title: Internal astigmatism and its correlation to corneal and refractive astigmatism.
abstract: OBJECTIVE
  To evaluate the internal astigmatism and its relationship to corneal and refractive astigmatism in a refractive surgery patient population.
  
  
  METHODS
  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).
  
  
  CONCLUSIONS
  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.
authors.affiliation: Department of Ophthalmology, Faculty of Medicine, Siriraj Hospital, Mahidol University, Bangkok 10700, Thailand.
authors.forename: Sabong
authors.initials: S
authors.lastname: Srivannaboon
mesh_terms: D000328:Adult; D001251:Astigmatism / Q000175:diagnosis* / Q000601:surgery*; D015331:Cohort Studies; D003315:Cornea / Q000503:physiopathology; D005260:Female; D005500:Follow-Up Studies; D006801:Humans; D020731:Keratomileusis, Laser In Situ / Q000379:methods*; D008297:Male; D008875:Middle Aged; D009216:Myopia / Q000175:diagnosis / Q000601:surgery*; D011183:Postoperative Complications; D011184:Postoperative Period; D011300:Preoperative Care; D011336:Probability; D012029:Refraction, Ocular; D012044:Regression Analysis; D018570:Risk Assessment; D012720:Severity of Illness Index; D016896:Treatment Outcome; D014792:Visual Acuity