Convergence reserves at distance appear subnormal in children with intermittent exotropia

The authors of this retrospective case series evaluated fusional convergence and its association with control in children with intermittent exotropia. They found that convergence reserves were subnormal in children with intermittent exotropia, particularly at distance fixation, whereas total convergence amplitudes were often higher than in visually normal children.

They identified 64 children ranging in age from 3 to 17 years (median, 7 years) with intermittent exotropia with measures of angle of deviation, control (using a 0-to-5-point scale) and fusional convergence taken during a single examination. They calculated total convergence as the sum of the angle of deviation plus additional convergence in reserve. Mean values were compared with published data from visually normal children. A fusion reserve ratio was calculated as convergence reserve divided by angle of deviation.

The authors found that convergence in children with intermittent exotropia differed from children without extropia. Total convergence was higher at distance (33 prism diopters [pd] vs. 17 pd, P < 0.0001) and near (38 vs. 18 pd, P < 0.0001), whereas convergence reserve was lower at distance (7 vs. 17 pd; P < 0.0001). There was a strong correlation between fusion reserve ratio and control score at distance (R = ­0.75, P < 0.0001) and near (R = -0.66, P < 0.0001).

The authors conclude that the fusion reserve ratio correlates well with control and may be useful in grading the severity of intermittent exotropia, although further study of fusional convergence in intermittent exotropia is required. They say that the strong correlation between convergence reserve and control in this study suggests that exodeviation control may be influenced by the amount of convergence available in reserve. The improved fusional control often observed at near fixation may be attributable to the added effects of proximal, tonic and accommodative convergence. They conclude that in children with intermittent extropia, fusional convergence ability may change not only with fixation distance but also with the type of visual stimulus and visual environment.