Pentacam and Corneal topography

Different methods for assessment of corneal topography with special details on pentacam and its interpretation

1. CORNEAL TOPOGRAPHY & PentaCam BY- Dr. PRIYANKA RAJ

2. CORNEAL TOPOGRAPHY Corneal topography refers to study of the shape of corneal surface provides us with a detailed description of various curvature and shape characteristics of the cornea. This information is very helpful for the illustration of corneal astigmatism detection of corneal pathologies perfection of contact lens fitting cataract and keratorefractive surgeries

3. ASSESSMENT OF CORNEAL SURFACE

4. CORNEAL WINDOW REFLEX

5. keratometry Devised by Helmholtz, who originally used the term ophthalmometer Calculations are based on the geometry of a spherical reflecting surface Based on fact that anterior surface of cornea acts as convex mirror and size of image varies with curvature Object of known size and distance is reflected off the corneal surface to determine the size of the reflected image with a measuring telescope Helps to measure the radius of curvature of anterior corneal surface from 4 reflected points within central 3mm of cornea Helmholtz keratometer Bausch and lomb keratometer Javal schiotz keratometer

6. BAUSCH & LOMB Object size constant Amount of doubling varied to produce the image of fixed size JAVAL SCHIOTZ Amount of image doubling is constant Measures the object size needed to produce an image of fixed size

7. RANGE OF KERATOMETRIC READING • Dioptric Power : 36D to 52D • Radius of Curvature : 6.5mm to 9.38mm

8. APPLICATIONS Objective method for determining curvature of the cornea. To estimate the amount and direction of corneal astigmatism The ocular biometery for the IOL power calculation To monitor pre and post surgical astigmatism. Differential diagnosis of axial versus refractive anisometropia. To diagnose and monitor keratoconus and other corneal diseases. For contact lens fitting by base curve selection LIMITATIONS Assumes cornea to be spherical Details of only central 3mm ignoring peripheral corneal zones Loses accuracy when measuring very steep or flat corneas Small corneal irregularities preclude the use due to irregular astigmatism.

9. Keratoscopy • Refers to the evaluation of topographic abnormalities of the corneal surface by direct observation of images of mires reflected from the surface of cornea. • Keratoscope: instrument that projects multiple concentric rings (mires) on the cornea • Keratoscopy: direct visualization of the rings Placido-disc keratoscope Photokeratoscope : when a still camera is added to photograph the mires Videokeratoscope : when a video camera is added

10. Rasterstereography • Uses a direct image on the corneal surface. • It projects a calibrated grid onto the fluorescein stained tear film. • The advantage is that it includes information across the whole cornea. • The projected nature of the test does not allow interference due to corneal surface or stromal defects.

11. Interferometry • Uses the technique of light wave interference. • This includes both holography and moire fringe techniques. • This method is not in widespread clinical use.

12. MODERN TOPOGRAPHY PLACIDO DISC SYSTEMS project a series of concentric rings of light on the anterior corneal surface. The corneal shape or curvature is directly measured in diopters of curvature along thousands of points on the rings. do not actually measure elevation they derive anterior corneal elevation data by reconstructing actual anterior curvature measurements via sophisticated algorithms SLIT-SCANNING DEVICES directly measure the elevation of both the anterior and posterior cornea via time domain or light-based analysis. These devices process elevation data along several points on the anterior and posterior corneal surfaces. This data is then converted into anterior and posterior curvature in diopters as well as corneal thickness or pachymetry in microns

13. PLACIDO DISC SYSTEMS Small-cone Placido disc topographers- project more rings on the cornea and have a shorter working distance Medmont E300 (Medmont) Scout and Keratron (EyeQuip) Magellan Mapper (Nidek) Large-cone Placido disc systems use a longer working distance and project fewer rings onto the cornea than small-cone topographers and are more forgiving when measuring patients with very deep set eyes. ATLAS 995 and 9000 (Carl Zeiss Meditec) ReSeeVit (Veatch Ophthalmic Instruments) SLIT-SCANNING DEVICES Orbscan (Bausch & Lomb) Visante OCT (Carl Zeiss Meditec). Scheimpflug imaging- uses a rotating camera to photograph corneal cross-sections illuminated by slit beams at different angles . This method corrects for the non-planar shape of the cornea and, thus, allows greater accuracy and resolution in creating a 3-D map of the cornea Pentacam (Oculus)

14. NEW GENERATION ORBSCAN PENTACAM VISANTE

15. Scheimpflug-based Devices • There are four devices adopting the Scheimpflug principle and using the Scheimpflug camera. These devices are 1. TMS-5 (Tomey, Nagoya, Japan), 2. Pentacam® HR (OCULUS, Wetzlar, Germany), 3. Sirius® (CSO Florence, Italy), 4. Galilei® (Ziemer, Port, Switzerland).

16. TMS 5 Galilei Sirius 3D

17. PENTACAM

18. Comprehensive anterior aegment analyser. Perform following 5 functions 1. Scheimpflug image of anterior segment 2. Three dimensional anterior chamber analyser 3. Pachymetry 4. Corneal topography 5. Cataract analyser

19. THE SCHEIMPFLUG PRINCIPLE The Scheimpflug principle, first introduced by Theodor Scheimpflug, a cartographer of the Austrian navy, describesan optical imaging condition, which allows documentation of an obliquely tilted object with the maximally possible depth of focus and minimal image distortion under given conditions. It is a geometric rule that describes the orientation of the plane of focus of an optical system (such as a camera) when the lens plane is not parallel to the image plane

20. Normally, the lens and image (film or sensor) planes of a camera are parallel and the plane of focus (PoF) is parallel to the lens and image planes . If a planar subject is also parallel to the image plane, it can coincide with the PoF, and the entire subject can be rendered sharply. If the subject plane is not parallel to the image plane, it will be in focus only along a line where it intersects the PoF, as illustrated in Figure 1B. When an oblique tangent is extended from the image plane, and another is extended from the lens plane, they meet at a line through which the PoF also passes, as illustrated in Figure 1C (referred as Scheimpflug line). With this condition, a planar subject that is not parallel to the image plane can be completely in focus. The Scheimpflug principle has been applied in ophthalmology to obtain optical sections of the entire anterior segment of the eye, from the anterior surface of the cornea to the posterior surface of the lens

21. A nonrotating Scheimpflug camera was first produced by the Oxford group, later on marketed as Oxford CASE 2000 The Zeiss SLC was the first rotating video Scheimpflug system. The EAS 1000 (Eye analysis system) was the first electronic rotating Scheimpflug camera, which recorded a retroillumination image as well. The latest development among ophthalmic camera systems based on Scheimpflug’s principle is the Pentacam (Oculus, Wetzlar, Germany) The GALILEI™ Dual Scheimpflug Analyzer (Zeimer Ophthalmic systems) is another high precision optical system for corneal topography and three dimensional analysis of the anterior eye segment, also based on a Revolving Dual Channel Scheimpflug Camera and a Placido Disk

22. The device The Pentacam obtains images of the anterior segment by a rotating Scheimpflug camera (digital CCD camera with synchronous pixel sampling) measurement. The light source - UV-free blue LED’s (wavelength=475 nm). 2 cameras  1 st -located in the center for the purposes of detection of the size and orientation of the pupil and to control fixation. 2 nd - mounted on rotating wheel to capture images from the anterior segment. This rotating process supplies pictures in three dimensions and also allows the center of the cornea to be measured precisely.

23. The slit images are photographed on an angle form 0 to 180 degrees to avoid shadows from nose. Every picture is a complete image through the cornea at a specific angle, combination of such slit images creates a real 360 degrees image of the anterior segment. The software utilizes a ray tracing algorithm to construct and calculate the anterior segment. It acquires a total of 50 images in approximately two seconds, extracting 2,760 true elevation points from these images which in turn generates 138,000 true elevation points for both the corneal front and back surfaces, from limbus to limbus, including the center of the cornea, a major advantage over keratometers and Placido-based corneal topographers. The measurement process lasts less than 2 seconds and minute eye movements are captured and corrected simultaneously.

24. CLINICAL APPLICATION Pachymetry Scheimpflug Imaging in LASIK Topography and contact lenses. Topography in RK. Post keratoplasty astigmatism. Keraoconus screening Corneal Pathologies Anterior Chamber  implantation of phakic IOLs Assessment of Lens Density + anterior and posterior subcapsular cataracts Capsular bag distension syndrome (CBDS) Posterior Capsule Opacification (PCO)

25. Improved IOL Calculations identifying intralenticular foreign bodies Glaucoma Screening Effect of pilocarpine on anterior chamber depth and anterior chamber volume in eyes with narrow angle and open angles Anterior chamber Volume has been found to be a good screening tool for diagnosing eyes with narrow angles dynamics of the anterior chamber including the ACV can be studied following procedures like laser peripheral iridotomy (PLI).

26. PAChYMETRIC MAPS The remaining map on Pentacam four-map view is called topometric map, or pachymetric map. determines central or paracentral corneal thickness + describes distribution of corneal thickness throughout entire corneal diameter Pachymetric data is useful in screening refractive surgery candidates, as it assists in the estimation of residual stromal bed thickness. Provides invaluable data when ruling out subclinical keratoconus (FFKC), as it distinguishes whether thinnest point corresponds with corneal apex.

27. corneal tomography • corneal tomography consisting of two parts: • corneal parameters on the left side, • 4-view refractive composite map on the right

28. CORNEAL PARAMETERS

29. Corneal Parameters Qs: Quality specification It specifies the quality of the tomographic capture Should be “OK”, otherwise there is some missed information which was virtually reproduced (extrapolated) by the computer; in this case, the capture should preferably be repeated. Q-val: Q Value Represents the asphericity of the anterior surface of the cornea. The ideal value is measured within the 6-mm central zone as shown between two brackets. Normal value is (–1 to 0). Plus Q (>0)  oblate corneas (e.g. after > –3 D myopic photoablation and after radial keratotomy). Over minus Q (<–1)  hyperprolate corneas (e.g. after > +3 D hyperopic photoablation and in keratoconus). Both oblate and hyperprolate corneas produce spherical aberrations.

30. K1: (Kf): Curvature power of the flat meridian of the anterior surface of the cornea measured within the 3-mm central zone (Sim-K) and expressed in diopters (D). Normal K1 = > 34 D. It should be considered in myopic correction; each –1 D correction reduces flat K by 0.75 D to 0.8 D. Final flat K should be > 34 D, otherwise positive spherical aberrations will be induced. K2: (Ks): Curvature power of the steep meridian of the anterior surface of the cornea measured within the 3- mm central zone (Sim-K) and expressed in diopters (D). Normal K2 is < 49 D. It should be considered in hyperopic correction; each +1 D correction will add 1.2 D to steep K. Final steep K should be < 49 D

31. Km: (K-avg): Mean curvature power of the anterior surface of the cornea within the 3-mm central zone (Sim-K) and expressed in diopters (D). It should be considered to avoid flap complications. Km <40 D  free-flap complication may occur Km > 46 D  button-hole complication. K-max: Maximum curvature power of the whole anterior surface of the cornea expressed in diopters (D). Normal K-max  < 49 D Normal difference in Kmax between the two eyes is < 2 D Normal (Kmax - K2) difference in the same eye is < 1 D. Whenever the difference is ≥ 1D, K-max should be used instead of K2 into the calculations for hyperopic correction to avoid postphotorefractive irregularities

32. Astig: Amount of corneal (topographic) astigmatism (TA) on the anterior surface of the cornea, i.e. the difference between the two curvature radii (K2 – K1) within the 3-mm central zone (Sim- K). TA should be compared with the manifest astigmatism (MA). • Axis: The axis of anterior corneal astigmatism within the 3-mm central zone. It should be compared with the axis of MA. • Pachy Apex: It represents thickness at the apex of the cornea. The computer considers the apex as the origin of the coordinates, where X and Y are horizontal and vertical meridians respectively. Zero is displayed in both squares of pachy apex coordinates. The direction of axis X is from the patient’s right to his/her left when the patient is seated opposite to the physician. The direction of axis Y is from the bottom up. Example: a point “ e” in the left cornea is located at “+0.2,–0.4” position, i.e. this point is located 0.2 mm temporal to and 0.4 mm inferior to corneal apex.

33. Pupil Center: Corneal thickness corresponding to pupil center location and its coordinates. Pupil center coordinates are necessary for the decentration technique when treating hyperopia, astigmatism or corneal irregularities. They are also important to evaluate angle kappa normal x-coordinate—in absolute value—is ≤ 200 μm (or ≤ 5°). Pupil diameter: It is the diameter of pupil in the circumstance of capture (photopic, mesopic or scotopic according to the amount of illumination). It is necessary for adjusting optical zone (OZ) diameter, which should be at least 0.5 mm larger than the scotopic pupil size.

34. • Thinnest location (TL): Thickness and location of the thinnest point of the cornea. The new definition of thin cornea is a cornea below 470 μm with normal tomography, or a cornea below 500 μm with abnormal tomography. The normal difference in thickness at the TL between the two eyes is < 30 μm. The difference in thickness between TL and pachy apex is normally ≤ 10 μ m. Y-coordinate is most often normal, suspected or abnormal when it is < 0.500 mm, 0.500 mm to 1.000 mm, or >1.000 mm respectively; the important algebraic sign is the minus indicating inferior displacement of the TL.

35. Anterior Chamber Volume (ACV), Angle (ACA) and Depth (ACD): • Anterior Chambers with ACV < 100 mm 3 ACA < 24° ACD < 2.1 mm may have the risk to develop angle closure glaucoma. On the other hand, safe parameters for phakic IOL (PIOL) implantation are- ACD ≥ 3.0 mm ACA > 30° ACV ≥ 100 mm 3 .

36. CORNEAL MAPS

37. The four most important tomographic maps are the anterior curvature sagittal map, the anterior and posterior elevation maps, and the pachymetry map In each map, both shape and parameters should be studied. It is necessary sometimes to study the anterior curvature tangential map.

38. ANTERIOR SAGITTAL map Steep areas  hot colours (red and orange), while flat areas  cold colours (green and blue). The cross point of this segmentation represents apex (anatomical center) of the cornea. Parameters studied particularly on the steep axis at the 5-mm central circle. The normal pattern is the symmetric bowtie (SB) .

39. The two segments (a) and (b) are equal in size, and their axes are aligned. The two opposing points (S and I) on the 5-mm central circle on the steep axis. Normally, the inferior (I) point has a higher value than the superior (S) one, and the I-S difference should be < 1.5 D. The superior point may rarely have a higher value than the inferior one  the S-I difference should be < 2.5D The SB pattern represents regular astigmatism, which can be with-the-rule (WTR), against-the-rule (ATR) or oblique according to the orientation of the SB.

40. WTR astigmatism  SB is on or within ―15° of the vertical meridian of the cornea . ATR astigmatism  SB is on or within ―15° of the Horizontal meridian of the cornea . Oblique astigmatism  SB is neither vertical nor horizontal . The SB pattern can be encountered in KC when K readings are abnormally high .

41. Abnormal patterns • They include the following: • 1. Round (R) . • 2. Oval (O) . • 3. Superior Steep (SS) . • 4. Inferior Steep (IS) . • 5. Irregular (Irr) .

42. Anterior Tangential Map This map helps in describing corneal irregularities. It is also useful for determining morphologic patterns of the cone in ectatic corneal disorders. Depending on this map, there are three patterns of the cone: nipple, oval and globus.

43. The Elevation Maps

44. Reference Body The computer adjusts the reference surface with the measured surface. The computer considers all points above reference surface as elevations, being displayed as positive values, and considers all points below the reference surface as depressions, being displayed as negative values ,all values are in microns. The coincidence points between reference surface and measured surfaceare displayed as zeros, i.e. exactly like the sea level .

45. The Elevation Maps An elevation map describes the height details of the measured corneal surface by matching it with a reference surface (RS). Points above the reference surface are considered elevations and expressed in plus values, and those below the RS are considered depressions and expressed in minus values . In corneal astigmatism, one meridian is steeper than the other and is located under the RS taking minus values, contrary to the flatter meridian which takes plus values .

46. Reference Body • The computer of the camera proposes a reference body for each corneal surface being captured . The reference body of the front surface may differ from that of the back surface, although both surfaces are of the same cornea

47. Reference Body Types Toric Ellipsoid Body • It is an aspherical shape which is rotationally symmetric according to two axes, major and minor. But it has a coronal elliptical cross-section , i.e. there are two perpendicular axes, one is steeper than the other. advantage  very good approach to the real course of, e.g. astigmatic corneal surface. •

48. Spherical Body • It is better than the previous bodies in highlighting cornealirregularities since the normal cornea has a toric ellipsoid shape. It is well known that to recognize something, it should be matched with other different things. Therefore, if we want to show the details of an abnormal cornea, we should relate it to a spherical reference body.

49. Float Mode • The reference body can be adjusted with examined surface of the cornea in various locations . Accordingly, details of central part might appear (or disappear). If the reference body is adjusted in contact with apex of the cornea, it is called “no float mode” . when the reference body is represented to be optimized with respect to the cornea, it is called “float mode” , i.e. the distance between the two bodies (corneal surface and reference body) should be equal in sum and minimum. The float mode is most commonly used as a standard to compare examinations carried out by various topographic systems. • very early stages of keratoconus (KC) are difficult to recognize on the float shape due to distance optimized adjustment.

50. • In general, we have to use both the Best Fit Sphere (BFS) and the Best Fit Toric Ellipsoid (BFTE). • The BFS is important for three reasons: (1) To see the shape of the cornea (2) To search for an important risk factor, that is the isolated island or the tongue like extension (3) To locate the cone in KC . • the BFTE is important for two reasons: (1) To evaluate the details of corneal surface (2) To evaluate the severity of the cone in KC .

51. The Pachymetry Map • The pachymetry map has three main landmarks : cornea apex (orange arrow) Thinnest location TL (red arrow) two opposing points on the vertical meridian at the central 5-mm circle (white dotted arrows). The normal difference between the superior (S) and inferior (I) points is ≤ 30 μm. Shape: The normal pachymetry map has a concentric shape .

52. ABNORMAL SHAPES include • a. Horizontal displacement of the thinnest Location . • b. Dome shape. The Thinnest Location is vertically displaced . • c. Bell shape. There is a thin band in the inferior part of the cornea . It is a hallmark for Pellucid Marginal Degeneration (PMD). • d. Keratoglobus. A generalized thinning reaching the limbus .

53. Thickness Profiles • These profiles are only displayed in the Pentacam. There are two pachymetry profiles: Corneal Thickness Spatial Profile (CTSP)  describes the average progression of thickness starting from the Thinnest Location to corneal periphery in relation to zones concentric with the Thinnest Location Percentage Thickness Increase (PTI)  describes the percentage of progression of the same.

54. Normal profile  curved line plotted in red, following (but not necessarily within) the course of the normative black dotted curves, with an average of 0.8–1.1 . • When there is a fast transition of thickness between the Thinnest location and corneal periphery, the average will be high, and vice versa e.g. in an oedematous cornea, the average will be low and the curve will be flat.

55. Abnormal profiles include: • a. Quick Slope . The red curve leaves its course before 6-mm zone. It is encountered in FFKC & ectatic disorders. The average is usually high > 1.1 . • b. S-shape . The red curve has a shape of an “S”. It is encountered in FFKC and ectatic disorders. The average is usually high > 1.1 . • c. Flat shape . The red curve takes a straight course. It is encountered in diseased thickened (oedematous) corneas such as Fuch’s dystrophy & cornea Guttata. The average is low < 0.8 . • d. Inverted . The red curve follows an upward course. It is encountered in some cases of PMD. The average is very low < 0.8 and may take a minus value.

56. QUICK 12 POINT READING ON PENTACAM

57. MODERATE RISK HIGH RISK

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