Unilateral Amblyopia Affects Two Eyes: Fellow Eye Deficits in Amblyopia

Abstract

Unilateral amblyopia is a visual disorder that arises after selective disruption of visual input to one eye during critical periods of development. In the clinic, amblyopia is understood as poor visual acuity in an eye that was deprived of pattern vision early in life. By its nature, however, amblyopia has an adverse effect on the development of a binocular visual system and the interactions between signals from two eyes. Visual functions aside from visual acuity are impacted, and many studies have indicated compromised sensitivity in the fellow eye even though it demonstrates normal visual acuity. While these fellow eye deficits have been noted, no overarching theory has been proposed to describe why and under what conditions the fellow eye is impacted by amblyopia. Here, we consider four explanations that may account for decreased fellow eye sensitivity: the fellow eye is adversely impacted by treatment for amblyopia; the maturation of the fellow eye is delayed by amblyopia; fellow eye sensitivity is impacted for visual functions that rely on binocular cortex; and fellow eye deficits reflect an adaptive mechanism that works to equalize the sensitivity of the two eyes. To evaluate these ideas, we describe five visual functions that are commonly reported to be deficient in the amblyopic eye (hyperacuity, contrast sensitivity, spatial integration, global motion, and motion-defined form), and unify the current evidence for fellow eye deficits. Further research targeted at exploring fellow eye deficits in amblyopia will provide us with a broader understanding of normal visual development and how amblyopia impacts the developing visual system.

The functional impact of amblyopia

“…Amblyopia principally affects one eye; nonetheless, the non‐amblyopic eye often has an array of small but measurable deficits of spatial, positional and motion sensitivity . Clinically, high contrast recognition visual acuity is the predominant measure that leads to a diagnosis of amblyopia, based on two or more lines difference in visual acuity measured between eyes.…”

Dichoptic Attentive Motion Tracking is Biased Toward the Nonamblyopic Eye in Strabismic Amblyopia

“…[8][9][10][11][12][13][14][15][16][17] A number of these deficits also extend to the nonamblyopic fellow eye. 8,12,14,[18][19][20][21] Beyond the loss of monocular visual function, amblyopia also impairs binocular vision, in part because visual information from the amblyopic eye is suppressed from conscious awareness when both eyes are viewing. [22][23][24][25][26][27] Suppression can affect large areas of the amblyopic eye visual field 28,29 and may play an important role in the visual deficits experienced by patients with amblyopia.…”

Optimal Audiovisual Integration in the Ventriloquism Effect But Pervasive Deficits in Unisensory Spatial Localization in Amblyopia

“…Contrary to clinical dogma, vision in the fellow eye is not entirely normal. 46 Careful psychophysical studies have shown that the fellow eye has reduced optotype 4,47 and Vernier acuity, 48 and perhaps more importantly, greater spatial uncertainty and distortion affecting both foveal and extra-foveal vision. 9,10 Such deficits in spatial vision, affecting both the amblyopic and fellow eyes, may therefore contribute to the binocular visual localization deficit observed in the present study.…”