What Is Corneal Topography?
While keratometry measures only the central 3 mm of the cornea at two points, corneal topography maps curvature and power across the entire anterior corneal surface. This comprehensive assessment detects irregularities, asymmetries, and subtle shape changes that keratometry cannot identify. It is an indispensable tool for refractive surgery planning, keratoconus detection, and complex contact lens fitting.
How Corneal Topography Works
The most common type of topographer uses Placido disc technology. The device projects a series of concentric illuminated rings (Placido rings) onto the cornea. A camera captures the reflected ring pattern, and software analyzes how the rings distort compared to a perfect sphere.
- Evenly spaced, circular rings indicate a smooth, spherical cornea
- Closer-together rings indicate steeper areas (higher power)
- Farther-apart rings indicate flatter areas (lower power)
- Irregular or wavy rings indicate surface irregularities
The software converts ring spacing data into a color-coded topographic map that displays power or curvature values across the corneal surface.
Reading Topographic Maps
Understanding the color scale is essential for interpreting topographic maps:
- Warm colors (red, orange, yellow): Represent steeper curvature and higher dioptric power
- Cool colors (blue, green): Represent flatter curvature and lower dioptric power
The scale typically covers a range from about 35 D to 55 D, with each color band representing a specific dioptric interval (commonly 0.5 or 1.5 D per step).
Normal Topographic Patterns
- Round (spherical): Uniform color across the map, indicating minimal astigmatism
- Oval: Slightly elongated pattern, typical of low regular astigmatism
- Symmetric bowtie: Classic pattern of regular astigmatism with symmetrical steep and flat meridians
- Asymmetric bowtie: May suggest early corneal ectasia and warrants further evaluation
Clinical Applications
Keratoconus Detection
Keratoconus is a progressive corneal ectasia (thinning and bulging) that produces characteristic topographic findings:
- Inferior or inferotemporal steepening (a "hot spot" of red/orange below center)
- Asymmetric bowtie pattern
- Steep K values exceeding 47-48 D
- Large difference between superior and inferior curvature
Topography can detect keratoconus before clinical signs become apparent on slit-lamp examination, making it critical for screening refractive surgery candidates.
Refractive Surgery Planning
Before LASIK, PRK, or other refractive procedures, topography confirms that the corneal shape is regular and symmetric. Post-surgically, it documents the treatment zone and detects complications like decentered ablations or central islands of untreated tissue.
Contact Lens Fitting
Topography guides fitting decisions for specialty lenses in irregular corneas, helping determine lens diameter, base curve, and vault parameters for scleral or GP lenses.
Topography vs. Keratometry
| Feature | Keratometry | Topography |
|---|---|---|
| Coverage area | Central 3 mm only | Entire anterior cornea |
| Measurement points | 2 meridians | Thousands of points |
| Irregularity detection | Limited (mire quality) | Excellent (color maps) |
| Output format | Numerical values | Color-coded maps |
| Keratoconus screening | Late-stage only | Early detection possible |
Key Takeaways
- Corneal topography maps curvature across the entire anterior corneal surface using Placido ring technology
- Warm colors (red/orange) represent steep areas; cool colors (blue/green) represent flat areas
- An asymmetric bowtie with inferior steepening is the classic early keratoconus pattern
- Topography is essential for refractive surgery screening, keratoconus detection, and specialty contact lens fitting
- Always check the color scale before interpreting any topographic map