In this study, better baseline stereoacuity was associated with better baseline amblyopic eye visual acuity, less anisometropia, and anisometropia due to astigmatism alone among subjects with anisometropic amblyopia and no heterotropia observed by cover test. Better outcome stereoacuity was associated with better baseline stereoacuity and better outcome amblyopic eye acuity.
We also observed some differences in baseline stereoacuity among age groups. Subjects aged 3 to <5 years had poorer baseline stereoacuity compared to that of subjects aged 5 to <10 years, and subjects aged 10 to <18 had poorer baseline stereoacuity compared to subjects aged 7 to <10 years after adjustment for potential confounders. The relatively poor stereoacuity of the youngest group might be explained by undetected microtropia or poorer testing performance. Despite the trends observed in e-Supplement 3
, baseline stereoacuity among subjects aged 7 to <10 years was not statistically different from that of the 3 to <5 or 5 to <7 age groups. These comparisons were inadequately powered to detect an age effect and were dependent on a single protocol that enrolled subjects in all three age groups. Subjects ages 10 years and older might have had worse baseline stereoacuity because of undetected microtropia with identity, or such children who present at this age may more commonly have a resistant type of amblyopia, particularly if they had not improved with prior amblyopia treatment.
Factors predictive of outcome stereoacuity were not identical to those predictive of baseline stereoacuity. One important reason for this disparity is that the outcome stereoacuity model included an adjustment for baseline stereoacuity. As a result, any factor found to be associated with outcome stereoacuity in this model is by definition associated with change in stereoacuity with treatment.
The relationship between stereoacuity and amblyopic eye acuity may be because better visual acuity allows for better discrimination of stereoacuity test targets. Odell and colleagues12
found that stereoacuity was progressively degraded by increasing levels of induced monocular blur in 15 normal adults, and that random dot stereo test performance is particularly sensitive to degradation by monocular blur.
Other investigators have examined the relationship between stereoacuity and amblyopic eye visual acuity. Caputo and colleagues3
found that better baseline stereoacuity is predictive of improvement in amblyopic eye acuity, and they hypothesized that the presence of binocular vision at the first evaluation was a good prognostic indicator for visual recovery with optical correction alone. Lee and colleagues4
reported a significant linear relationship between stereoacuity improvement measured by the Titmus test and visual acuity improvement in patients with and without small angle or intermittent strabismus.
Better baseline stereoacuity was associated with less anisometropia when expressed as vector dioptric difference. These data agree with findings of other investigators who found that baseline stereoacuity in amblyopic patients is associated with magnitude of anisometropia.1,2
Weakley reported that higher amounts of spherical anisometropia and higher amounts of cylindrical anisometropia were each associated with decreased stereoacuity in spectacle correction.1,2
Rutstein and Corliss2
found that binocularity of spectacle-corrected hyperopic patients decreased as the degree of anisometropia increased.1
One explanation for these findings is that anisometropia and aniseikonia (image size disparity) are obstacles to sensory fusion and development of stereoacuity. Lubkin and colleagues25
showed that anisometropia is associated with amblyopia, and the combination of anisometropia and aniseikonia is strongly associated with amblyopia. Jimenez and colleagues26
and Oguchi and colleagues27
reported an association between induced aniseikonia and reduced stereoacuity. Dobson and colleages15
found that small interocular refractive error differences disrupt stereoacuity, whereas larger interocular refractive error differences are necessary to produce differences in interocular, best-corrected recognition acuity. They hypothesized that development of stereoacuity is particularly dependent on similarity in refractive error between fellow eyes, even in the absence of unilateral amblyopia.15
Among subjects whose amblyopic eye acuity at outcome was 20/25 or better and within one line of the fellow eye visual acuity, stereoacuity was worse than that of children with normal vision of the same age. These data support the premise that anisometropic amblyopia is associated with subnormal binocular development. Although visual acuity in the amblyopic eye can improve with treatment to normal or near-normal levels, a significant deficit in binocular function, as implied by reduced stereoacuity, persists in many children. This deficit in stereoacuity, despite normal visual acuity, might result from a central insult induced by anisometropia early in life, possibly manifesting as ongoing partial foveal suppression or a central limitation of binocular potential, and/or from a combination of other unknown factors; however, this analysis does not address whether stereoacuity would improve further with complete resolution of amblyopia and/or longer treatment duration. It is also possible that stereoacuity was limited because of undetected microtropia in an unknown number of children in our cohort.
It is noteworthy that only 48 of our 248 subjects with anisometropic amblyopia and no heterotropia observed on cover testing (19%) demonstrated marked improvement of visual acuity to 20/25 or better in the amblyopic eye. In contrast, Agervi and colleagues28
reported that 62 of 66 children (94%) with anisometropic amblyopia achieved resolution of amblyopia (interocular difference of one line or less) after one year of treatment with spectacles alone or with spectacles in addition to Bangerter filter. They may have observed a higher rate of amblyopia resolution because their subjects were younger, had less severe amblyopia, and had longer treatment duration than our subjects. The protocols included in our analyses were not designed to produce the maximal possible improvement with treatment.
In conclusion, better baseline stereoacuity is associated with better baseline amblyopic eye visual acuity, less anisometropia, and anisometropia due to astigmatism alone. Better stereoacuity after amblyopia treatment is associated with better baseline stereoacuity and better amblyopic eye acuity at outcome. For many children with anisometropic amblyopia, subnormal stereoacuity persists after a course of treatment, even when their visual acuity deficit effectively resolves.