Assessing Suppression in Amblyopic Children With a Dichoptic Eye Chart

Abstract

Purpose: Suppression has a key role in the etiology of amblyopia, and contrast-balanced binocular treatment can overcome suppression and improve visual acuity. Quantitative assessment of suppression could have a role in managing amblyopia. We describe a novel eye chart to assess suppression in children.

Methods: We enrolled 100 children (7-12 years; 63 amblyopic, 25 nonamblyopic with strabismus or anisometropia, 12 controls) in the primary cohort and 22 children (3-6 years; 13 amblyopic, 9 nonamblyopic) in a secondary cohort. Letters were presented on a dichoptic display (5 letters per line). Children wore polarized glasses so that each eye saw a different letter chart. At each position, the identity of the letter and its contrast on each eye's chart differed. Children read 8 lines of letters for each of 3 letter sizes. The contrast balance ratio was the ratio at which 50% of letters seen by the amblyopic eye were reported.

Results: Amblyopic children had significantly higher contrast balance ratios for all letter sizes compared to nonamblyopic children and controls, requiring 4.6 to 5.6 times more contrast in the amblyopic eye compared to the fellow eye (P < 0.0001). Amblyopic eye visual acuity was correlated with contrast balance ratio (r ranged from 0.49-0.57 for the 3 letter sizes). Change in visual acuity with amblyopia treatment was correlated with change in contrast balance ratio (r ranged from 0.43-0.62 for the 3 letter sizes).

Conclusions: Severity of suppression can be monitored as part of a routine clinical exam in the management of amblyopia in children.

Figure 1

Schematic diagram of the dichoptic eye chart. Letters were band-pass filtered Sloan letters chosen from the e-ETDRS letter set. Five letters were presented in a line with varying letter contrast on a gray background. A different letter chart was presented to each eye using an LG passive 3D video display. At each letter position, the identity and interocular contrast-ratio of the letter on each chart differed while the spatial-frequency content of the letter remained the same. Children were instructed to read the 5 letters left to right and completed 8 lines (40 letter/contrast combinations) for each letter size. Contrast balance ratio was defined as the contrast ratio (amblyopic eye contrast/fellow eye contrast) at which 50% of letters seen by the amblyopic eye were reported. For nonamblyopic children with a history of treated amblyopia, the contrast ratio was calculated as previously amblyopic eye contrast/fellow eye contrast. For nonamblyopic children who were never amblyopic and for normal controls, the contrast balance ratio was calculated as right eye contrast/left eye contrast. Three letter sizes were tested, 1.25 (20/480), 2.5 (20/240), and 5 c/deg (20/120), for a total of 120 trials.

Figure 2

Contrast balance ratios (mean ± SEM) of 7- to 12-year-old amblyopic children, nonamblyopic children who had been treated for strabismus and/or anisometropia, and normal control children.

Figure 3

Contrast balance ratios (mean ± SEM) of 7- to 12-year-old children with anisometropic amblyopia, strabismic amblyopia, and combined mechanism amblyopia.

Figure 4

Contrast balance ratios (mean ± SEM) of 3- to 6-year-old amblyopic children and nonamblyopic children who had been treated for strabismus and/or anisometropia.

Figure 5

Amblyopic eye visual acuity as a function of contrast balance ratio (amblyopic eye/fellow eye).

Figure 6

Changes in contrast balance ratio (baseline − outcome ratio) and change in logMAR visual acuity (baseline − outcome) for 30 children tested on two visits. For both changes, positive numbers indicate improvement, while negative numbers indicate regression.