To elucidate the pathophysiology of dissociated horizontal deviation.

The reversed fixation test was performed prospectively in 28 patients who developed consecutive exotropia following horizontal extraocular muscle surgery for infantile esotropia. All patients were assessed for the presence of adduction weakness, latent nystagmus, dissociated vertical divergence, and neurologic disease.

A positive reversed fixation test, indicating the presence of dissociated horizontal deviation, was found in 14 of 28 patients (50%) with consecutive exotropia. In patients with dissociated horizontal deviation, the exodeviation was usually smaller with the nonpreferred eye fixating than with the preferred eye fixating, and smaller with the preferred eye fixating than during periods of visual inattention or under general anesthesia. Dissociated horizontal deviation correlated with the findings of dissociated vertical divergence, but not with asymmetric adduction weakness, latent nystagmus, or neurologic disease.

Using reversed fixation testing, dissociated horizontal deviation can be detected in 50% of patients who develop consecutive exotropia following surgery for infantile esotropia. In this setting, monocular fixation with either eye superimposes a dissociated esotonus upon a baseline exodeviation. Fixation with the nonpreferred eye usually exerts greater esotonus than fixation with the preferred eye, producing an asymmetrical exodeviation during prism and alternate cover testing. Depending on the baseline anatomical position of the eyes, this dissociated esotonus can manifest as an intermittent exodeviation or an intermittent esodeviation. This unrecognized form of ocular motor dissociation may contribute to the pathogenesis of infantile esotropia.