This study followed the tenets of the Declaration of Helsinki and Local Ethics Committee approved the methodology. We retrospectively reviewed the records of 133 right eyes of 133 patients with a confirmed diagnosis of keratoconus (keratoconus group) and 101 right eyes of 101 healthy controls (control group) aged between 10-40 years. Both keratoconus and control groups were divided into two age subgroups as pediatric (age< 18 years) and adult (age≥ 18 years). In the keratoconus group, age subgroups were matched with each other in terms of gender and keratoconus stage.
Inclusion criteria for the keratoconus group were biomicroscopic examination and corneal topography consistent with keratoconus according to the criteria of Collaborative Longitudinal Evaluation of Keratoconus Study Group, 11 inferiorsuperior (I-S) keratometric asymmetry >1.5, skewing of the steepest radial axes above and below the horizontal meridian, stromal thinning, Fleischer's ring, Munson's sign, Rizzuti's phenomenon and scissoring reflex on dilated retinoscopy.
The control group consisted of age- and gender- matched healthy subjects with normal ophthalmological examination and corneal topography (except regular astigmatism, if any).
Eyes with subclinical or form fruste keratoconus, history of prior corneal surgery, trauma or scarring were excluded from the study.
In the keratoconus group, disease severity was graded according to the Amsler-Krumeich Classification System 12 as follows;
Stage 1: Eccentric steepening; myopia, induced astigmatism, or both <5.00 D; mean central K < 48 D
Stage 2: Myopia, induced astigmatism, or both from 5.00 to 8.00 D; mean central K readings < 53.00 D; absence of scarring; corneal thickness >400 microns
Stage 3: Myopia, induced astigmatism, or both from 8.00 to 10.00 D; mean central K readings >53.00 D, absence of scarring; corneal thickness 300 400 microns
Stage 4: Refraction not measurable; mean central K readings >55.00 D; central corneal scarring, corneal thickness < 200 microns
All patients underwent detailed ophthalmological examinations included best corrected distance visual acuity (CDVA, including contact lens) measurement with Snellen charts, slit-lamp biomicroscopic examination, applanation tonometry, dilated fundus examination (with +90 D lens) and retinoscopy. Eyes with stage 4 keratoconus were not included into the study to prevent potential effect of corneal scarring on the Scheimpflug measurements.
A single experienced technician performed the anterior segment measurements using the Oculus Pentacam (Oculus Optikgerate GmbH, Wetzlar, Germany). Images were captured in automatic mode under scotopic conditions with undilated pupils. A single test with the highest quality score (over 95%) was used for the statistical analysis.
Anterior chamber depth (ACD, distance from the corneal endothelium to the anterior lens surface with undilated pupil along the optical axis), anterior chamber volume (ACV, calculated from the corneal endothelium to the anterior lens surface with undilated pupil in a 12 mm diameter around the corneal apex), anterior chamber angle (ACA), pupil-center pachymetry, apical pachymetry, thinnest pachymetry, corneal volume (CV) and Kmax were obtained for each eye.
The differences in Scheimpflug measurements between pediatric and adult subgroups were investigated within the keratoconus and control groups separately. Pediatric vs. pediatric and adult vs. adult comparisons were also performed regarding the Scheimpflug parameters between the keratoconus and control groups. Moreover, in each keratoconus subgroup (pediatric and adult), we sought for differences in the Scheimpflug measurements based on the keratoconus stage.
The sample size in this study was calculated using the PASS software version 11.0.1 (NSCC, LLC, Utah, USA). Statistical analysis was performed with the Statistical Package for Social Sciences software version 16.0 (SPSS Inc, Chicago, IL, USA). Results were expressed as mean ± standard deviation (SD). The differences in gender and keratoconus stage between the two age subgroups were analyzed using the Chi Square test. An independent samples t test was used to analyze the differences regarding quantitative variables (Scheimpflug system measurements) between the pediatric and adult subgroups in the keratoconus and control groups.
However, a Mann-Whitney U test (nonparametric) or an independent samples t test (parametric) was performed to compare two independent subgroups [Pediatric vs. pediatric and adult vs. adult comparisons (keratoconus vs. control groups), and comparisons between eyes with stage 2 and 3 keratoconus] in terms of quantitative variables (Scheimpflug parameters).
Pearson correlation coefficients were used to analyze relations between age and Scheimpflug parameters. A p value less than 0.05 was considered statistically significant at 95 % confidence interval.