes (n = 20) Strabismic Amblyopes (n = 16) Anisometropic, and Strabismic Amblyopes (n = 14) All Amblyopes (n = 50)
CAT 0.485 ± 0.2 0.511 ± 0.2 0.487 ± 0.2 0.499 ± 0.2
SLA 0.708 ± 0.3 0.679 ± 0.3 0.692 ± 0.1 0.663 ± 0.2
LC 0.428 ± 0.2 0.386 ± 0.2 0.350 ± 0.2 0.322 ± 0.2
RRL 0.603 ± 0.3 0.557 ± 0.2 0.575 ± 0.3 0.567 ± 0.3
Table 3.
 
Identification of the Amblyopes with Primary Functional Visual Deficit as Determined by the Tests Outlined
Table 3.
 
Identification of the Amblyopes with Primary Functional Visual Deficit as Determined by the Tests Outlined
Primary Visual Deficit Determined by Number of Amblyopic Subjects
Group A CAT 12
Reduced High Contrast Visual Acuity (4 A, 5 S, 3 S+ A)
Group B LC 10
Reduced Low Contrast Visual Acuity (4 A, 2 S, 4 S+ A)
Group C OMDT 13
Reduced Hyperacuity Thresholds AT (6 A, 4 S, 3 S+ A)
Group D RRL > CAT 8
Increased Fixation Instability (3 A, 3 S, 2 S+ A)
Group E SLA–CAT 7
Increased Contour Interaction (3 A, 2 S, 2 S+ A)
Table 4.
 
Mean Visual Function Thresholds Pretherapy and Posttherapy (Final Visit) + Standard Deviation (Modified LogMAR) and Hyperacuity Thresholds + Standard Deviation (Seconds of Arc), for All Amblyopic Subjects Grouped with Respect to the Primary Visual Deficit (see Table 3 )
Table 4.
 
Mean Visual Function Thresholds Pretherapy and Posttherapy (Final Visit) + Standard Deviation (Modified LogMAR) and Hyperacuity Thresholds + Standard Deviation (Seconds of Arc), for All Amblyopic Subjects Grouped with Respect to the Primary Visual Deficit (see Table 3 )
Test of Visual Function Primary Visual Deficit
A (n = 12) B (n = 10) C(n = 13) D(n = 8) E(n = 7)
CAT 0.522 ± 0.193 0.490 ± 0.096 0.648 ± 0.138 0.447 ± 0.119 0.232 ± 0.079
0.89± 0.07 0.782 ± 0.169 0.783 ± 0.137 0.835 ± 0.113 0.731 ± 0.116 0.675 ± 0.102
SLA 0.710 ± 0.201 0.518 ± 0.093 0.777 ± 0.162 0.703 ± 0.099 0.532 ± 0.168
1.02± 0.08 0.873 ± 0.137 0.830 ± 0.122 0.873 ± 0.125 0.891 ± 1.120 0.811 ± 0.117
LC 0.408 ± 0.175 0.096 ± 0.158 0.369 ± 0.139 0.412 ± 0.171 0.153 ± 0.123
0.62± 0.09 0.562 ± 0.134 0.397 ± 0.098 0.625 ± 0.091 0.575 ± 0.127 0.479 ± 0.094
RRL 0.619 ± 0.216 0.400 ± 0.234 0.719 ± 0.150 0.659 ± 0.180 0.329 ± 0.074
0.96± 0.07 0.829 ± 0.173 0.792 ± 0.147 0.814 ± 0.117 0.791 ± 0.125 0.714 ± 0.117
OMDT 100.63 ± 21.60 102.12 ± 44.01 124.14 ± 25.33 97.32 ± 21.19 86.29 ± 22.19
59.69± 8.59 64.72 ± 21.31 72.01 ± 48.42 67.59 ± 25.33 61.78 ± 18.15 74.25 ± 25.59
AT 211.85 ± 78.44 241.26 ± 70.84 268.92 ± 95.73 226.68 ± 45.08 236.09 ± 94.77
69.18± 19.38 137.08 ± 73.15 176.09 ± 68.48 183.55 ± 46.73 116.12 ± 51.75 180.14 ± 95.61
Table 5.
 
Comparison of Pre- and Posttherapy findings
Table 5.
 
Comparison of Pre- and Posttherapy findings
Test Pretherapy Posttherapy P
CAT 0.499 ± 0.193 0.736 ± 0.140 0.0001
SLA 0.663 ± 0.182 0.873 ± 0.124 0.0001
LC 0.322 ± 0.206 0.564 ± 0.120 0.0001
RRL 0.567 ± 0.231 0.795 ± 0.140 0.0001
OMDT 100.62 ± 26.44 69.79 ± 14.72 0.0001
AT 234.37 ± 83.92 166.54 ± 28.69 0.0001
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