20.3 Corneal Topography, Scheimpflug Tomography & Keratoconus Screening

Corneal Topography, Scheimpflug Tomography & Keratoconus Screening

Core Clinical Mandate: The detection of subclinical corneal ectasia (forme fruste keratoconus) is the single most critical safety screen in refractive surgery and premium intraocular lens implantation. Performing LASIK or PRK on an undiagnosed ectatic cornea induces biomechanical collapse and progressive, vision-threatening keratoectasia. Technologists must master the optical distinction between anterior Placido reflection and full-thickness Scheimpflug tomography, elevation reference geometries, and multi-metric ectasia screening displays.

Fundamentals of Corneal Surface Evaluation: Placido vs. Scheimpflug

Clinical corneal diagnostics are broadly categorized into reflection-based topography and optical cross-sectional tomography.

Placido Disk Topography (Anterior Curvature Reflection)

• Optical Physics: A Placido disk system projects an array of alternating black-and-white concentric rings of light onto the pre-corneal tear film. The anterior tear film acts as a convex mirror, reflecting the virtual image of the rings (Purkinje image I) back into a central telecentric camera lens. Dedicated software tracks the spatial distance and distortion between consecutive ring margins.
• What Placido Measures: Placido systems calculate the slope and curvature of the anterior corneal surface exclusively.
• Inherent Limitations of Placido Topography:
  1. Blind to the Posterior Cornea: Placido reflection cannot capture light returning from the posterior corneal surface. Because early keratoconic ectasia manifests on the posterior cornea long before the anterior surface deforms, Placido systems frequently miss early subclinical ectasia.
  2. Tear Film Dependency: The reflected image is formed by the pre-corneal tear film, not the stroma. Dry eye disease, tear film break-up, or mucus plaques generate severe mire distortion, producing pseudokeratoconic steepening artifacts.
  3. No True Pachymetric Mapping: Cannot measure corneal thickness distribution from limbus to limbus.
  4. Data Dropout in Severe Irregularity: Highly distorted or scarred cone apices cause mire crossover and loss of reflection, preventing data acquisition at the most critical anatomical zone.

Scheimpflug Principle and 3D Optical Tomography

• Representative System: Oculus Pentacam, Ziemer Galilei, CSO Sirius.
• The Scheimpflug Optical Principle: In conventional photography, the film plane, the lens plane, and the object plane are all mutually parallel, resulting in a narrow depth of focus. In a Scheimpflug optical camera, the lens plane is tilted relative to the image plane such that the plane of the object, the plane of the lens, and the plane of the image all intersect along a single common line (the Scheimpflug line). This creates an infinite depth of focus, allowing the instrument to capture razor-sharp, optically undistorted cross-sectional optical slit sections from the anterior corneal surface through the crystalline lens.
• Rotating Slit-Beam Acquisition: In the Oculus Pentacam, a blue LED slit-beam (475 nm) rotates 180° around the corneal optical axis within 1 to 2 seconds, capturing 25 to 50 individual radial cross-sectional optical slices and measuring up to 138,000 true elevation points.
• True 3-Dimensional Tomography: Scheimpflug imaging generates a comprehensive volumetric reconstruction of the entire anterior segment:
  1. True Anterior Corneal Elevation
  2. True Posterior Corneal Elevation
  3. Full Limbus-to-Limbus Pachymetric Distribution
  4. Anterior Chamber Depth (ACD) and Anterior Chamber Volume (ACV)
  5. Corneal Optical Density (Densitometry)

Topographic and Tomographic Map Interpretation

Modern tomographic software displays data across four primary map presentations (the "Quad Map"): Sagittal (Axial) Curvature, Tangential (Instantaneous) Curvature, Elevation, and Pachymetry.

1. Sagittal (Axial) Curvature Map

• Mathematical Model: Calculates curvature at any point based on the perpendicular distance to the optical axis (center of curvature fixed on the reference axis).
• Clinical Application: Provides a mathematically smoothed, global overview of corneal power. Excellent for identifying generalized regular astigmatism (orthogonal bowtie) or macro-level steepening. However, its mathematical averaging smooths out and obscures small, localized ectatic cones.

2. Tangential (Instantaneous / True) Curvature Map

• Mathematical Model: Measures the local radius of curvature at every discrete point independently, without reference to the central optical axis.
• Clinical Application: Provides exceptional local spatial resolution. It does not smooth data, making it the ideal map for pinpointing the exact size, true apex, and anatomical boundaries of a keratoconic cone, peripheral corneal thinning, or surgical ablation margins.

3. Elevation Maps (Anterior and Posterior)

• The Concept of the Reference Surface: Elevation does not measure optical power (diopters); it measures physical spatial height in microns (µm). To display corneal shape, the software fits a mathematical reference body—most commonly a Best-Fit Sphere (BFS) or Best-Fit Toric Aspheric Ellipsoid (BFTE)—to the measured corneal data.
• Interpreting Elevation Color Scales:
  • Warm Colors (Red / Orange): Points that sit above (anterior to) the reference sphere (positive values, representing protrusions or steep meridians).
  • Cool Colors (Blue / Violet): Points that sit below (posterior to) the reference sphere (negative values, representing depressions or flat meridians).
  • Green Colors: Points that align closely (within ±5 µm) with the reference sphere.
• The Clinical Supremacy of Posterior Elevation: Because the anterior cornea is rigid and bound by Bowman's layer and epithelial remodeling, early biomechanical weakening forces the posterior corneal surface to bulge forward first. Elevated posterior protrusion (>+13 to +16 µm above the BFS) is the earliest, most reliable biomarker for subclinical keratoconus, detectable years before anterior curvature changes appear.

4. Pachymetric Map

• Limbus-to-Limbus Thickness Profile: Scheimpflug tomography maps corneal thickness across the entire corneal diameter, identifying the central corneal thickness (CCT), the apex thickness, and the thinnest corneal point (TP).
• Thinnest Point Coordinates: In a healthy cornea, the thinnest point is located within 0.5 mm of the geometric center. In keratoconus and ectasia, the thinnest point is displaced inferotemporally and exhibits progressive local thinning.

The Belin-Ambrósio Enhanced Ectasia Display (BAD)

Developed by Dr. Michael Belin and Dr. Renato Ambrósio Jr., the BAD display is the universally recognized international benchmark for refractive surgery screening and ectasia risk assessment on the Oculus Pentacam.

The Flaw of the Standard Best-Fit Sphere

When a standard Best-Fit Sphere is computed across the central 8 mm of an ectatic cornea, the cone itself distorts the reference body. Because the cone is steep and protruding, the software creates a steeper reference sphere that artificially hugs the cone, dramatically reducing the apparent elevation height of the ectasia and masking early disease.

The Enhanced Reference Sphere Concept

The BAD software overcomes this masking effect through an innovative algorithm:

1. The software locates the exact coordinates of the cornea's thinnest point.
2. It defines an exclusion zone of 3.5 mm centered directly on the thinnest point.
3. It recalculates the reference sphere using all corneal data outside the exclusion zone—yielding the Enhanced Best-Fit Sphere (Enhanced BFS).
4. The Clinical Effect: By calculating the reference surface exclusively from healthy, non-ectatic peripheral stroma, the enhanced BFS does not steepen. When the patient's actual cornea is compared to this enhanced reference body, the ectatic cone stands out with dramatic prominence.

Differential Elevation Maps (Δ Elevation)

The BAD display presents differential maps showing the change in elevation when shifting from the standard BFS to the enhanced BFS:

• Normal Anterior Δ Elevation: $< +7$ µm
• Suspicious Anterior Δ Elevation: $+7\text{ to } +8$ µm
• Pathologic Anterior Δ Elevation: $> +8$ µm
• Normal Posterior Δ Elevation: $< +13$ µm
• Suspicious Posterior Δ Elevation: $+13\text{ to } +16$ µm
• Pathologic Posterior Δ Elevation: $> +16$ µm

Corneal Thickness Spatial Profile (CTSP) and Percentage Thickness Increase (PTI)

The BAD software tracks pachymetric changes along concentric imaginary rings starting at the thinnest point and expanding toward the periphery:

• Corneal Thickness Spatial Profile (CTSP): Plots absolute corneal thickness as a function of distance from the thinnest point. In normal eyes, thickness increases steadily and gradually from center to periphery. In keratoconus, the curve demonstrates a steep, rapid divergence from the normal population envelope.
• Percentage Thickness Increase (PTI): Plots the percentage rate of thickness increase from the thinnest point outwards. Ectatic corneas show abnormally accelerated thickness progression rates.

The Composite Final D Score

The BAD display synthesizes five separate standard deviation indices into a single, global multi-variate score—the Final $D$ (Belin-Ambrósio $D$):

1. $D_f$: Change in anterior elevation
2. $D_b$: Change in posterior elevation
3. $D_t$: Corneal thickness at the thinnest point
4. $D_p$: Pachymetric progression index
5. $D_y$: Vertical displacement of the thinnest point from the geometric center

Clinical Interpretation of Final D:
• Final D < 1.6 Standard Deviations (SD) ──> NORMAL (Displayed in White/Green)
• 1.6 ≤ Final D < 3.0 SD                  ──> SUSPICIOUS / BORDERLINE (Displayed in Yellow)
• Final D ≥ 3.0 SD                        ──> PATHOLOGIC ECTASIA (Displayed in Red)

Keratoconus and Ectasia Classification and Differential Diagnosis

Quantitative Keratoconus Diagnostic Hallmarks

1. Asymmetric Bowtie with Skewed Radial Axes (SRAX): On axial curvature maps, regular astigmatism displays a symmetric bowtie with hemimeridians aligned at 180° to each other. In keratoconus, the bowtie lobes are asymmetric (inferior lobe much larger than superior), and the axes of the two lobes are tilted or angled relative to each other. A SRAX angle > 22° is highly sensitive for keratoconus.
2. Inferior-Superior (I-S) Value: The difference in average refractive power between five points situated in the inferior corneal paracentral zone (3 mm from center) and five corresponding points in the superior zone. An $I-S$ value > 1.4 D indicates significant inferior steepening.
3. Steep Keratometry ($K_{max}$): Central or paracentral maximum keratometry exceeding 47.0 to 48.0 D warrants immediate investigation.

Morphological Cone Classifications

• Nipple Cone: Small diameter ($< 5.0$ mm), round, located near the visual axis or paracentral. Easily fitted with specialized rigid gas-permeable (RGP) contact lenses.
• Oval Cone: Larger diameter ($5.0\text{ to } 6.0$ mm), oval-shaped, typically displaced inferotemporally. Most common clinical presentation; prone to progressive corneal thinning.
• Globus Cone: Massive ectasia involving > 75% of the corneal surface ($> 6.0$ mm diameter), with generalized thinning extending nearly to the limbus.

Pellucid Marginal Degeneration (PMD) vs. Keratoconus

Technologists must definitively differentiate keratoconus from Pellucid Marginal Degeneration (PMD), as their surgical and contact lens approaches differ fundamentally:

• Anatomical Thinning Zone: In PMD, the thinning occurs in a narrow, crescent-shaped peripheral band situated 1.0 to 2.0 mm from the inferior limbus, extending between the 4 o'clock and 8 o'clock positions. The central cornea maintains normal thickness.
• Topographic Pattern: On axial curvature maps, PMD generates a pathognomonic "crab-claw" or "butterfly" (kissing-doves) pattern, characterized by marked peripheral steepening that wraps around a relatively flat central corridor.
• Refractive Profile: Induces severe, high against-the-rule (ATR) astigmatism with vertical flattening.