Risk of consecutive esotropia after surgery for intermittent exotropia according to passive duction force

Purpose To determine the relationship between consecutive esotropia (ET) and passive duction force (PDF) in patients with intermittent exotropia (XT). Methods The study enrolled 70 patients in whom PDF was measured under general anesthesia prior to XT surgery. The preferred eye for fixation (PE) and the nonpreferred eye for fixation (NPE) were determined using a cover–uncover test. The patients were subdivided into two groups according to the angle of deviation at 1 month postoperation: (1) consecutive ET (CET group), >10 prism diopters (PD) of ET; and (2) non-CET (NCET group), ≤10 ET or residual exodeviation. The relative PDF of the medial rectus muscle (MRM) was obtained by subtracting the ipsilateral PDF of the lateral rectus muscle (LRM) from the PDF of the MRM. Results The PDFs for the LRM in the PE in the CET and NCET groups were 47.28 g and 58.59 g, respectively (p = 0.147), and 56.18 g and 46.59 g for the MRM (p = 0.11), and in the NPE were 59.84 g and 55.25 g, respectively, for the LRM (p = 0.993), and 49.12 g and 50.53 g, respectively, for the MRM (p = 0.81). However, in the PE, the PDF in the MRM was larger in the CET group than in the NCET group (p = 0.045), which was positively associated with the postoperatively overcorrected angle of deviation (p = 0.017). Conclusions An increased relative PDF in the MRM in the PE was a risk factor for consecutive ET after XT surgery. Quantitative evaluation of the PDF could be considered when planning strabismus surgery to achieve the desired surgical outcome.