Quantitative measurement of passive duction force tension in intermittent exotropia and its clinical implications.

Purpose To evaluate the passive duction force (PDF) in extraocular muscles (EOMs) in patients with intermittent exotropia (IXT) using a quantitative tension-measuring device. Methods This prospective, case-control study enrolled 25 patients with IXT and 26 age- and sex-matched controls. PDF was measured under general anesthesia as the eyeball was rotated medially or laterally away from the direction of the force being tested. The preferred eye for fixation was determined using a cover-uncover test. Results The PDF in the IXT and control groups were 60.9 g and 52.1 g, respectively, for the lateral rectus (LR) (p = 0.046) and 53.0 g and 48.8 g for the medial rectus (MR) (p = 0.293). When the eyes were examined separately in the IXT group, the PDF of LR was larger in the nonpreferred eye for fixation than in the control group (p = 0.039), whereas there was no difference in the preferred eye for fixation (p = 0.216). Additionally, the relative PDF of LR in the nonpreferred eye compared to the ipsilateral PDF of MR was positively associated with the duration of manifest deviation (p = 0.042) and the average angle of the near and far deviations (p = 0.023). Conclusions The PDF in the LR in patients with IXT in the nonpreferred eye for fixation was larger than normal and could increase with the duration of manifest deviation and the angle of deviation. Evaluating the PDF in EOMs could provide information that is useful for managing strabismus and understanding its pathophysiology.