How do I interpret a corneal topography report?

How to Interpret a Corneal Topography Report

Corneal topography interpretation requires systematic evaluation of curvature maps, elevation maps, and pachymetry data to identify patterns of corneal disease, particularly ectasia, irregular astigmatism, and surgical planning considerations. 1

Essential Components to Evaluate

Topographic Power/Curvature Maps

• Look for corneal steepening patterns: Normal corneas typically measure less than 46.0 diopters (D); values exceeding 46.0 D suggest possible ectasia 1
• Calculate the I/S ratio (inferior to superior corneal power): An I/S ratio of 1.2 or greater indicates inferior steepening typical of keratoconus 1
• Assess radial axis skewing: Skewing greater than 21° from the expected pattern is characteristic of keratoconus 1
• Identify the pattern of astigmatism: Regular astigmatism appears as symmetric bow-tie patterns, while irregular astigmatism shows asymmetric or distorted patterns 1
• Note the limitation: Placido-based systems only capture approximately 60% of the corneal surface and may miss peripheral pathology like pellucid marginal degeneration 1

Elevation Maps (Anterior and Posterior)

• Search for isolated islands of elevation: These appear on either the anterior surface, posterior surface, or both, and are highly suggestive of keratoconus or post-refractive surgery ectasia 1
• Prioritize posterior elevation analysis: Posterior elevation mapping has relatively high sensitivity and specificity for detecting keratoconus, though less reliable for subclinical disease 1
• Evaluate the reference surface: Elevation maps compare the actual corneal surface to a best-fit sphere or toric surface; deviations indicate abnormal corneal shape 2

Pachymetry (Thickness) Maps

• Identify the location of thinnest point: In keratoconus, maximum protrusion typically occurs at the area of maximal thinning 1
• Distinguish pellucid marginal degeneration: The area of maximal protrusion is superior to the band of thinning, with an inferior band of thinning separated 1-2 mm from the limbus 1
• Assess pachymetric pattern regularity: Loss of regular isopachs (contour lines of equal thickness) and displacement of the thinnest point suggest pathology 1
• Consider measurement variability: OCT measurements are systematically 7-26 μm lower than ultrasound measurements; different devices are not directly comparable 1

Clinical Context Integration

For Refractive Surgery Screening

• Comprehensive anterior and posterior surface evaluation is mandatory to screen for forme fruste keratoconus and prevent post-LASIK ectasia 1, 3
• Slit-scanning and Scheimpflug systems are necessary for proper refractive surgery candidate screening 1
• Evaluate corneal higher-order aberrations (HOAs): Significant HOAs predict poor outcomes with premium IOLs and multifocal lenses 4

For Irregular Astigmatism Assessment

• Quantify topographic irregularity: Normal eyes average 0.4 diopters of irregularity, while advanced keratoconus shows 3.0 diopters or more 5
• Determine if irregular astigmatism is correctable: Improvement with rigid gas permeable (RGP) lens trial suggests surface irregularity as the primary cause of reduced vision 1
• Assess impact on visual function: Topographic irregularity provides a measure of irregular astigmatism that keratometry cannot capture on abnormal corneas 5

For Premium IOL Selection

• Screen for corneal shape abnormalities: Post-refractive surgery corneas require appropriate IOL power calculation formulae 4
• Verify regular astigmatism for toric IOLs: Check both regular astigmatism magnitude and absence of significant HOAs before toric IOL implantation 4
• Confirm spherical aberration is within normal range: Before implanting aspheric IOLs, ensure corneal spherical aberration is not below normal 4
• Exclude multifocal IOLs in eyes with significant HOAs: Patients should be counseled about the effect of corneal HOAs on postoperative visual function 4

Common Pitfalls to Avoid

• False-positive keratoconus patterns: Eyes with positive angle kappa may show asymmetric bow-tie patterns on Placido-based topography that falsely suggest keratoconus 1
• Incomplete peripheral assessment: Relying solely on Placido-based systems misses peripheral pathology; use Scheimpflug or slit-scanning for complete evaluation 1
• Ignoring posterior surface: Anterior surface analysis alone has insufficient sensitivity for subclinical ectasia detection 1
• Comparing measurements across devices: Different technologies (ultrasound, OCT, Scheimpflug) produce systematically different values that cannot be directly compared 1

Advanced Considerations

• Epithelial thickness mapping: In ectasia, the epithelium remodels over stromal irregularities, creating a characteristic "donut-shaped" pattern with thinning at the cone and peripheral thickening 1
• Serial monitoring: Topography is essential for following disease progression in keratoconus, post-surgical ectasia, and corneal transplants 6, 3, 2
• Focal posterior surface depression: On posterior elevation maps, this finding helps predict prognosis in Fuchs endothelial corneal dystrophy 1