INVESTIGATION OF THE USE OF CORNEAL SUBBASAL NERVE LENGTH AND TORTUOSITY TO DETECT FORME FRUSTE KERATOCONUS AND STAGING KERATOCONUS: AN IN VIVO CORNEAL CONFOCAL MICROSCOPY STUDY

To investigate whether corneal subbasal nerve length (CSNL) and corneal subbasal nerve tortuosity (CSNT) measured by in vivo corneal confocal microscopy (IVCM) could detect forme fruste keratoconus (FFKC) and classify KC severity.

University of Health Sciences, Dışkapı Yıldırım Beyazıt Training and Research Hospital, Ankara, Turkey.

The IVCM (HRT 3Rostock Cornea Module, Heidelberg Engineering, Germany) and corneal topography (Sirius; Costruzione Strumenti Oftalmici (CSO), Florence, Italy) measurements of 14 eyes of 14 KC patients, 4 eyes of 4 FFKC patients, and as a control group 11 eyes of 11 healthy patients were evaluated. Patients in the control group had no systemic and ocular pathology other than myopia and astigmatism. KC was classified according to the modified Amsler-Krumeisch classification (stage 1, KC1: 5 patients, stage 2, KC2: 5 patients, stage 3, KC3: 4 patients). CSNL and CSNT were calculated using the 3 clearest non-overlapping images obtained from IVCM with the help of Image J and its plugin Neuron J open-source software. The CSNT was evaluated by calculating the amplitude of the curve (CSNTA) and the area under the curve (CSNTAUC) formed by the CSN. After checking the normality of the groups with the Shapiro-Wilk test and the homogeneity of the variances with the Levene test, the groups were compared with one-way ANOVA (Welch or Fischer) and Kruskal-Wallis tests. The post-hoc Tukey and Games-Howell tests were used to investigate pairwise differences within the groups.

The mean ages of the control group, the FFKC group and the KC group were 22.9±4.30, 21.3±1.71, and 22.6±3.61 years, respectively (p=0.47). The gender distribution was not different between the groups (p=0.27). CSNL was statistically different between the control group and the FFKC group (p=0.03). Furthermore, it was different between the FFKC group and the KC group (p<0.001). CSNTAUC was also statistically different both between FFKC and the control group and between FFKC and KC group (p<0.001). When the control group, FFKC group, and KC1 group were compared, CSNL, CSNTA, and CSNTAUC were different among the groups (p<0.001, p=0.003 and p<0.001, respectively). When the KC1, KC2 and KC3 groups were compared, the pairwise comparisons of the groups were not statistically different with respect to CSNL, CSNTA, and CSNTAUC.

CSNL and CSNT obtained from IVCM could be useful tools to determine FFKC patients. However, they were found to be insufficient to classify the KC severity.