Objective: To investigate the effect of horizontal rectus muscle surgery on astigmatism.
Material and Method: A prospective descriptive study was conducted between July 2004 and June 2005, we recruited 16 strabismic patients with bilateral recession or recession of one horizontal muscle or monocular recession and resection for this study at Srinagarind Hospital. The authors measured the patients’ bestcorrected visual acuity and ocular alignment using the prism cover test and the Modified Krimsky tests, respectively. Auto-refraction was performed on the pre- operative and first post operative day by a technician not apprised of the type and/or amount of surgery. Any differences between the pre-operative and post operative astigmatism were calculated using the x-y coordinate system and the mean value for astigmatism. Descriptive statistics were used to analyze the data.
Results: Included were 23 eyes from 16 patients who met the inclusion criteria; 8 females; 8 males; average age 14.75±11.76 (range, 4-40) years. Seven patients (43.75%) had esotropia and nine (56.25%) exotropia. For all procedures, the respective mean cylinder of the pre- operative and first post-operative day were 0.74±1.20 D in 67.56° axis and 0.69±0.86 D in 56.30°axis, in the same direction as the cylinders for each type. There was no statistically significant change in the refractive error between the pre- operative and first postoperative day.
Conclusion: In contrast to previous studies, astigmatism was not significantly affected by horizontal rectus muscle surgery; however, the present study population was small. Therefore, a larger prospective control trial would be necessary to definitively answer this question.

Keywords: Astigmatism, Horizontal strabismus surgery