Fixation instability in anisometropic children with reduced stereopsis

Abstract

Background: Hyperopic anisometropia in children can be associated with abnormal stereoacuity and "microstrabismus," a small temporalward "flick" as each eye assumes fixation on cover testing. The prevailing hypothesis is that abnormal sensory experience leads to foveal suppression and, subsequently, secondary microstrabismus. This study investigated the hypothesis that disruption of bifoveal fusion by anisometropia directly affects ocular motor function.

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.

Copyright © 2013 American Association for Pediatric Ophthalmology and Strabismus. Published by Mosby, Inc. All rights reserved.

Figures

FIG 1

Examples of MP-1 data from 3 children with hyperopic anisometropia acquired during 30 seconds of fixation. A, A child with stable fixation and nearly all fixation points inside the 1-deg circle. B, A more typical anisometropic child, with moderate instability. C, A child with extreme fixation instability. Red 1-deg circle shows the location of the fixation target in the reference frame. A small blue dot marks the location of fixation every 0.04 seconds; for the 30-second test, this resulted in a cloud of 750 fixation blue fixation points. Pale blue BCEs on each image enclose 68%, 95%, and 99% of the fixation points. The 95% BCE has been highlighted in yellow; the 95% BCEA is 0.8 deg2 (A), 5.9 deg2 (B), and 10.8 deg2 (C).

FIG 2

Mean (±SD) fixation instability (bivariate contour ellipse area) of 94 anisometropic children (5–13 years old). Children were divided into 3 subgroups based on stereoacuity. Also shown is the mean fixation instability of 43 age-matched healthy control children (horizontal line) and 95% tolerance limits for the normative data (gray area). Aniso-metropic children with subnormal stereoacuity showed a trend toward greater fixation instability than children with normal stereoacuity (P = 0.08) and anisometropic children with nil stereoacuity had significantly more fixation instability than either of the other two groups (P<0.001 for both comparisons).

FIG 3

Eye movements recorded by the Nidek MP-1 during attempted steady fixation from two anisometropic children. A, An 8-year-old anisometropic boy with stable fixation (same boy shown in Figure 1A). Overall, his fixation was accurate, with the exception of a brief saccadic oscillation. B, A 6-year-old anisometropic girl with significant fixation instability (same girl shown in Figure 1C). Her eye position record showed the classic waveform of FMNS syndrome, that is, slow drifts nasalward, with rapid refixating temporalward saccades.

FIG 4

Percentage of children with FMNS waveforms identified in eye movement records obtained with the Nidek MP-1 during 30 seconds of attempted steady fixation. Children are grouped by their stereoacuity measured with the Randot Preschool Stereoacuity Test.