Abstract
Purpose: :
To determine the ranges of Corneal Resistance Factor (CRF), Corneal Hysteresis (CH), among the Mexican population with different stages of keratoconus.
Methods: :
Measurements of CH and CRF with the Ocular Response Analizer (ORA) were taken in patients with clinical and topographic diagnosis of Keratoconus who consulted the Cornea or Refractive Surgery Clinic at the Instituto de Oftalmologia Fundación Conde de Valenciana during the first 6 months of 2007. Four measurements were taken in each eye and the average was reported. The keratoconus were ranked from I to IV, according to the Amsler-Krumeich classification. Those patients with topographic images of keratoconus but with keratometric values of astigmatism or pachimetry under the I stage of this classification where considered subclinical. We used Kruskal-Wallis One Way Analysis of Variance on Ranks, for statistical analysis (ANOVA), and Dunn's Method for Pairwise Multiple Comparison.
Results: :
Data from 103 eyes of 62 patients with diagnosis of keratoconus, with a mean age of 26.98 years. There were 7 eyes with form frustre keratoconus, with mean CRF of 8.30 mmHg (6.87-10.19), and CH of 7.95 mmHg (6.78-9.24). Eyes with keratoconus stage I were 23, with mean values of CRF 7.07 mmHg (5.99-7.72), and CH 6.875 mmHg (6.26-7.85). There were 32 eyes with keratoconus stage II, and mean values of CRF 6.30 mmHg (5.48-6.96) and CH 6.81 mmHg (5.76-7.44). 19 Eyes had Keratoconus stage III, with mean values of CRF of 6.60 mmHg (5.06-7.53), and CH 7.55 mmHg (6.30-8.51). Keratoconus stage IV was found in 22 eyes with mean values of CRF 4.45 mmHg (3.40-7.15) y CH 5.91 mmHg (4.82-7.64). (P= <0.001). When we compared the CRF and CH from the different groups of keratoconus with a normal Mexican population, we found statiscally significance difference with all single groups ( p<0.05) except with subclinical keratoconus.
Conclusions: :
The average of CH and CRF values are reduced in patients with keratoconus compared to the normal Mexican population. Ranking the keratoconus in stages gave higher values of CRF and CH in subclinical and stage I Keratoconus. Stage II keratoconus CH mean values are less than Stage III mean values because 3 cases in the latter group were abnormally high, outside the mean. Even though the tendency showed lower values of CRF and CH as the keratoconus was more advanced. Curiously, within keratoconus stage IV, 3 cases with abnormally high CRF and CH levels were found, related with corneas with central dense scars. When we compared groups there were no differences between Normal population and form frustre, due to the small amount of cases in the latter.