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Astigmatism induced by spherical photorefractive keratectomy corrections
S Shah1, A Chatterjee, S J Doyle
1Optimax Laser Eye Clinics, Manchester, England.
Purpose:
The purpose of the study is to evaluate the induced astigmatism after spherical photorefractive keratectomy on the Summit Omnimed (Summit Instruments, Waltham, MA) and the Nidek EC-5000 (Nidek Co. Ltd, Aichi, Japan) excimer lasers.
Methods:
A total of 4269 eyes of 3289 patients were treated with a 5-mm optical zone using the Summit Omnimed excimer laser and 1825 eyes of 1303 patients treated with the Nidek EC-5000 excimer laser. The final astigmatic refractive outcome was compared with the initial refraction by vector analysis (Alpin and Jaffe method).
Results:
Subjective astigmatic refraction for the Summit laser reduced from a mean of -0.39 diopter (D) +/- standard deviation (SD) 0.33 D (range, 0 to -2.50 D) to -0.33 D +/- SD 0.41 D (range, 0 to -3.00 D). Surgically induced astigmatism (SIA) had a mean of 0.42 +/- SD 0.34 D (range, 0 to 2.89 D). Mean SIA increased with increasing preoperative astigmatism by 0.60 D SIA for every 1.00 D of preoperative cylinder. For the Nidek laser, subjective astigmatic refraction changed from a mean of -0.18 D +/- SD 0.21 D (range, 0 to -1.25 D) to -0.30 D +/- SD 0.33 D (range, 0 to -3.00 D). Surgically induced astigmatism had a mean of -0.32 D +/- SD 0.29 (range, 0 to 3.05 D). Mean SIA increased with increasing preoperative astigmatism by 0.47 D SIA for every 1.00 D of preoperative cylinder.
Conclusions:
The authors show that spherical photorefractive keratectomy corrections can induce significant astigmatic change, particularly if a large amount of preoperative astigmatism is present.