Is accelerated corneal cross-linking for keratoconus the way forward? Yes or No

Sir,

While I congratulate the Journal for encouraging such interesting debates and the authors for their hard work in presenting their points of view, I feel it is necessary to point out two inaccuracies presented and repeated in both articles.^{1, 2}

The first is equating the degree, depth, and safety of cross-linking to the depth of the demarcation line. There is currently no evidence to support this direct correlation. The so-called stromal demarcation line, first described by Seiler and Hafezi,³ can be easily delineated by anterior segment optical coherence tomography, has been shown to possibly be shallower in older patients and those with more severe ectatic disease.⁴ It has been found to be thicker centrally and thinner peripherally⁵ and possibly related to an increased density of the extracellular matrix.⁶ Although a deeper demarcation line has been associated with a larger decrease in corneal thickness,⁷ its depth has not been shown to be correlated to either visual or keratometric changes at 6 months post-operatively.⁴ It may simply represent natural wound healing responses rather than delineate the true area between cross-linked and uncross-linked tissue. Clearly a lot more research is required to ascertain the true nature of this demarcation line and its relationship with the actual cross-linking process.

Finally, in both articles it is stated that keratoconus in its early stages is a posterior corneal disease. Although posterior corneal curvature changes can indeed be detected before anterior alterations in sub-clinical disease, this is almost certainly due to the epithelium masking early anterior changes. This has been elegantly demonstrated by Reinstein et al⁸ using high-resolution ultrasound.