Corneal Topography and Imaging

Corneal Topography Basics - Peeking at Peaks

• Definition: Non-invasive imaging technique mapping the cornea's surface curvature and shape.
• Purpose:
  • Detecting corneal ectasias (e.g., keratoconus, pellucid marginal degeneration).
  • Planning refractive surgery (LASIK, PRK).
  • Fitting contact lenses.
  • Evaluating astigmatism.
  • Monitoring corneal disease progression.
• Map Types:
  • Axial (Sagittal): Curvature relative to the optical axis; good for overall view.
  • Tangential (Instantaneous): True local curvature; better for peripheral details & ectasia localization.
  • Elevation: Height relative to a reference sphere.
  • Refractive Power: Displays corneal power in diopters.

⭐ Placido Disk Reflection: Most topography systems use the reflection of concentric rings (Placido mires) off the tear film to analyze corneal curvature. Irregular mires indicate irregular astigmatism or ectasia. 📌 Mnemonic: Placido Peeks at Protrusions.

Topography Technologies - The Image Makers

• Placido Disc Reflectors:
  • Principle: Projects concentric rings (mires); analyzes reflected pattern from anterior corneal surface.
  • Measures: Anterior curvature primarily.
  • Limitation: Infers posterior shape; less accurate for posterior ectasia or post-refractive surgery.
• Scheimpflug Imaging:
  • Principle: Rotating camera captures multiple corneal optical cross-sections based on Scheimpflug principle.
  • Measures: Anterior & posterior curvature, true pachymetry, anterior chamber depth (ACD).
  • Examples: Pentacam, Sirius, Galilei (dual Scheimpflug + Placido).

  ⭐ Scheimpflug imaging is crucial for detecting early keratoconus by providing true pachymetry and detailed posterior corneal surface data, unlike Placido-based systems.

• Slit-Scanning Technology:
  • Principle: Projects multiple light slits; reconstructs corneal surface (anterior & posterior elevation) from reflected/scattered light.
  • Example: Orbscan (often combines slit-scanning with Placido disc for curvature).

Advanced Corneal Imaging - Deeper Dive Views

• Anterior Segment OCT (AS-OCT):
  • Non-contact, high-resolution cross-sections.
  • Measures: Pachymetry, epithelial thickness, LASIK flap.
  • Visualizes: Anterior chamber angle, iris, lens.
  • Uses: Keratoconus (epithelial mapping), glaucoma, cataract surgery.
• Confocal Microscopy:
  • In-vivo cellular imaging of all corneal layers.
  • Detects: Endothelial cells, keratocytes, nerves, pathogens (Acanthamoeba, fungi).
  • Aids in: Dystrophies, keratitis, post-refractive surgery evaluation.
• Ultrasound Biomicroscopy (UBM):
  • High-frequency ultrasound (35-50 MHz).
  • Images anterior segment when cornea is opaque.
  • Details: Ciliary body, sulcus, posterior chamber, IOL position.
  • Key for: Angle-closure glaucoma, tumors, trauma.

⭐ AS-OCT epithelial mapping is highly sensitive for subclinical keratoconus, revealing focal thinning often before other signs.

Clinical Applications & Patterns - Diagnosing with Dots

• Core Principle: Maps corneal curvature. Utilizes Placido disc (anterior curvature) or Scheimpflug imaging (anterior/posterior curvature, pachymetry).
• Keratoconus (KCN):
  • Inferior steepening, asymmetric bow-tie (ABT), skewed radial axes (SRAX).
  • ↑ Kmax, ↑ I-S value (threshold >1.4 D), central/paracentral thinning.
  • Indices: KISA%, Belin/Ambrosio Display (BAD).
• Pellucid Marginal Degeneration (PMD):
  • "Crab claw" or "kissing doves" pattern.
  • Inferior peripheral thinning (1-2 mm from limbus), high against-the-rule (ATR) astigmatism.
• Post-Refractive Surgery Assessment:
  • Myopic ablation: Central flattening. Hyperopic ablation: Central steepening.
  • Detecting ectasia (progressive steepening, KCN-like), decentered ablation.
• Other uses: Contact lens fitting, pre-operative astigmatism analysis.

⭐ An I-S (Inferior-Superior dioptric asymmetry) value > 1.4 D on corneal topography is a key indicator for keratoconus.

High‑Yield Points - ⚡ Biggest Takeaways

• Placido topographers map anterior corneal curvature; Scheimpflug systems (e.g., Pentacam) analyze anterior/posterior elevation and pachymetry.
• Early keratoconus detection relies on inferior steepening, posterior elevation, and corneal thinning, vital for refractive surgery screening.
• AS-OCT offers high-resolution cross-sections, precise pachymetry, and angle details.
• Essential for diagnosing irregular astigmatism, monitoring ectasias, and IOL power calculations.
• Axial maps show overall curvature; Tangential maps show local curvature changes.