Internal astigmatism and its correlation to corneal and refractive astigmatism

Objective: To evaluate the internal astigmatism and its relationship to corneal and refractive astigmatism in a refractive surgery patient population. Method: Patients who underwent pre-operative evaluation for Laser in situ Keratomileusis (LASIK) at Excimer Laser Clinic, Siriraj Hospital, Mahidol University were included. Pre-operative data including age, manifest refraction and corneal topography obtained by Orbscan Corneal Topography were performed. Regression Analysis was done to find the correlation between refractive and corneal astigmatism. Results: 110 patients (220 eyes: 110 right eyes and 110 left eyes) were included. The mean age was 31.14 ± 7.00 year (range 19 to 48 years). The mean astigmatism measured by manifest refraction (refractive astigmatism) was 0.76 ± 0.72 diopters (range 0 to 3.50 diopters). The mean astigmatism measured by Orbscan Corneal Topographer (corneal astigmatism) was 1.38 ± 0.72 diopters (range 0.20 to 5.30 diopters). The mean difference in magnitude of refractive and corneal astigmatism (internal astigmatism) was 0.62 ± 0.67 diopters (range 0 to 3.00 diopters) and 74 per cent were within ± 1.00 diopters difference. The mean difference in axis of astigmatism was 0.95 ± 23 degree (range 0 to 85 degree) and 79.6 per cent were within ± 15 degree difference. There was low correlation between corneal and internal astigmatism (R² = 0.12, p < 0.05), also low correlation between refractive and internal astigmatism (R² = 0.27, p < 0.05). There was a statistically significant difference between magnitude of corneal and refractive astigmatism (p < 0.05) but no difference in the axis of astigmatism (p = 0.55). Conclusion: This study demonstrated non-mutual agreement between refractive and corneal astigmatism (presence of internal astigmatism). High value (> 1.00 diopter) of internal astigmatism was demonstrated in 1/3 of the cases. Kerato-refractive surgery that attempts to correct refractive astigmatism at corneal plane may effect long-term evaluation of the astigmatism.