Changes in binocular alignment after surgery for concomitant and pattern intermittent exotropia.

Introduction: Although early post-surgical over-correction for intermittent exotropia is widely advised, post-operative drift has not been well quantified in concomitant intermittent exotropia, and has not been described specifically with A and V patterns. While such patterns have been proposed to result from abnormal locations of the rectus muscle pulleys, others have suggested that A and V patterns may result from the disruption of fusion arising from exotropia itself.

Methods: We prospectively performed Hess screen analysis in 20 exotropic patients (mean age 42 +/- 16 yrs) before and two to six times after strabismus surgery, with a post-operative follow-up of 2-108 weeks. Primary surgery cases included medial rectus resection (2) and lateral rectus recession (10), combined resection/recession (6), and superior oblique tenectomy (2). Alignment trends in primary and secondary gazes were analyzed for concomitant, pattern, and re-operated subgroups. Results were also analyzed by type of surgery performed.

Results: Mean pre-operative central gaze exotropia was 8.6 +/- 7.1 degrees . Twelve cases were concomitant, while 8 exhibited A or V patterns. Twelve cases were re-operations. In initial surgery for concomitant exotropia, there was a well-defined exotropic drift approaching 5 degrees by 30 weeks post-operatively (linear regression, r = 0.43, p = 0.01). There was similar exo drift in re-operations. However, in pattern exotropia, post-operative drift was more variable, with mean esotropic drift of approximately 5 degrees (r = 0.18, p = 0.43). For all patients, final post-operative central gaze exotropia was 1.9 +/- 5.8 degrees , with significant pattern collapse (p < 0.01).

Discussion: Post-operative exo-shift of about 5 degrees occurs in initial and re-operated concomitant exotropia. However, in A and V patterns, there is no definitive direction of post-operative drift, suggesting that pattern strabismus may be more likely due to mechanical factors in the orbit than to neural factors associated with fusion disruption.

Conclusions: Alignment following strabismus surgery differs in concomitant vs. pattern exotropia. Initial over-correction of about 5 degrees is advisable for concomitant exotropia, but should be avoided in A and V patterns.