Progressive inferior keratoconus in a patient misdiagnosed as pellucid marginal corneal degeneration

A patient in their early 40s presented with a bilateral gradual painless decrease in vision. Best-corrected visual acuity (BCVA) was 20/40p OD and 20/20 OS. Tomography using Pentacam (OCULUS) demonstrated a bilateral claw-like appearance on keratometry maps with inferior peripheral thinning, suggestive of pellucid marginal corneal degeneration (PMCD). Given the patient's age is more typical for PMCD and the crab-claw pattern, a diagnosis of PMCD was made. Vision and tomography were stable over 2 years. At year 3, OD developed Vogt's striae and Fleischer's ring with tomography showing increased inferior steepening and loss of the characteristic vertical 'flat' band between inferior lobes; BCVA was 20/50 OD and 20/20p in OS. The diagnosis was revised to inferior keratoconus (KC) OD and epithelium-off collagen cross-linking (CXL). Progressive inferior KC can present in the fifth decade of life, and a case of early inferior KC at this age is likely to get misdiagnosed as PMCD due to the crab-claw appearance.