Motion perimetry in anisometropic amblyopia: elevated size thresholds extend into the midperiphery

The Initiation of Smooth Pursuit is Delayed in Anisometropic Amblyopia

Purpose

Several behavioral studies have shown that the reaction times of visually guided movements are slower in people with amblyopia, particularly during amblyopic eye viewing. Here, we tested the hypothesis that the initiation of smooth pursuit eye movements, which are responsible for accurately keeping moving objects on the fovea, is delayed in people with anisometropic amblyopia.

Methods

Eleven participants with anisometropic amblyopia and 14 visually normal observers were asked to track a step-ramp target moving at ±15°/s horizontally as quickly and as accurately as possible. The experiment was conducted under three viewing conditions: amblyopic/nondominant eye, binocular, and fellow/dominant eye viewing. Outcome measures were smooth pursuit latency, open-loop gain, steady state gain, and catch-up saccade frequency.

Results

Participants with anisometropic amblyopia initiated smooth pursuit significantly slower during amblyopic eye viewing (206 ± 20 ms) than visually normal observers viewing with their nondominant eye (183 ± 17 ms, P = 0.002). However, mean pursuit latency in the anisometropic amblyopia group during binocular and monocular fellow eye viewing was comparable to the visually normal group. Mean open-loop gain, steady state gain, and catch-up saccade frequency were similar between the two groups, but participants with anisometropic amblyopia exhibited more variable steady state gain (P = 0.045).

Conclusions

This study provides evidence of temporally delayed smooth pursuit initiation in anisometropic amblyopia. After initiation, the smooth pursuit velocity profile in anisometropic amblyopia participants is similar to visually normal controls. This finding differs from what has been observed previously in participants with strabismic amblyopia who exhibit reduced smooth pursuit velocity gains with more catch-up saccades.

Amblyopia: a mini review of the literature

Amblyopia is a form of cerebral visual impairment in the absence of an organic cause. It is considered to derive from the degradation of the retinal image associated with abnormal visual experience during the developmental period of the visual system in infancy and early childhood. Amblyopia is a significant cause of unilateral visual deficit in childhood and is still considered as one of the most common causes of persistent unilateral visual impairment in adulthood. The following review aims at presenting the contemporary literature regarding the prevalence, the aetiology, the neural correlates, the period of critical development, the treatment, the prognosis and the disability associated with this visual deficit.

Decreased cortical activation in response to a motion stimulus in anisometropic amblyopic eyes using functional magnetic resonance imaging

PURPOSE

Motion perception abnormalities and extrastriate abnormalities have been suggested in amblyopia. Functional MRI (fMRI) and motion stimuli were used to study whether interocular differences in activation are detectable in motion-sensitive cortical areas in patients with anisometropic amblyopia.

METHODS

We performed fMRI at 1.5 T 4 control subjects (20/20 OU), 1 with monocular suppression (20/25), and 2 with anisometropic amblyopia (20/60, 20/800). Monocular suppression was thought to be form fruste of amblyopia. The experimental stimulus consisted of expanding and contracting concentric rings, whereas the control condition consisted of stationary concentric rings. Activation was determined by contrasting the 2 conditions for each eye.

RESULTS

Significant fMRI activation and comparable right and left eye activation was found in V3a and V5 in all control subjects (Average z-values in L vs R contrast 0.42, 0.43) and in the subject with monocular suppression (z = 0.19). The anisometropes exhibited decreased extrastriate activation in their amblyopic eyes compared with the fellow eyes (zs = 2.12, 2.76).

CONCLUSIONS

Our data suggest motion-sensitive cortical structures may be less active when anisometropic amblyopic eyes are stimulated with moving rings. These results support the hypothesis that extrastriate cortex is affected in anisometropic amblyopia. Although suggestive of a magnocellular defect, the exact mechanism is unclear.

Amblyopia characterization, treatment, and prophylaxis

Amblyopia has a 1.6-3.6% prevalence, higher in the medically underserved. It is more complex than simply visual acuity loss and the better eye has sub-clinical deficits. Functional limitations appear more extensive and loss of vision in the better eye of amblyopes more prevalent than previously thought. Amblyopia screening and treatment are efficacious, but cost-effectiveness concerns remain. Refractive correction alone may successfully treat anisometropic amblyopia and it, minimal occlusion, and/or catecholamine treatment can provide initial vision improvement that may improve compliance with subsequent long-duration treatment. Atropine penalization appears as effective as occlusion for moderate amblyopia, with limited-day penalization as effective as full-time. Cytidin-5'-diphosphocholine may hold promise as a medical treatment. Interpretation of much of the amblyopia literature is made difficult by: inaccurate visual acuity measurement at initial visit, lack of adequate refractive correction prior to and during treatment, and lack of long-term follow-up results. Successful treatment can be achieved in at most 63-83% of patients. Treatment outcome is a function of initial visual acuity and type of amblyopia, and a reciprocal product of treatment efficacy, duration, and compliance. Age at treatment onset is not predictive of outcome in many studies but detection under versus over 2-3 years of age may be. Multiple screenings prior to that age, and prompt treatment, reduce prevalence. Would a single early cycloplegic photoscreening be as, or more, successful at detection or prediction than the multiple screenings, and more cost-effective? Penalization and occlusion have minimal incidence of reverse amblyopia and/or side-effects, no significant influence on emmetropization, and no consistent effect on sign or size of post-treatment changes in strabismic deviation. There may be a physiologic basis for better age-indifferent outcome than tapped by current treatment methodologies. Infant refractive correction substantially reduces accommodative esotropia and amblyopia incidence without interference with emmetropization. Compensatory prism, alone or post-operatively, and/or minus lens treatment, and/or wide-field fusional amplitude training, may reduce risk of early onset esotropia. Multivariate screening using continuous-scale measurements may be more effective than traditional single-test dichotomous pass/fail measures. Pigmentation may be one parameter because Caucasians are at higher risk for esotropia than non-whites.

Disorders of motion and depth

Damage to the human homologue of area MT produces a motion perception deficit similar to that found in the monkey with MT lesions. Even temporary disruption of MT processing with transcranial magnetic stimulation can produce a temporary akinetopsia [127]. Motion perception deficits, however, also are found with a variety of subcortical lesions and other neurologic disorders that can best be described as causing a disconnection within the motion processing stream. The precise role of these subcortical structures, such as the cerebellum, remains to be determined. Simple motion perception, moreover, is only a part of MT function. It undoubtedly has an important role in the perception of depth from motion and stereopsis [112]. Psychophysical studies using aftereffects in normal observers suggest a link between stereo mechanisms and the perception of depth from motion [9-11]. There is even a simple correlation between stereo acuity and the perception of depth from motion [128]. Future studies of patients with cortical lesions will take a closer look at depth perception in association with motion perception and should provide a better understanding of how motion and depth are processed together.

Automated perimetry in amblyopia: a generalized depression

PURPOSE

To quantitate the visual field abnormalities associated with amblyopia.

METHODS

In a prospective study, 37 amblyopic patients (11 anisometropic, 13 strabismic, 9 combination, 4 deprivation) performed automated perimetry in each eye using the Humphrey 30-2 program. Primary outcome measures were foveal threshold, mean deviation, and average threshold.

RESULTS

When the probability plots were examined, 21 visual fields in amblyopic eyes were normal, 8 had central scotomas, and 7 had diffuse depressions. No focal defects other than mild central scotomas were seen. However, the foveal threshold of amblyopic eyes was decreased by an average of 7.2 +/- 8.0 dB (P < .0001) compared with fellow eyes; intereye differences in mean deviation (3.2 +/- 5.4 dB; P < .001) and average threshold (2.9 +/- 5.3 dB; P < .005) were also seen. This decrease in sensitivity for the amblyopic eye occurred for all types of amblyopia. The depression in threshold was greatest at the fovea but was detectable and significant at all eccentricities of the 30-degree field. The average threshold in the amblyopic eye was highly correlated with visual acuity (r = .839; P < .001).

CONCLUSIONS

Although automated visual fields in amblyopic eyes typically appear normal, all four types of amblyopia are associated with a generalized depression of light sensitivity, which is proportionately greatest at the fovea and highly correlated with visual acuity loss. In general, amblyopia is not associated with any area of focal loss of threshold light sensitivity. If a focal defect is present in the visual field of the amblyopic eye, organic causes of visual loss should be suspected. The Humphrey visual field analyzer STATPAC program (Allergan-Humphrey, Inc, San Leandro, California) may artifactually transform small and generalized full-field depressions in a manner that makes them appear to be isolated central defects.