Keratoconus Evaluation: How We Diagnose and Stage It

Who should be evaluated

Anyone with a prescription that keeps changing, particularly increasing astigmatism in the teens or twenties; anyone whose glasses never seem quite right or who sees ghosted, doubled or smeared images; anyone told at a LASIK screening that their corneal map was abnormal; and relatives of patients with keratoconus. Eye rubbing, allergies, Down syndrome and connective tissue conditions raise the likelihood.

The measurements

• Corneal tomography: a rotating camera images the cornea in cross section and builds maps of front curvature, back curvature and thickness at thousands of points. Keratoconus shows as localized steepening, usually below center, with matching thinning and a steepened back surface. This is the definitive test and the one that detects early disease before vision is affected
• Pachymetry: the thinnest point of the cornea, tracked over time
• Refraction: the prescription, including irregular astigmatism that glasses cannot fully correct, and the best vision achievable with glasses versus a rigid lens
• Slit lamp examination: for the signs of established disease, such as stromal thinning, a ring of iron deposition, fine stress lines and scarring
• Biomechanical assessment where available, which measures how the cornea deforms under a puff of air

Staging

Keratoconus is staged from the tomography: the steepest curvature, the thickness at the thinnest point and the degree of irregularity, alongside the vision achievable with glasses. Early disease may be detectable only on the maps. Moderate disease reduces glasses-corrected vision and usually needs specialty lenses. Advanced disease shows marked thinning and scarring. Staging sets expectations and guides the choice of lens.

Progression: the question that matters

Cross-linking is recommended for keratoconus that is progressing, and progression is judged by comparing tomography over time: an increase in the steepest curvature, a decrease in thickness, or a worsening of the back surface beyond the measurement's normal variation. Young patients are re-measured every three to six months, because they progress fastest; older patients annually. Where previous maps exist from another practice, bringing them speeds the decision. In young patients with clear early disease, the cornea specialist may discuss cross-linking before progression is fully documented, because progression is common at that age; insurance coverage generally follows documented progression.

Before you come

Rigid gas permeable and scleral lenses mold the cornea and usually need to be out for two to three weeks before tomography, soft lenses for a few days. Wear glasses. Bring your previous prescriptions and any earlier corneal maps.