Corneal Topographic versus Manifest Refractive Astigmatism in Patients with Keratoconus: A Retrospective Cross-Sectional Study

PURPOSE: To compare the subjective manifest astigmatism to the corneal topographic astigmatism in patients with keratoconus. METHODS: This retrospective study included data of 230 keratoconic eyes of 115 patients. Topographic corneal astigmatism (TA), which was measured by pentacam, was compared and correlated to manifest refractive cylinder (MRC) in terms of power, axis, vector components, and mean vector. The difference between TA and MRC was correlated to the maximum keratometric reading (Kmax) and the thinnest pachymetry location (TL), as indicators of keratoconus severity. RESULTS: There was a significant positive correlation between MRC power and TA power (p < 0.001; r = 0.58). TA power was significantly higher than MRC power (p < 0.001). A significant correlation was present between the axis of TA and MRC (r = 0.73; p < 0.001) with the axis of the MRC tending to be more vertical (more against the rule) than the axis of TA. The vector difference between TA and MRC is correlated to the Kmax (p < 0.001; r = 0.62) and TL (p < 0.001; r = 0.3). CONCLUSIONS: A significant difference is present between TA and MRC in keratoconic eyes, the power of MRC tends to be less and the axis tends to be more vertical than those of TA; this difference increases as keratoconus becomes more advanced.