Corneal higher order aberrations: a method to grade keratoconus.

PURPOSE: To use the anterior corneal surface higher order aberrations as a tool to detect and grade keratoconus using corneal map analysis videokeratoscopy.

METHODS: A prospective observational comparative study of 80 eyes was performed. The eyes were divided into two groups. Group A comprised 40 eyes of 20 asymptomatic individuals with no ocular pathology. Mean sphere was -0.03 diopters (D) (range: +0.75 to -0.75 D), mean cylinder was -0.27 D, mean average K was 43.28 D, and mean uncorrected visual acuity (UCVA) was 1.01. Group B comprised 40 eyes of 25 patients with keratoconus. Mean sphere was -3.70 D (range: +2.00 to -10.00 D), mean cylinder was -3.82 D, mean average K was 49.29, and mean best spectacle-corrected visual acuity (BSCVA) was 0.61.

RESULTS: In group A, mean root-mean-square (RMS) of spherical (Z4 and Z6), coma-like (Z3, Z5, and Z7), and higher order aberrations (Z3-7) were 0.38 microm, 0.35 microm, and 0.52 microm, respectively. In group B, mean RMS of spherical, coma-like, and higher order aberrations were 1.06 microm, 2.90 microm, and 3.14 microm, respectively, for a 6.0-mm simulated pupil diameter. Mean RMS differences between the two groups were 0.68 microm (P < or = .0002), 2.55 microm (P < or = .0001), and 2.61 microm (P < or = .0001) for spherical, coma-like, and total higher order aberrations, respectively. In group B, according to Amsler-Krumeich classification, the mean RMS of coma-like aberration was 1.87 microm in grade I (14 eyes), 2.97 microm in grade II (11 eyes), 3.46 microm in grade III (12 eyes), and 5.20 microm in grade IV (3 eyes).

CONCLUSIONS: Corneal higher order aberrations, especially coma-like aberrations, are significantly higher in eyes with keratoconus than normal eyes. Coma-like aberrations, with the aid of a corneal aberrometry map, are good indicators for early detection and grading of keratoconus.