Comparison of the Clinical Effectiveness of Correcting Different Types of Astigmatism with Small Incision Lenticule Extraction

Few studies have reported the differential outcomes of Small Incision Lenticule Extraction (SMILE) on myopic astigmatism. Given this, we examined the effectiveness of SMILE for up to one year, comparing with-the-rule (WTR), against-the-rule (ATR), and oblique astigmatism, conducting a retrospective review of patients who underwent correction of myopic astigmatism using the 500-kHz VisuMax femtosecond laser (Carl Zeiss Meditec) at two refractive clinics in Poland between 2016-2017. Patients were aged >= 21 with stable refractive errors between -0.5 and -10.0 diopter (D) with astigmatism up to 5D. The mean age of the 209 patients (355 eyes) available was 32 years; 58.4% were female. Of these, 247 had WTR, 62 oblique, and 46 ATR astigmatism. The mean pre-operative spherical equivalent (SE) was -5.4 +/- 2.57D and the cylinder -1.7 +/- 1.0D. The mean SE for WTR reduced from -5.60 +/- 2.37D to -0.31 +/- 0.67D at 2 months and -0.38 +/- 0.70D at 12 months; the mean cylinder improved from -1.90 +/- 1.10D to -0.31 +/- 0.39D and -0.36 +/- 0.43D, respectively. Eyes with oblique astigmatism also improved from a mean SE of -5.8 +/- 3.4 D to -0.82 +/- 1.50D and -0.69 +/- 1.15D and a cylinder of -1.4 +/- 0.73D to -0.17 +/- 0.33D at 2 months and -0.1 +/- 0.32D at 12. For ATR, the mean SE improved from -4.0 +/- 1.8D to -0.08 +/- 0.22D and -0.04 +/- 0.12D; and the mean cylinder from -1.25 +/- 0.53 to -0.02 +/- 0.09D -0.08 +/- 0.21D at 2 and 12 months, respectively. There were statistically significant improvements in SE, manifest sphere and cylinder refraction, and UDVA and CDVA scores for each cylinder type at 2 months with ATR cylinders having better outcomes. Although missing data limited interpretation at one year, differences were maintained. The magnitude of error calculations suggests that WTR was more prone to under-correction, particularly for high astigmatism (>1.5D). SMILE for myopic astigmatism reliably corrects SE, irrespective of the subtype of astigmatism.