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 withAandVpatterns. 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 exodrift 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., 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.