Factors associated with changes in horizontal deviation after inferior oblique weakening surgery

To identify the factors affecting postoperative horizontal deviation following isolated inferior oblique (IO) weakening surgery.

The medical records of patients who had undergone isolated IO weakening surgery between February 2010 and September 2021, with a minimum follow-up period of 6 months, were retrospectively reviewed. Patients were divided into two groups, eso-shift, and non-eso-shift, based on the change in horizontal deviation at postoperative 6 months. Eso-shift or exo-shift was defined as a change of 2Δ or more.

A total of 77 patients were included. Among them, 49 patients (63.6%) showed changes in horizontal deviation: 37 (48.0%) eso-shift, 12 (15.6%) exo-shift. Twenty-eight patients (36.4%) showed no change in alignment. The eso-shift group showed the following characteristics compared to the non-eso-shift group: older age (P < 0.001), higher proportion of patients with preoperative exo-deviation (P < 0.001), and greater preoperative hypertropia in primary gaze (P < 0.001), down gaze (P = 0.031), ipsilateral gaze (P < 0.001), and ipsilateral head tilt (P = 0.002). Regarding the postoperative changes in horizontal deviation based on prior horizontal strabismus, patients with preoperative exo-deviation showed a statistically significant eso-shift. The patients with prior eso-deviation tended toward exo-shift, while those without prior horizontal deviation rarely changed.

Changes in postoperative horizontal deviation are associated with preoperative horizontal and vertical deviation, which might result from changes in the rectus muscle due to oblique muscle dysfunction. Also, this study suggests that isolated IO weakening surgery could effectively correct small-angle horizontal deviation in patients with inferior oblique overaction (IOOA) and horizontal strabismus.