What Should the Earliest Age be for Clinical Trials of Corneal Crosslinking for Keratoconus?

Keratoconus is often diagnosed in the second or third decade of life, with a younger mean age at diagnosis more likely among those of Middle Eastern and Asian descent (1). Studies have shown that patients with severe forms of keratoconus present at a younger age (usually in the second decade of life), and these patients have more rapid progression of disease (2-6). Pediatric keratoconus is generally attributed to disease manifesting in those less than 18 years of age, however studies that looked at progression in different age groups used varying age criteria. Léoni-Mesplié et al. found that at diagnosis, keratoconus is often more advanced in children (defined as ?15 years) than in adults (?27 years), while Tuft et al. found that patients ?18 years at time of diagnosis progressed to transplantation faster than patients >18 years of age (5-6). McMahon et al. found that the rate of change in corneal curvature was substantially greater in patients <20 years old and slowed down dramatically in those ?20 years old (7). Until the late 1990s when corneal crosslinking (CXL) was initially developed (8) there were no effective means to halt or slow progression, and keratoplasty was the definitive treatment. There was some assertion in the literature that certain contact lenses and intracorneal rings may help slow progression, but no definitive evidence was ever presented (9-11). Ring segments have been shown to improve best corrected visual acuity as well as contact lens tolerance, but do not alter progression of disease (9, 11).