Analysis of total corneal astigmatism with a rotating Scheimpflug camera in keratoconus

Background: To analyze mean corneal powers and astigmatisms on anterior, posterior, and total cornea in patients with keratoconus as calculated according to various keratometric measurements using a Scheimpflug camera. Methods: We examined the left eyes of 64 patients (41 males and 23 females; mean age 29.94 +/- 6.63 years) with keratoconus. We measured simulated K (Sim-K), posterior K, true net power (TNP) and four types of total corneal refractive powers (TCRP). We then used the obtained values to analyze mean K, and corneal astigmatism. TCRP were measured at 2.0 +/- 5.0 mm. Results: Mean corneal powers from Sim K, posterior K, and TNP were 49.12 +/- 3.99, - 7.39 +/- 0.79, and 47.78 +/- 4.09 diopters, respectively. For TCRP centered on the pupil, mean K tended to decrease with measurement area (all p < 0.01). While, both mean K and astigmatism measured using TCRP centered on the apex decreased with measurement area (all p < 0.001). TCRP centered on the apex were greater than those centered on the pupil for mean K values calculated using TCRP (all p < 0.001). The proportion of WTR was greatest on the anterior and total cornea. As the measurement area moved to the periphery, the proportion of WTR increased. Conclusions: Mean corneal powers and astigmatisms on total cornea with keratoconus change depending on calculation methods and measurement areas.