Beneficial Effects of Alternate Occlusion on Binocular Motion Processing in Infantile Esotropia

We are evaluating the hypothesis that one can preserve the potential for developing normal sensory binocular mechanisms by full-time alternate occlusion. In order to test this hypothesis without harm, we used the objective, quantifiable tool of motion processing (a sensory binocular test) by a new measure of visually evoked potential (VEP) easily applicable to infants as well as to adults. It is important to know whether the hypothesis is correct, since alternate occlusion prior to surgical realignment would have very beneficial effects in the management of the commonly occurring infantile strabismus.

Full-time alternate occlusion for patients with infantile esotropia (crossed eyes) has been advocated as a means to prevent abnormal binocular interaction from occurring prior to the achievement of therapeutic realignment. It was suggested that full-time alternate occlusion would "keep the binocular slate clean" and, thus, preserve the normal development potential for binocularity. Indirect clinical evidence from enhanced final treatment results suggests a beneficial role for pre-operative alternate occlusion therapy in equal visioned infantile esotropes, but there is no direct evidence that alternate occlusion has either a positive or negative effect, per se, on visual system development.

Motion Processing Defects

It is important to note that directional asymmetries of monocular optokinetic nystagmus and monocular smooth pursuit have long been known to be associated with the lack of binocularity in infantile esotropes. What is striking is that monocular tests can give valuable information about binocular sensory development. The VEP is generated in the visual cortex and thus reflects solely the cortical sensory part of the sensory motor arc.

Normal infants show monocular motion asymmetries both in optokinetic nystagmus and in the motion VEP. The magnitude of the motion asymmetry declines rapidly during the first 6 months of life. Full adult maturation of symmetric motion processing may take as long as 2-3 years for the most rapid motions and for the motion of fine detailed targets. It is important to recognize that normal development here requires normal eye alignment for the development sequence to progress from asymmetry to symmetry. Infantile esotropes aligned for the first time after 2 years show a marked and persistent asymmetry, which is a lifelong marker of the infantile esotrope, even though the eyes may have been aligned at a later date.

Experimental Evaluation

Patients were selected with onset of esotropia prior to 4 months of age (without any significant accommodative component); had habitual alternate fixation in each eye; had at least 12 weeks of full-time alternate occlusion; and had more than 1 motion VEP test before and after, or during, alternate occlusion therapy.

Motion VEPs were measured monocularly in response to vertical sinusoidal gratings displayed on a video monitor. The gratings were square-wave alternated between two positions separated by 90 degrees of spatial phase, and presented for 10 seconds per trial (with multiple recorded trials).

Fourier analysis was used to extract the amplitude and phase of the evoked response, according to methods devised by Smith-Kettlewell researchers. Results are shown in , which is a composite of the 3 cases the authors studied, compared with the normal developmental sequence for motion VEP. Like normal infants, these patients progressed from a greater degree to a lesser degree of motion asymmetry while under alternate occlusion. From the cases presented here it is not possible to determine whether the motion recovery was complete or only partial.

The data support the hypothesis that full-time alternate occlusion is a no-binocular-stimulus condition that prevents abnormal binocular interactions, and allows normal sensory development to proceed.

The adequate stimulus for the development of normal binocular motion processing consists of two components, a) equal bilateral visual inputs, with b) the absence of abnormal binocular interactions.

The new tool for assessing binocular sensory motion processing in infants and adults, by an objective monocular measurement, is a useful method for providing information about the developmental status of binocular mechanisms.

Conclusion

Thus, it appears that full-time alternate occlusion for strabismic infantile esotropes does indeed "keep the binocular slate clean" and preserves the normal development potential for binocularity. Surprisingly and unexpectedly, this study also indicated that there is a normal innate development from asymmetric motion to symmetric motion without the necessity of perfect alignment, unless one interferes with this normal development with either manifest strabismus, or unequal visual inputs in the two visual systems.

Collaborators: Anthony Norcia and Russell Hamer.