Corneal Topography and Tomography PDF

Corneal Zoeanne Schinas OD Nova Southeastern Topography University College of Optometry  Aids not only in the diagnosis of corneal disease, but in monitoring the progression of corneal disease as well Clini...

Corneal Zoeanne Schinas OD Nova Southeastern Topography University College of Optometry  Aids not only in the diagnosis of corneal disease, but in monitoring the progression of corneal disease as well Clinical Uses Contact Lens Fitting (especially GP’s); more accurate than Keratometry of Topography Screening and monitoring of refractive surgery; LASIK, PRK, etc; including pre operative IOL selection Ocular surface disorder evaluation (pterygium, corneal scars, Salzmann's nodular, etc)  Keratometry Topography Why not just Measures central 3-4 mm Measures 7mm up to the limbus use a Measures on 4 central data points Depending on the Keratometer? instruments, evaluates Irregular corneas may be 8,000-10,000 points missed across the entire surface It's so much Gives a quantitative Allows for both cheaper to evaluation of curvature quantitative and qualitative assessment purchase! You can charge for it! Types of Topographers: Placido based Projects a series of concentric rings of light onto the anterior corneal surface Creates a virtual image Shape and spacing between the concentric rings are used to calculate radius of curvature at thousands of points. Smaller spaces between rings = steeper Wider spaces between rings = flatter Computer then extrapolates the height to determine elevation via an algorithm Examples: Medmont https://medmont.com.au/e300- corneal-topographer/ Humphrey https://eyewiki.aao.org/Corneal_Topography  For Advance corneal Types of Topographer: Projection based Projection or elevation-based or slit-scan topography measures both the anterior and posterior corneal surface by both time domain and light-based analysis Measures specific points on the cornea and determines the height From the height, it measures the slope and radius of curvature Also measures corneal thickness or pachymetry in microns Examples Orbscan slit-scan imaging combined with placido disc imaging Pentacam combined device consisting of a slit illumination system and a Scheimpflug camera, which rotate together around the eye to acquire data. Quick summary Placido based Projection based Topography Topography *anterior and posterior corneal surface elevation *only anterior corneal surface evaluation Contact Lens simulation Contact Lens simulation with GPs (some, not all) Wavefront aberrations Wavefront aberrations (some, not all) 3 D image of anterior segment (Pentacam) 3-D image of topography (Medmont) Tomography (Pentacam) o *The Scheimpflug image is a cross-sectional image showing the cornea, anterior chamber, iris, and lens. Pachymetry / Corneal Thickness Other options available / useful for surgeons and for monitoring progression in disease. (Pentacam) Normal Corneas Aspheric Q < 0 (in 80 % of cases) The Q value tells us about the sphericity of the anterior surface of cornea A value between 0 to -1 is considered normal and indicates a prolate shape A value >0 indicates an oblate cornea (flat center and steep periphery) A value < -1 indicates a hyperprolate cornea as seen in corneal ectasia Central keratometry 40 D (8.4 mm) to 47 D (7.2 mm); mean 43 D (7.8 mm) Symmetry between the 2 eyes Corneal topography in clinical practice. Ghemame et al. Journal Français d'Ophtalmologie. Volume 42, Issue 10, December 2019, Pages e439-e451 Corneal Shapes Oblate Prolate Flatter in the center, steepens With the steepest radii of towards periphery curvature (warm colors) at the Post Refractive surgery apex with progressively flatter (Myopic) such as Lasik/PRK shape (cool colors) out to the Ortho-K (temporary) periphery Some s/p transplants Normal corneal Keratoconus (hyperprolate)  Patterns As described by Rabinowitz et al Color Coded Scale Curvature Maps The higher the dioptric value, the warmer the color and the steeper the curvature Curvature Map Colors The warmer colors represent steeper corneal curves The cooler colors represent flatter curves  Old pentrated plasty Oblate or Prolate?  Hyperprolate Oblate or Prolate or ? Regular Astigmatism: WTR or ATR?  Remember that you still need to look at the patient as a whole Refraction, Slit lamp findings, Pachymetry Topography is a tool The exception may be subclinical Keratoconus (ectasia) --> posterior float/elevation of the cornea, no anterior corneal findings  Curvature Maps: Axial curvature map Obtains global view of the cornea Which map Useful in initial diagnosis Often used in monitoring should I Tangential curvature map Draws attention to the location that may require special attention --> esp useful in specialty CL use for fittings Elevation maps : what? Anterior/Posterior Elevation Maps Designing custom lenses due to the estimation of the true shape of the cornea Useful in keratoconus diagnosis Refractive surgery evaluation (pre/post op) Curvature Maps Axial vs Tangential Maps Curvature vs Elevation Maps Curvature maps DO NOT represent elevation! https://www.pentacam.com/fileadmin/user_upload/pentacam.de/downloads/publikationen/artikel/2 008-Article__Pentacam_An_introduction_understand_elevation_based_topography.pdf Elevation Maps Best Fit Sphere https://www.pentacam.com/fileadmin/user_upload/pentacam.de/downloads/publikatio Calculation is based on the normal cornea --> nen/artikel/2008- Article__Pentacam_An_introduction_understand_elevation_based_topography.pdf aspheric & prolate Steep/more curved meridian is below the best fit sphere (shown as the red curved line here) Flat/less curved meridian is above best sphere (shown as the blue curve line here)  Steep in this image is red but is below/ depressed from the best fit sphere. STEEP On an elevation map it would be doesn't a cool color (blue) instead. always mean high elevation https://www.pentacam.co m/fileadmin/user_upload/pentacam.de/do wnloads/p ublikatio nen/artikel/ 2008-Article__Pentacam_An_introduction_u nderstand_elevation_based_topography.pdf Posterior Elevation Map Elevation and curvature is different Flat in this image is blue line but is above/elevated from the best fit sphere. On an elevation map it would be a red/orange color instead. https://www.pentacam.com/fileadmin/user_upload/pentacam.de/downloads/publikationen/artikel/2 008-Article__Pentacam_An_introduction_understand_elevation_based_topography.pdf Posterior Elevation Map Elevation Maps Applying the BFS Steep/more curved meridian is An astigmatic pattern below the best fit sphere. It is will have the flat depicted in the blue/green vertical meridian raised above meridian on the elevation map. the BFS (best fit sphere) and the steep meridian Flatter/ less curved meridian is below. above the best fit sphere. It is depicted in the red/orange horizontal meridian on the elevation map. https://www.pentacam.com/fileadmin/user_upload/pentacam.de/downloads/publikationen/artikel/2008- Article__Pentacam_An_introduction_understand_elevation_b ased_topography.pdf https://www.pentacam.com/fileadmin/user_upload/pentacam.de/downloads/publikationen/artikel/2 008-Article__Pentacam_An_introduction_understand_elevation_based_topography.pdf Elevation in microns Points above the BFS are considered elevations and expressed in plus values (+ is above) Points below the BFS are considered depressions and expressed in minus values (- is below)  Anterior and Posterior Elevation Maps Look for the highest plus value within the central 5mm *these values will vary depending on the size of BFS diameter *these values may also coincide to the area Anterior Posterior of interest on axial/tangential curve map & Pachymetry map when using to diagnose Elevation Elevation Map ectactic disease Map Based on 8-mm diameter BFS >+8 abnormal > +16 abnormal Information provided by Pentacam Elevation Map Shapes SHAPE The normal shape is a symmetric hourglass Abnormal shapes: Irregular Tongue-like extension Isolated island Elevation Map: Keratoconus with BFS and Enhanced Best Fit Sphere https://www.oculus.de/uploads/media/belin.pdf Belin MW, Khachikian SS. An introduction to understanding elevation-based topography: how elevation data are displayed – a review. Clinical & Experimental Ophthalmology 2009; 37: 14–29  Early ectatic changes may be seen solely on the posterior cornea prior to any changes in the anterior corneal surface Posterior Keratoconus or post LASIK ectasia Elevation Map: Recognizing Early These patients have abnormal corneas in spite of having excellent BCVA --> Ectatic Changes detecting subclinical disease Therefore, the posterior corneal surface may serve as an earlier indicator of ectatic changes than the anterior corneal surface  This best-fit sphere (BFS) model compares the measured elevation data to a best-fit sphere (BFS) within the central 8-mm zone It then takes the measured data and Enhanced excludes a small-diameter optical zone, Best Fit typically 3 mm in keratoconic corneas, centered on the thinnest portion of the Sphere cornea and recomputes the BFS reference (EBFS) shape o This will enhance the area of the abnormal cornea if truly pathological o Normal corneas will look the same BFS vs EBFS  Using elevation maps in GP fitting Good way to predict the NaFL pattern Example depicted in these photos: Dumbell pattern observed with a spherical GP on a highly toric WTR cornea https://www.pentacam.com/fileadmin/user_upload/pentacam.de/do wnloads/publikationen/artikel/2008- Article__Pentacam_An_introduction_understand_elevation_based_top ography.pdf  Scleral Lens Fitting: CSP Report Cornea Scleral Profile (CSP) scan Display of sagittal heights of cornea and sclera Profile of sclera for scleral lens fitting https://www.pentacam.com/int/ophthalmologist-diagnostic-without- pentacam/models/pentacamr/optional-software.html Pachymetry Map Patterns: Normal pattern: Concentric Abnormal patterns: Displaced, dome-like, globus, bell shaped Color distribution: Warm colors = thin Cool colors = thick ***Displacement of the thinnest point from the apex of cornea raises suspicion for corneal ectasia if it co-exists with corneal steepening*** Abnormal Pachymetry Indices  Abnormal Parameters Criteria: Less than 470 at the thinnest location (TL) with a NORMAL topography Abnormal Less than 500 at the thinnest location (TL) with an ABNORMAL topography values for Greater than 10 microns between the apex pachy Pachymetry and the thinnest location (TL) Greater than 30 microns of difference at the thinnest location (TL) between two eyes At 5mm circle, the difference between the Superior and Inferior readings are An irregularity may be over-emphasized The Normalized (Adjusted) Scale https://www.artoptical.com/consultation-services/topography-scales Absolute (Standard) vs Normalized (Adjusted) True pathology will still show up on the Absolute scale R/O Decentered apex https://eyewiki.aao.org/Optical_Axes_and_Angle_Kappa www.pentacam.com Decentered Apex Corneal Topography REVIEW Let's test your knowledge  Patient A Tangential Map  Patient A 4 Map Refractive  Patient A Bellin Ambrosio Display  Patient B 4 Map Refractive  Patient B  Patient C  Patient C  Patient D 20/20 DVA Sim Ks: 41.5/44.2 @112 Pachymetry: 515 apex ???  Patient D What do you want to review before you make your diagnosis?  Patient D What other Keratoconus Index (KI): ratio between mean information radius values in the upper half and lower half of cornea. KI >1.07 is abnormal and/or can I get pathological. from Inferior Superior Asymmetry (I-S) index: Topography: Calculated as the difference between inferior and superior average dioptric (D) values. The Keratoconus average of five superior points above the horizontal meridian are compared to the average of five inferior points. Any value >1.4 is Indices suggestive of keratoconus.  Topography is a great tool to AID in Topography diagnosis and management does not MAKE the Definitive diagnosis of corneal disease should be based on diagnosis patient's symptoms, slit lamp examination and other testing (ex Topography) Keratoconus Clinical Signs Vogt's striae Fleisher's ring (partial, complete) Apical Scarring Munson's sign Scissoring ret reflex Rizzuti's sign Always upload and interpret 4 Maps Refractive BAD display This includes PEDs department too!