Effect of Vertical Transposition of the Horizontal Muscles on Vertical Strabismus Associated with Exotropia

Purpose: To evaluate the correction of vertical strabismus associated with exotropia by vertical transposition of the horizontal muscle to an extent equal to half the muscle width combined with muscle recession-resection. Methods: We enrolled patients with vertical strabismus of 4-15 prism diopters (PD) associated with exotropia who were not scheduled for vertical rectus or oblique muscle surgery. Regardless of the strabismus angle, the insertion site of the horizontal muscle was vertically transposed by half the muscle width during surgery. Success was defined as a vertical strabismus angle of 2 PD or less. Results: The preoperative mean exodeviation far angle in 41 patients was 27.8 PD and the average vertical strabismus far angle 8.1 PD. The success rate of exotropia surgery was 68.3%. The mean vertical strabismus angle was 2.8 PD on postoperative day 1 and the success rate of vertical strabismus surgery 92.7%. Undercorrection was evident in 4.9% of patients and overcorrection in 2.4%. The preoperative vertical strabismus angle was strongly associated with the residual angle after surgery (r = 0.386, <i>p</i> = 0.013). Receiver operating curve analysis revealed that the preoperative vertical strabismus far angle cutoff was 11 PDs. No patient factor significantly influenced vertical strabismus correction. Conclusions: Vertical transposition of the horizontal muscle by half of the muscle width may correct vertical strabismus of far angle 11 PDs or less.