The Epithelial Thickness Ratio – A New Parameter For Quantitative Evaluation Of Ectatic Changes Based On Oct Corneal Epithelial Maps

The purpose of our study was to a) develop an objective parameter for evaluating epithelial thickness changes in keratoconus and b) assess this parameter in eyes with borderline tomographic maps and to assess its agreeability with BAD-D scores.

Patients undergoing refractive surgery with a single surgeon in a Canadian ophthalmology practice during a 12-month period were included provided they met inclusion criteria.

Phase one consisted of characterizing and validating the epithelial thickness ratio (ETR) – a parameter to evaluate epithelial corneal thickness centered around the thinnest point of the cornea using tomographic (Pentacam) and epithelial maps of patients with confirmed keratoconus vs patients with normal corneas. The second phase examined the ETR in patients with borderline BAD-D scores (1-3). Analyses were conducted based on two systems of categorization. First, ETR was compared between eyes with BAD-D scores of ≤ 1.6 (presumably normal) and > 1.6 (presumably ectatic). Second, ETR was also compared between eyes diagnosed as ectatic vs normal based on expert analysis of tomographic and epithelial maps (blinded to ETR and BAD-D score).  

Phase one revealed that a radius of 2mm for 4 points centered around the thinnest point of the cornea was the most specific and sensitive based on the analysis of 93 eyes (Total AUC 94.7%). In phase two, the ETR was calculated for 277 corneas with BAD-D scores between 1-3. Pearson correlation revealed no statistical significance between ETR and BAD-D score.  Unpaired t-tests were conducted to examine ETR and BAD-D score. ETR was not significantly different in eyes with BAD-D scores ≤ 1.6 vs >1.6 (p > 0.05). However, ETR was found to be significantly higher in eyes deemed to be suspicious for keratoconus by expert opinion compared to those deemed normal (p = 0.0038)

In the field of refractive surgery, early diagnosis of keratoconus is critical to identify patients at risk of post-operative ectasia. It is a challenge to ascertain a diagnosis of ectasia in eyes with suspicious and borderline tomographic data. As such, epithelial thickness mapping has become useful as an adjunct diagnostic tool. To our knowledge, we have developed the first objective parameter, the ETR, to assess changes in epithelial thickness centered around the thinnest point of the cornea to assist refractive surgeons in detecting early cases of keratoconus.  This parameter may be of special relevance in cases with borderline BAD-D scores.