Role of the equator in the early overcorrection of intermittent exotropia

Purpose: To determine the cause of consecutive esotropia on the basis of the relationship between the location of the equator and a new insertional site of the recessed lateral rectus muscle in cases of intermittent exotropia. Methods: Ninety-two patients with intermittent exotropia, all of whom underwent recessions of both lateral rectus muscles (5 to 8 mm), were included. The distances from the limbus to the equator (LE distance) and from the equator to the new insertion of the recessed lateral rectus muscle (EIN distance) were calculated using the following formula: (LE = axial lengthπ/4 - corneal diameter/2). Overcorrection was defined as esophoria in excess of 5 prism diopters, and all patients were followed up for at least 3 months after surgery. Results: In all patients, the mean LE distance was 12.7 mm in the right eye and 12.6 mm in the left eye. The mean EIN distance in the successfully corrected patients was -0.82 mm in the right eye and -0.95 mm in the left eye. A negative distance value indicates a new insertion anterior to the equator. Overcorrection was detected in 15 patients (16.3%), whose mean EIN distances were -0.93 mm in the right eye and -0.78 mm in the left eye. No significant differences were determined to exist between the corrected and overcorrected patients with regard to the affected eye (P > .05). Conclusions: Overcorrection of intermittent exotropia did not appear to be related to the location of new insertions of the lateral rectus muscle of up to 8 mm of recession. This may indicate the redistributing of relatively more innervational inputs to the medial rectus muscle after the completion of lateral rectus recession in consecutive esotropia.