Interpreting Corneal Topography: Axial, Tangential & Elevation Maps for Keratoconus

A comprehensive Scheimpflug topography report (the Quad Map) displays four simultaneous viewpoints:

1. Axial (Sagittal) Curvature Map: Calculates curvature relative to the center optical reference axis. It provides a smoothed, globally averaged overview of corneal power, ideal for identifying regular vs. irregular astigmatism.
2. Tangential (Instantaneous) Curvature Map: Calculates local surface curvature independently at every microscopic coordinate without center-referencing. It provides razor-sharp edge resolution, pinpointing the exact geographical apex and boundary of a keratoconic cone.
3. Anterior Elevation Map: Measures the physical height in microns of the corneal front surface compared to a mathematical reference sphere (Best Fit Sphere, BFS). Warm red zones indicate tissue bulging above the sphere ($> +12\ \mu\text{m}$ is suspicious).
4. Posterior Elevation Map: Measures the back surface of the cornea. Keratoconus almost always manifests on the posterior cornea first! Elevation $> +15\text{ to } +18\ \mu\text{m}$ confirms early ectasia.

Topographic patterns reveal specific corneal diseases:

• Symmetric Bowtie: Normal regular corneal astigmatism. The lobes of the bowtie are equal in size and aligned $180^\circ$ opposite each other.
• Asymmetric Bowtie with Skewed Radial Axes (AB/SRA): The hallmark of early Keratoconus. The inferior lobe is significantly hotter and larger than the superior lobe, and the axes are angled.
• 'Crab-Claw' or Butterfly Pattern: Classic for Pellucid Marginal Degeneration (PMD), showing severe inferior peripheral band thinning between 4 and 8 o'clock.

The report lists Simulated Keratometry metrics:

$$K_1 \text{ (Flat K)} \quad | \quad K_2 \text{ (Steep K)} \quad | \quad K_{\max} \text{ (Maximum Peak Diopters)}$$

Normal corneas exhibit $K_{\max}$ between $42.00\ \text{and } 45.00\ \text{D}$. A $K_{\max} > 47.00\ \text{D}$—or an inferior-superior asymmetry ($I-S$ value) $> 1.4\ \text{D}$—is a strong indicator of ectasia requiring clinical monitoring.