The effect of botulinum toxin injection on the hang-back recession of rectus muscles

Background: Hang-back recession and botulinum toxin injection are performed as one treatment option for the correction of large angles of deviation. However, the effect of botulinum toxin on hang-back recession has not been thoroughly evaluated. The purpose of this study is to investigate the effect of botulinum toxin on the hang-back recession of extraocular muscles. Methods: Under general anesthesia, 6-mm hang-back recession of the superior rectus muscle (SR) was performed in all eyes of 12 rabbits. Botulinum toxin (5 IU, 0.1 ml) was injected into the SR of one eye in each rabbit. The inferior rectus muscles (IR) of both eyes were excised to model severe paralysis. The distance between the superior limbus and the new insertion was measured immediately after the procedure and 3 months postoperatively. Changes in the location of the insertion were evaluated and compared between both eyes. Results: After excision of the IR, upward deviation was observed in all eyes. The insertion site of the recessed SR immediately after the procedure and 3 months postoperatively were significantly different in all rabbit eyes with or without botulinum injection (P=0.002, 0.002); SR advancement was observed in both cases. The amount of advancement was 1.38±0.41 mm in the SR without botulinum toxin injection and 2.29±0.45 mm in the SR with injection; this difference was statistically significant (P=0.002). Conclusions: Botulinum toxin injection reduced the amount of hang-back recession in deviated rabbit eyes. Surgeons should be aware that botulinum toxin can diminish the effects of hang-back recession when used simultaneously.