The oval of adequate binocular alignment: just how straight do the eyes of an infantile esotropia have to be maintained to preserve stereopsis and binocular vision?

PURPOSE

To measure the maximum exotropic and hypertropic deviations compatible with at least gross stereopsis in a group of treated infantile esotropes.

METHODS

Over a ten year period (1992-2002) all former (i.e. surgically corrected) infantile esotropes now ages 10 to 23 years old coming into my office for routine long term followup visits were tested for the presence of at least gross stereopsis on the classic Titmus Stereo Test (Stereo Optical Co., Inc., 3539 N Kenton Ave, Chicago IL 60641). For the approximately 40% who demonstrated at least gross stereopsis on this test at near in either the primary position or in downgaze, further stereo testing was performed if the patient had a manifest deviation in other positions of gazes that was more than two prism diopters more than the position in which stereopsis was originally found. Testing in these other positions of gaze was performed no more than 20 degrees away from the primary position so that the standard Polaroid glasses could be used with the test. The deviation at which the patient reported that the "fly went flat" was taken as the end point.

RESULTS

Among 22 with more XT in a position of gaze away from the position where they demonstrated stereopsis, 2 lost stereopsis at 5 prism diopters (pd) XT, 10 lost stereopsis at 6 pd XT, 8 lost stereopsis at 7 pd XT, 1 lost stereopsis at 8 pd XT, and 1 preserved stereopsis until the deviation was 9 pd XT. Among 8 with more HT in a position of gaze away from the position where they demonstrated stereopsis, 1 lost stereopsis at 2 pd HT, 1 at 5 pd HT, 4 at 6 pd HT, 1 at 7 pd HT, and 1 at 8 pd HT. The averages were 6.5 pd XT and 6.3 pd HT (with the 2 pd HT excluded as an outlier).

CONCLUSIONS

To maintain stereopsis among those infantile esotropes for whom stereopsis is possible, an exotropia of more than 6 pd XT or a hypertropia of more than 6 pd HT should not be tolerated. These limits can be combined with criteria from other studies to produce a diagram, the oval of adequate binocular alignment.