Detection of SCKC/FFKC has always been a challenge for ophthalmologists, especially when there are no clinical signs or symptoms in the patient.
The rotary camera Scheimpflug (Pentacam®) 13, 14, 20, 23, 25, 26, 30, has been used to diagnose keratoconus in daily clinical practice. The topographic parameters of clinical keratoconus are recognizable. However, it is not easy to diagnose subclinical keratoconus based on topographic variables. This study calculates a diagnostic model based on the aberrometry data of the anterior and posterior corneal surface provided by the Pentacam
The selection of the sample was made that there were no differences between the age groups12 − 15, 17, 19, 21, 24, 29, sex14, 21, 22, 29, and laterality21, 22. In Koçamis et al. study22, there were significant differences for age between keratoconus (26.19 ± 7.90) and healthy (30.88 ± 7.57).
Pupillary dilatation is a parameter modifying aberrometry results27. In this study, it was prefixed in 6 mm. In previous studies8, 12−14, 20, 23, 29. Hondur et al27 established it in 5 mm.
Many studies have been made between healthy patients with Keratoconus8, 10, 12−14, 20, 22, 24, 27 or healthy patients with SCKC/FFKC15, 17, 19, 21, 23, 26, 28−31. The purpose in most of them was to analyze the topographic parameters to find differences between a healthy patient and an incipient corneal ectasia without symptoms.
Different classification methods have been used: Amsler-Krumeich24, 27, 28, Alió and Shabayek13, 20, KISA % index21 or KSS29.
All this methodological variability leads to an outstanding selection and classification bias that it must be taken into account when making comparisons between studies.
If we analyze the refractive parameters of our study, statistically significant differences were obtained between the three groups analyzed for the sphere, the cylinder and the spherical equivalent (p < 0.05, Kruskal-Wallis), as in other studies19, 30. However, when comparing normal corneas with SCKC/FFKC, we obtained statistically significant differences only for the sphere (p = 0.012, U Mann-Whitney).
Saadand Gatinel17obtained that the mean of the sphere was significantly higher in their normal group than in their SCKC/FFKC group (p < 0.001). Reddy et al19obtained statistically significant differences for the cylinder (p < 0.001) not obtaining these differences for the sphere (p = 0.08). However, Naderanet al29found no statistically significant differences for sphere (p = 0.136) or cylinder (p = 0.108).
Statistically significant differences were observed between the visual acuity of the three groups, not existing between normal corneas and SCKC/FFKC. These values are consistent with previous studies8, 20, 22, 27, 30, 31.
A bivariate analysis has been performed between normal corneas and SCKC/FFKC. Statistically significant differences were only obtained for variables of vertical asymmetry, coma to 90º and corneal thickness (p < 0.05).
According to Bührenet al15, the anterior coma to 90º would be the most useful parameter to differentiate normal corneas from SCKC/FFKC. Other parameters such as the posterior coma to 90º and the minimum corneal thickness would not exceed the value of the anterior surface.
When the corneal coma to 90º was analyzed in absolute value, we found that it was higher in SCKC/FFKC (|-0.404| ± 0.319) than in normal (0.0123 ± 0.209), but lower than in keratoconus (|-1.877| ± 1.413). This value indicates that group 2 included those patients with a very early stage of keratoconus and that the parameter corneal coma to 90º had increased with the natural history of the disease17. The negative sign of the corneal coma to 90º refers to the lower decentration of the cone in the axis of ordinate17.
More recently, Naderanet al29 y Xu et al30 indicated the importance of posterior surface aberrations to differentiate normal SCKC/FFKC corneas. In the first study, they obtained that the values for posterior coma to 90º of the healthy group were 0.032 ± 0.363 and for the SCKC/FFKC group were 0.193 ± 0.264 with statistically significant differences between groups (p = 0.003, U Mann-Whitney). In our database, the posterior coma to 90º for normal corneas were − 0.008 ± 0.049 and for SCKC/FFKC were 0.112 ± 0.103, (p < 0.05, U Mann-Whitney).
The relationship between coma-like aberrations of the anterior surface and the degree of manifest keratoconus is well known.8, 12, 22, 24−27 Piñero et al13 were the first to attempt to characterize the posterior corneal surface and its aberrations in patients with normal corneas and keratoconus, finding results that were not concordant by the optical theory of the corneal surface.
In this study in the healthy patients were obtained values of anterior coma to 90º of 0.001 ± 0.225 and posterior coma to 90º of 0.319 ± 0.372 while in keratoconus were − 1.754 ± 0.976 and − 3.692 ± 1.81 respectively. If we analyze the results of our study, in healthy patients the anterior coma to 90º were 0.009 ± 0.200 and posterior coma to 90º were-0.008 ± 0.049, and in keratoconus, we obtained − 2.073 ± 1.513 and 0.536 ± 0.386 respectively. In our case, the anterior parameters, in absolute value, were higher than the posterior ones, which is concordant with the corneal optical theory.
For Buhrenet al15, the minimum corneal thickness was the most discriminating pachymetric parameter between normal corneas and SCKC/FFKC. However, he concluded that the posterior surface was not discriminate as to the anterior surface, and this surface was not sufficient for the diagnosis of the subclinical entity. Safarzadeh et al28 reflected that minimum corneal thickness and posterior corneal elevation would be the best parameters for differentiating suspicious keratoconus from healthy eyes. Although other authors17, 21, 30 have established binary logistic models, it is the first report of a simple diagnostic model to obtain the probability of having or not having subclinical keratoconus using parameters obtained from the Pentacam® topographer.