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Corneal Topography

This document discusses corneal topography, which is the examination and mapping of the shape and curvature of the cornea. It describes different techniques for measuring corneal topography including keratometry, photokeratoscopy, and videokeratography. It outlines the major regions of the cornea and indices used to characterize topography maps. Examples of topography patterns are shown for normal, astigmatic, and diseased corneas. Clinical applications of topography including refractive surgery planning and evaluation, contact lens fitting, and diagnosing corneal conditions are also summarized.

Health & Medicine◦
Raman Prasad Sah Optometrist B.P. Koirala Lions Centre for Ophthalmic Studies Institute of Medicine, Kathmandu, Nepal
Study of shape 0f corneal surface. Configuration or description of surface. Aspheric – elliptical shape. Eccentricity: 0.41 – o.58 ( Aver:0.47 )
1. Central Region/Corneal Cap/Apical Zone/Optic Cap 3-4 mm 2. Paracentral / Mid Peripheral Region 4-8mm from cornea. 3. Peripheral Region 8-11mm Regions
Keratometer (Ophthalmometer ) Keratoscope : • Placido disc • Photokeratoscopy • Video Keratoscopy (Computer assisted topographic analysis.)
 Examination of curvature of anterior surface of cornea Nidek keratoscope Kera corneoscope
Principle Use of the first Purkinje image.  Consists of equally spaced alternating black & white lines.
 A luminous object (target of rings) is placed in front of patient’s cornea.  Image size produced in the corneal reflection is measured
 Circular Rings -Spherical cornea  Oval Rings –Regular astigmatism .  WTR astigmatism  ATR astigmatism With long vertical axis – Against the Rule Astigmatism.
 Irregular astigmatism  Corneal anomaly– irregular cornea distorted cornea scar  Keratoconus
 Small degrees of abnormalities of corneal shape are not easily identifiable.  Not used in corneas with epithelial defects and stromal ulcers .  Clinically significant amounts of corneal cylinder (up to 3.00 D) may not be diagnosed.  Subtle irregularities cannot be detected.
 Measurement is restricted to a small central corneal area (3-3.5mm) in keratometry.  Videokeratoscopes evaluate several thousands of points from nearly the entire corneal surface. They measure the entire corneal contour.
 LSU Corneal Topographic System (LSUCTS)  PAR CTS  Topographic Modelling System (TMS)
 Corneal Lens Analysis System (CLAS)II Unit  Computerized Corneal Topographic EH-270  The EyeSys 2000 Corneal Analysis System  ORBSCAN  PENTACAM
 Process of building a topographic map of cornea from keratoscopic data . Capture video images of the keratoscope rings. Measure angular size of points on the rings. Reconstruct the corneal surface point by point. Assign dioptric or other descriptors for each surface. Present surface descriptors in a color topographic map.
 Numerical power plots  Simulated keratometry view  Photokeratoscopic view  Surface Elevation Maps  Color Coded Maps
 1 .Color coding  2. Scale  3. Quantitative indices
Cool colors (black, blue, azure) Flatter surfaces Warm colors (orange, red, white) Steeper surfaces Normal (green, yellow) Normal surfaces
Range: 28.o0 D to 65.00 D Interval : 1.5 D
Simulated keratometry (Sim K ) Surface Regularity Index (SRI) Surface Asymmetric Index (SAI) Irregular astigmatic index (IAI ) Differential sector index(DSI) Opposite sector index(OSI) Centre/surround index (CSI ) Analyzed area (AA)
- Measure differences in corneal power & compares asymmetry. -Provides the power and location of the steepest and flattest meridians - Equivalent to Conventional Keratometry Reading. 
 Surface Regularity Index (SRI) -Local fluctuation in central corneal power -Local surface irregularities  Surface Asymmetric Index (SAI) -Measures differences in corneal power between corresponding points at each ring & compares symmetry. - Used to monitor changes caused by contact Lens warpage or keratoplasty.
Round: 22.6%
Bow- tie patterns indicate astigmatism
Small, near central ectasia, less than 5.0 mm in cord diameter May manifest as moderate to high with-the-rule corneal astigmatism
In advanced keratoconus. Corneal apex is displaced well below the midline resulting in varying degrees of inferior mid- peripheral steepening. Kissing pigeon pattern
 Hallmarked by a thinning of the inferior peripheral cornea.  The corneal thinning begins approximately 1.0 to 2.0 mm above the inferior limbus.
 High against the rule astigmatism.  Inferior mid-peripheral steepening at 4 & 8 o’clock position.  Kissing pigeon pattern (diagnostic of PMD)
 Typical with-the-rule astigmatism is induced.  Bow-tie pattern oriented vertically.
 Depends upon ◦ Location ◦ Severity(extent & depth) ◦ Type of trauma  Flattening along the meridian of laceration & steepening along 90* away.
Photorefractive Surgery Preoperative screening Surgical planning Assessment of surgical outcomes Detection & management of complications Refinement & development of surgical outcome Expected shape of cornea after refractive surgery
Evaluation Of intraocular Surgery  Cataract  Penetrating keratoplasty  Radial Keratotomy  Epikeratophakia
Diagnosis of Corneal degenerations & dystrophies. Keratoconus Keratoglobus Pellucid Marginal Degeneration Terrien’s Marginal degenerations
 Determination Of ◦ Refraction esp. for poor quality retinoscopic reflex. ◦ Curvature of cornea ◦ Amount and direction of corneal astigmatism ◦ Quality of corneal refracting surface ◦ Stability of corneal refracting surface.
 In cases of trauma  Progressive Myopia  Aphakia or high ametropia  To calculate the power of Intraocular Lens
Contact lens fitting Choice of trial lens. Verification of contact lens parameters Monitoring corneal shape Orthokeratology
Borish’s Clinical Refraction Benjamin Franklin Principle and practice of refractive surgery Elander Robin Corneal surgery, 3rd edition Fredrich Brightbill Refractive Eye Surgery ,2nd Edition Leo D. Bores Optics - Refraction & Contact Lens AAO-3 IACLE Contact lens module 1 & 9 Primary care optometry Theodore & Grosvenor
Corneal Topography