Table 2 Concise definitions of keratoconus severity stages [55, 56]

Stage 0 – clear cornea with normal thickness and corrected distance visual acuity (CDVA) of 6/6

Stage 1 – clear cornea with the potential presence of Fleischer's ring, mild corneal thinning evident on topography but not grossly, distorted reflex on retinoscopy, and CDVA less than 6/6

Stage 2 – Fleischer’s ring and Vogt’s striae, corneal thinning may be evident grossly, scissoring reflex on retinoscopy and CDVA below 6/12

Stage 3 – Initial manifestation of Munson's sign, significant corneal thinning with faint scarring, retinoscopy difficult to perform, spectacles distance visual acuity worse than 6/30, yet there is potential improvement to 6/6 with contact lenses

Stage 4 – corneal scarring and opacities at the apex, evident Munson's sign, retinoscopy impossible to perform, CDVA worse than 6/120 and not achieving 6/6 even with contact lenses