Challenge to the concept of surgical dose-response for moderate to large angle exotropia

PURPOSE: To determine whether the effect of three muscle surgery for moderate to large angle exotropia can be better predicted by surgical dosage, or by pre-operative exotropia angle. We examined cases where identical surgical dose was performed for a wide range of preoperative angles 30 to 55Δ. STUDY DESIGN: Retrospective, interventional case study. METHODS: Patients with exotropia 30-55Δ who underwent bilateral lateral rectus (BLR) muscle recession 7 mm and 4 mm unilateral medial rectus (UMR) muscle plication between 2014 and 2024 were included. Surgical effect was analyzed by linear regression against the amount of preoperative exotropia, as well as total millimeters of recession and plication. Success was defined as less than 10Δ exotropia or 5Δ esotropia. RESULTS: Thirty-four patients were included with 32 ± 27 (standard deviation, SD) years mean age. Mean preoperative exotropia at distance was 42 ± 6Δ (range 30-55Δ), and 40 ± 16Δ (range 6-70Δ) at near. Surgical effect varied from 29Δ to 75Δ, averaging 42 ± 10Δ, giving a mean dose response of 2 ± 1Δ/mm. Linear regression showed that average surgical effect was equal to pre-operative deviation, accounting for 34-36% of variation in surgical effect. Success rate of this operation for all exotropia angles was 76% at 2 months mean followup. CONCLUSIONS: Surgical effect for moderate to large angle exotropia does not strongly depend on surgical dose, so that successful outcomes measured 2 months post-operatively are obtained by performing a numerically identical procedure regardless of the pre-operative angle exceeding 30Δ. Since outcome is attributable more to the biological response than to amount of surgery, pre-operative measurements require only sufficient precision to confirm that the angle is moderate to large.