How is Keratoconus Diagnosed? Understanding the Tests

Because the early symptoms of keratoconus, like blurred vision and light sensitivity, mimic common refractive errors, the condition is often misdiagnosed as simple astigmatism. A standard "Which is better, one or two?" eye exam is often not enough to catch the subtle thinning of the cornea in its early stages.

To accurately diagnose keratoconus, eye care professionals use a suite of specialized, non-invasive imaging technologies. These tests allow doctors to see the structure of your eye in three dimensions, catching the "cone" long before it's visible to the naked eye.

The Gold Standard: Corneal Topography

Corneal Topography is the most critical tool in a specialist's office. It acts like a GPS for the eye, creating a detailed 3D map of the surface of the cornea.

• How it works: A computer projects a series of lighted rings (Placido discs) onto the surface of the eye. A camera measures how these rings are reflected back.
• What it shows: If the cornea is perfectly round, the rings look like perfect circles. If the cornea is bulging, the rings become distorted. The computer then translates this data into a color-coded "heat map."
• Interpreting the map: Cool colors (blues and greens) represent flatter areas, while warm colors (reds and oranges) indicate the steep, "coning" areas characteristic of keratoconus.

Corneal Pachymetry: Measuring Thickness

Since the hallmark of keratoconus is the thinning of the corneal tissue, measuring the thickness is essential. This test is called pachymetry.

A normal cornea is usually about 540 to 560 microns thick in the center. In a keratoconus patient, the "apex" of the cone may be significantly thinner. Monitoring thickness over time is how doctors determine if the disease is progressing and if a procedure like Corneal Cross-linking (CXL) is urgently needed.

Tomography: Looking at the Back Surface

While topography looks at the front of the eye, tomography (using devices like the Pentacam or Galilei) looks at the entire thickness of the cornea, including the back surface (posterior).

This is vital because, in many patients, the back of the cornea begins to bulge before the front does. By looking at the posterior elevation, specialists can diagnose "sub-clinical" or very early-stage keratoconus that a standard topographer might miss.

Slit-Lamp Examination

During a routine exam, the doctor will use a slit-lamp, a high-powered microscope with a bright thin beam of light. While this is a standard tool, for a keratoconus patient, the doctor is looking for specific physical markers, such as:

• Fleischer Rings: Tiny, brownish-yellow rings caused by iron deposits in the corneal tissue.
• Vogt's Striae: Small, vertical stress lines in the cornea caused by the stretching and thinning of the tissue.
• Corneal Scarring: Any visible opacities caused by the peak of the cone rubbing against the eyelid or previous "hydrops" events.

Epithelial Mapping

A newer diagnostic tool involves mapping the epithelium, which is the very thin outer "skin" of the cornea. The body often tries to compensate for the irregular shape of keratoconus by growing more epithelial cells in the "valleys" and thinning them out over the "peaks." Measuring this compensation helps doctors confirm a diagnosis in very borderline cases.

Conclusion: The Importance of a Baseline

If you have a family history of keratoconus or are seeing the visual "red flags," getting a baseline topography map is the most important step you can take. Even if the results are normal today, having that map on file allows your doctor to compare future tests and detect the slightest changes in your corneal shape.

Early diagnosis is the difference between simply needing specialty contact lenses and needing a corneal transplant. If your vision feels "off," don't settle for a basic eye exam, ask for a topography.