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Using a Nidek microperimeter, the authors assessed fixation instability in children with anisometropia and different levels of stereoacuity. Analysis of the eye movement records showed children with no stereoacuity had fixation instability that included fusion maldevelopment nystagmus, which is seen in a small temporalward flick on cover testing.

Methods: A total of 94 children with hyperopic anisometropia (ages 5-13 years) were evaluated prospectively between June 2010 and December 2012 with the use of the Nidek MP-1 microperimeter. Fixation instability was quantified by the area of the bivariate contour ellipse that included 95% of fixation points during a 30-second test interval. Each eye movement waveform during the 30-second test interval also was examined with the use of custom software and classified as normal, fusion maldevelopment nystagmus (FMNS), or infantile nystagmus. Finally, the Randot Preschool Stereoacuity Test (Stereo Optical Company Inc, Chicago, IL) was administered.

Results: Stereoacuity was correlated with fixation instability (Spearman r = 0.50; 95% CI, 0.33-0.64); visual acuity was more weakly correlated (r = 0.28). All children with normal stereoacuity had stable fixation, children with subnormal stereoacuity had fixation instability, and those with nil stereoacuity had the most instability. Eye movement records during attempted fixation were of sufficient quality for classification in 81 children; 61% of those with reduced stereoacuity and 88% of those with nil stereoacuity had FMNS eye movement waveforms.

Conclusions: Our data support the hypothesis that the binocular decorrelation caused by anisometropia can disrupt ocular motor development, resulting in FMNS and its temporalward refoveating “flicks” that may mimic microstrabismus.