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PENTACAMTOMOGRAPHYINDOREDRISHTI.WORDPRESS.COM

DR DINESH MITTALDR SONALEE MITTALDRISHTI EYE HOSP VIJAYNAGAR INDORE

Corneal topographyThe interpretation of corneal topography has become an important clinical skill for all eye care professionals as our ability to surgically alter the cornea has dramatically improved and patients visual expectations have risen to unprecedented levels. Despite advancements in automated indices and reference databases, interpretation of corneal topography remains an exercise in pattern recognition. In the case of pre-LASIK evaluation, for example, there is now a consensus that abnormality in the shape of the cornea preoperatively is perhaps the most important risk factor for the development of post-surgical ectasia. Therefore, understanding the difference between normal topography and abnormal topography has become increasingly important.

Corneal topographyCorneal topography initially gained prominence in the sphere of refractive surgery for the main purpose of excluding potential candidates for RK or LASIK due to sub-clinical keratoconus. As technology developed, corneal topography developed into a therapeutic corneal tool when the data was used to drive topography-guided photorefractive keratectomy (PRK).

Corneal topographyImproving your understanding of corneal topography/ tomography and its interpretation and clinical application is an excellent start on the pathway to making your cataract and refractive surgeries more successful and your patients more satisfied with their surgery. Skillful interpretation of corneal imaging is a key to successful refractive surgery . However, busy clinicians are challenged to find the time to keep abreast with the latest advances in topographic interpretation.

Corneal topographyModern cataract surgery has evolved into refractive surgery and now topography interpretation is an important part of understanding intraocular lens choice for lens replacement in cataract and clear lens extraction surgery. Multifocal IOLs would be contraindicated in a patient with irregular corneal topography that may present with normal Ks on keratometry. Likewise, good decisions can be made in terms of toric IOL selection using corneal topography.

Corneal topography historyTodays corneal imaging devices have come a long way from the 8-ring Placido-disk systems of the early 1990s, which were limited to topographical maps of the anterior surface and often contained more artifact than information. Todays devices provide a wealth of information about the cornea, and do so using multiple imaging sources, often obtained simultaneously.They minimize artifact using automated image capturing and sophisticated validation algorithms, and even provide diagnostic interpretations, which incorporate information about the anterior and posterior cornea, and corneal thickness.

Corneal topographyCorneal images are inherently technical and are often non-specific. The same map might be normal or abnormal depending on the clinical setting. For example, a pachymetry map showing paracentral corneal thickness of 450 microns might suggest keratoconus in a 21 year old but be of little concern in a 65 year old. Central corneal power of 47 Diopters may be expected after hyperopic LASIK but suggest corneal ectasia after a myopic ablation.

PATTERN RECOGNITIONThe authors use pattern recognition to create associations between corneal conditions and the maps they produce, so clinicians learn to recognize patterns rather than focus on isolated findings. Pattern recognition is fundamental to everyday life, from the earliest stages of development through the most advanced levels of learning.

PATTERN RECOGNITIONInfants use pattern recognition when they differentiate between familiar and unfamiliar faces. Master chess players can plan complex strategies after glancing at a chess board, not by studying the locations of individual pieces but by seeing patterns and layouts that are commonly repeated.Pattern recognition requires familiarity, which we come to recognize as expertise. Experienced automobile drivers come to recognize normal traffic patterns from potential threats in an instant, where new drivers may not. Closer to home, ophthalmologists can readily differentiate an eye that is red from a foreign body from a one with bacterial conjunctivitis, while the difference may not be as obvious to an internist.

Curvature-based Devices

PLACIDO DISC TOPOGRAPHYPlacido-disk topography has been the most commonly used technology since the late 1880s when the Javal-Placido target was developed. Placido-disk topography is based on the reflection of concentric mires (rings) on the cornea. The closer the mires, the steeper the curvature. The wider the mires, the flatter the curvature. Clear surfaces are required for clear reflection of mires.computerized interpretation of the reflections generated by these disks became a reality in the 1970s.

Curvature-Based InstrumentsKeratometryKeratoscopy or PhotokeratoscopyComputerized Videokeratoscopy

Curvature-based DevicesThe normal corneal outer surface is smooth; corneal irregularities being neutralized to some extent by the tear film layer. The anterior surface acts as an almost transparent convex mirror; it reflects part of the incident light. Many devices have been developed to measure the curvature of the anterior corneal surface using the first Purkinje image. These noncontact devices use light target (in different shapes) and a microscope or other optical systems. These curvature-based devices are either quantitative or qualitative, and are either reflection-based or projection-based.

KeratometryA keratometer is a quantitative reflection-based instrument. It measures the corneal radius in the central 3 mm by measuring the size of the reflected image, and converting the image size into corneal radius using the following mathematical relationship:r = 2 x a x (i/o) wherer: anterior corneal radiusa: distance from mire to cornea (75 mm in keratometer)i: image sizeo: mire size (64 mm in keratometer)The keratometer can convert from corneal radius r (measured in meters) into refracting power RP (in diopters) using the followingrelationship: RP = 337.5/ r .

Keratoscopy or PhotokeratoscopyBecause of the small area over which the keratometer can obtain measurements, additional imaging modalities which provided qualitative information about the shape of the entire cornea were needed. While the keratometer only analyses approximately 6% of the anterior corneal surface, the keratoscope measures 70% of the total anterior corneal area (limited by the optical system of the machine itself). A photokeratoscope is a qualitative reflection-based instrument.

Keratoscopy or PhotokeratoscopyThe projected light may be a simple flash light or a Placido disk target. The latter is a series of concentric rings (10 or 12 rings) or a cone with illuminated rings lining the internal surface of the instrument. The rings are concentric, regular and have uniform interspaces a normal cornea. According to the changes in the shape of the reflected rings and the spaces in between, one can take an idea about the shape of the cornea; for instance, small, narrow and closely spaced rings indicate steep regions with small radius of curvature .

Keratoscopy or PhotokeratoscopyThe use of the photokeratoscope is being abandoned; several computerized topographersallowing both qualitative and quantitative measurementsare being used. Some of the known disadvantages of the photokeratoscope are: It requires assumptions about the corneal shape It misses data from the central cornea (not all devices) Limited data points It measures the corneal curvature and not the height It measures only the anterior surface of the cornea Defocusing and misalignment The patient is exposed to high light It is severely affected by tear film disturbances.

Computerized VideokeratoscopyModern topographers are based on projected rather than reflected images. Basically, a projection-based topographer consists of a Placido disk or cone (small or large) which projects a mire of concentric light rings, a video camera that captures the reflected rings from the tear film layer and a software to analyze the data. The computer evaluates the distance between the concentric rings (dark and light areas) in a variable number of points. The shorter the distance, the higher is the corneal power, and vice versa. After analyzing the data, they are plotted by the computer as a color map.

Computerized VideokeratoscopyThe Placido cone may be large or small according to the manufacturer. The larger the cone the more are the rings and the wider is the estimated area. The mires of most systems exclude the very central cornea and paralimbal area. The reproducibility and validity of videokeratography measurements are mainly dependent on the accuracy of manual adjustment in the focal plane.

ELEVATION BASED TOPOGRAPHYPlacido-disk topography has been the standard methodology used for describing the curvature and power of the corneal surface. Placido-disk topography is based on a two dimensional image, however, and is not capable of accurately describing corneal shape. In order to accurately determine corneal shape, a measurement of the Z coordinate, or elevation, is required.

ELEVATION BASED TOPOGRAPHYSeveral systems have been developed over the years to measure the X, Y, and Z coordinates of the cornea in an attempt to determine corneal shape. Of the systems currently available for use, the Pentcam (OCULUS GmbH) is the most commonly encountered. Due to advantages that elevation-based topography and tomography hold over placido-disk topography, the Pentacam is rapidly becoming the standard for corneal imaging, particularly when screening candidates for refractive surgery.

Scheimpflug-based DevicesThere are four devices adopting the Scheimpflug principle and using the Scheimpflug camera. These devices are TMS-5 (Tomey, Nagoya, Japan), Pentacam HR (OCULUS, Wetzlar, Germany), Sirius (CSO Florence, Italy), Galilei (Ziemer, Port, Switzerland).

PENTACAMTOMOGRAPHYTHANKS TO MAZEN M SINZAB

PENTACAM TOMOGRAPHY

Placido-based or curvature-based systems rely on the data collected from anterior surface of cornea, such systems can be reflection-based or projection-based. Without the information about the posterior surface, complete pachymetric evaluation of the cornea is not possible. Ultrasonic pachymetry can give us central and few paracentral measurements, but full pachymetric map is mandatory in modern refractive surgeries .

PENTACAM TOMOGRAPHY

Moreover, the posterior surface of cornea is being more seen as a sensitive indicator of corneal ectasia and can often be abnormal in spite of a normal anterior corneal surface. It is now recognized that while refractive power of cornea is mostly determined by the anterior surface, the biomechanical behavior of the cornea is at least equally determined by both surfaces. In the curvature-based systems the elevation map of the anterior surface is derived from the curvature map, while it is directly calculated in the elevation-based systems.

Principle of a traditional camera. All planes are parallel

Principle of a Scheimpflug-based camera. The three planes intersect at one point

SCHEIMPFLUG CAMERA

The Pentacam system.

Description of the UnitThe OCULUS Pentacam is a rotating Scheimpflug camera . The rotational measuring procedure generates Scheimpflug images in three dimensions, with the dot matrix fine-meshed in the center due to the rotation. It takes a maximum of 2 seconds to generate a complete image of the anterior eye segment. Any eye movement is detected by a second camera and corrected for in the process to some extent. The Pentacam calculates a 3-dimensional model of the anterior eye segment from as many as 25000 (HR: 138000) true elevation points .

Description of the Unit Topography and pachymetry of entire anterior and posterior surfaces of cornea from limbus to limbus are calculated and depicted. The analysis of the anterior eye segment includes a calculation of the chamber angle, chamber volume and chamber depth . In a moveable virtual eye, images of the anterior and posterior surface of the cornea, the iris and the anterior and posterior surfaces of the lens are generated. The densitometry of the lens is automatically quantified.

Description of the UnitThe Scheimpflug images taken during the examination are digitalized in the main unit and all image data are transferred to PC. When examination is finished, the PC calculates a 3D virtual model of the anterior eye segment, from which all additional information is derived. The Scheimpflug law states: To get a higher depth of focus, move the three planes, provided that the picture plane, the objective plane and the film plane have to cut each others in one line or one point of intersection. Advantages of the Scheimpflug camera include higher depth of focus and sharp picture .

Pentacam vs Galilei vs SiriusThe Pentacam combines a rotating Scheimpflug camera with a static camera to acquire multiple photographs of the anterior eye segment. The Galilei Dual Scheimpflug Analyzer integrates a Placido disc and a dual rotating Scheimpflug system for corneal topography and three-dimensional analysis of the anterior eye segment.

Pentacam vs Galilei vs SiriusThe dual camera configuration captures two Scheimpflug slit images from opposite sides of the slit beam and simultaneously tracks decentration due to eye movements . The height data obtained from two corresponding slit images are averaged to improve the measurements of corneal elevation and thickness. The Sirius Scheimpflug Analyzer integrates a Placido disc and a mono rotating Scheimpflug system for corneal topography and three-dimensional analysis of the anterior eye segment.

Pentacam vs Galilei vs SiriusThe aim of integration of Placido disc into Galilei and Sirius is to enhance the analysis of the anterior corneal surface. Three devices have high repeatability. The Pentacam and Galilei could be considered interchangeable contrary to Sirius. The Pentacam has more repeatability and reproducibility than Galilei in measuring the curvature, astigmatism and corneal wavefront, and vice versa in measuring thickness, although both have good correlation and agreement with each other and with ultrasound pachymetry.

Pentacam vs Galilei vs SiriusOn the other hand, the Pentacam is twice more precise than Galilei but as equally precise as Sirius in measuring Sim-K (anterior curvature), while the three devices have same precision in measuring the curvature of the posterior surface. All maps in the Pentacam and Galilei are nearly comparable, while the elevation maps are displayed in Sirius in a special manner that makes them incomparable with those in the Pentacam and Galilei.

PACHYMETRIC MAPSThe remaining map on Pentacam four-map view is called topometric map, or pachymetric map. This map not only determines central or paracentral corneal thickness, as has been traditionally determined by ultrasound, but it also 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. It also provides invaluable data when ruling out subclinical keratoconus (FFKC), as it distinguishes whether thinnest point corresponds with corneal apex.

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

Corneal Parameters

Corneal Parameters QsQs: 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.

Corneal Parameters Q-valQ-val: Value of Q which 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) is found in oblate corneas (e.g. after > 3 D myopic photoablation and after radial keratotomy (RK). Over minus Q ( +3 D hyperopic photoablation and in keratoconus (KC). Both oblate and hyperprolate corneas produce spherical aberrations.

Corneal ParametersK1: (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 is > 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.

Corneal ParametersKm: (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. When Km is < 40 D, free-flap complication may occur; while Km > 46 D may result in a button-hole complication.K-max: Maximum curvature power of the whole anterior surface of the cornea expressed in diopters (D). Normal K-max is < 49 D, the normal difference in K-max between the two eyes is < 2 D, and the 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.

Corneal ParametersAstig: 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 patients 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.

Corneal ParametersPupil 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-coordinatein absolute valueis 200 m (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.

Corneal ParametersThinnest location (TL): Thickness and location of the thinnest point of the cornea. The new definition of thin cornea is a cornea below 470 m with normal tomography, or a cornea below 500 m with abnormal tomography. The normal difference in thickness at the TL between the two eyes is < 30 m. The difference in thickness between TL and pachy apex is normally 10 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.

Corneal ParametersAnterior Chamber Volume (ACV), Angle (ACA) and Depth (ACD):Anterior Chambers with ACV < 100 mm3, ACA < 24 or 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 and ACV 100 mm3.

Corneal Maps

Corneal Maps

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.

The ANTERIOR SAGITTAL mapSteep areas are displayed in hot colours (red and orange), while flat areas are displayed in cold colours (green and blue). On the other hand, red segments are displayed on steep areas, while blue segments are displayed on flat areas. The cross point of this segmentation represents apex (anatomical center) of the cornea. Beside the shape of the map, parameters should be studied particularly on the steep axis at the 5-mm central circle. The normal pattern is the symmetric bowtie (SB) .

The ANTERIOR SAGITTAL map The two segments (a) and (b) are equal in size, and their axes are aligned. Note 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; in this case, the S-I difference should be < 2.5 D. 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.

The ANTERIOR SAGITTAL map

a. In WTR astigmatism, the SB is on or within 15 of the vertical meridian of the cornea .b. In ATR astigmatism, the SB is on or within 15 of the Horizontal meridian of the cornea . c. In oblique astigmatism, the SB is neither vertical nor horizontal .The SB pattern can be encountered in KC when K readings are abnormally high .

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

Abnormal patterns6. Abnormal Symmetric Bowtie (SB) . K READING IS HIGH .7. Symmetric Bowtie with Skewed Radial Axis (SB/SRAX). The angle between the axes of the two lobes is >22 .8. Asymmetric Bowtie/Inferior Steep (AB/IS); the I-S difference is >1.5 D .9. Asymmetric Bowtie/Superior Steep (AB/SS); the S-I difference is >2.5 D .10. Asymmetric Bowtie with Skewed Radial Axis (AB/SRAX). The angle between the axes of the two lobes is >22 .

The Anterior Tangential MapThis 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.

The Elevation Maps

Reference BodyThe 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 surface are displayed as zeros, i.e. exactly like the sea level .

The Elevation MapsAn elevation map describes the height details of the measured corneal surface by matching it with a reference surface (RS). Points above the RS 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 .

Reference BodyThe 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

RELATIONSHIP BETWEEN REFERENCE BODY AND CORNEA

Reference Body TypesToric Ellipsoid BodyIt 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. Its advantage consists in the very good approach to the real course of, e.g. astigmatic corneal surface.Spherical BodyIt is better than the previous bodies in highlighting corneal irregularities 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.

Float ModeThe 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 . On the other hand, 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. Unfortunately, very early stages of keratoconus (KC) are difficult to recognize on the float shape due to distance optimized adjustment.

NO FLOAT AND FLOAT LOCATION

Which Reference Body Should we Use?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 .On the other hand, 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 .

The Enhanced Spherical Reference Body

WHICH REFERENCE BODY & MODE The most important are best fit sphere (BFS) which describes (qualifies) the shape of the measured surface, and best fit toric ellipsoid (BFTE) which estimates (quantifies) the parameters of that surface. For routine use in refractive surgery screening, the ideal diameter of the RS is 8 mm and the ideal mode is the float mode.

Shape (BFS float mode):The normal shape of a cornea with regular astigmatism is the symmetric hour glass . Vertical steeper meridian is shown as depressions ve value .

Abnormal shapes includeAbnormal shapes includea. Skewed hourglass . Normally, it can be seen with large angle Kappa and misalignment during taking the capture, otherwise it indicates an abnormal distorted cornea.b. Tongue-like extension and irregular hourglass . They are seen in abnormal distorted corneas.c. Isolated island . It is encountered in abnormal distorted corneas with central or paracentral protrusion.

Abnormal shapes include

Abnormal shapes include

Parameters (BFTE float mode):Look at the highest plus values within the central 5-mm zone . Abnormal values are > 12 m and > 15 m on the anterior and posterior elevation maps respectively.

Parameters (BFS float mode):look at values corresponding to the TL using the BFS float mode. This can be done by pointing with the cursor at the TL symbol on the elevation maps and left click on the mouse to display the corresponding values.

Cone location (BFS mode):In ectatic corneal disorders, the cone can be localized by the BFS float mode , and can be quantified by the BFTE float mode . On the BFS, the cone can be central, paracentral or peripheral when its apex is located within the central 3-mm zone, between 3-mm and 5-mm, or outside the central 5-mm zone, respectively .

Cone location (BFS mode):Whenthe cone is peripheral, the elevation map takes kissing birds sign . Classifying cone location is important for the treatment of KC.

Corneal Thickness Map

Main Elements

The computer displays the thickness map in two patterns:1. Five values: A central value representing the central thickness, and four values around at the 5 mm central circle .2. Distributed values all over the cornea : The distributed pattern is more important and valuable.However, those are not the main elements of the corneal thickness map. The main elements are three locations that appear on the main page : the thinnest location, the corneal apex and the pupil center. These three locations are displayed with their coordinates, where the corneal apex is the origin point (zero point). The direction of axis X is from the patients 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 +0.3, -0.5 position, i.e. this point is located 0.3 mm temporal to and 0.5 mm inferior to corneal apex.

The Pachymetry MapThe pachymetry map has three main landmarks : cornea apex (orange arrow), Thinnest location TL (red arrow), and the 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 m.Shape: The normal pachymetry map has a concentric shape .

Abnormal shapes includea. Horizontal displacement of the TL .b. Dome shape. The TL 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 .

Thickness ProfilesThese profiles are only displayed in the Pentacam. There are two pachymetry profiles: Corneal Thickness Spatial Profile (CTSP) and Percentage Thickness Increase (PTI). The former describes the average progression of thickness starting from the TL to corneal periphery in relation to zones concentric with the TL. The latter describes the percentage of progression of the same.

Thickness ProfilesThe normal profile is a curved line plotted in red, following (but not necessarily within) the course of the normative black dotted curves, with an average of 0.81.1 .When there is a fast transition of thickness between the TL 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.

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 Fuchs 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.

Corneal TopometryCorneal topometry measures the slope of the cornea. Corneal surface may take one of the four main shapes: spheric, aspheric oblate, aspheric prolate or aspheric hyperprolate . Q-value is positive (> 0) when the cornea is oblate, negative (< 0) when the cornea is prolate or hyperprolate, and = 0 when the cornea is spheric. The normal value is [1 , 0]. In KC, Q-value is highly negative; and after high myopic photoablation, Q-value is positive. Abnormal Q-value causes spherical aberrations. The least spherical aberrations are found when Q-value = 0.27.

Steps of Reading Corneal Tomography

Study the parameters carefully

Study the corneal parameters and focus on the parameters of anterior corneal surface, corneal thickness and anterior chamber Quality of the image (QS) is OK for both surfaces. K-readings are within the normal range; both K2 and K-max are < 49 D and (K-maxK2) is < 1 D. The amount and axis of TA should be compared with MA. Q-value of both surfaces is within the normal range [1 , 0]. TL thickness is > 500 m. Difference in thickness between the TL and pachy apex is < 10 m. There is no vertical displacement of the TL . Angle kappa is not significant ; x-coordinate is < 200 m in absolute value. ACV is > 100 mm3, ACA is normal and > 30, ACD is normal (> 2.1 mm) but < 3.0 mm.

Quality of the image (QS) is OK for both surfaces. K-max is > 49 D and (K-maxK2) is > 1 D. The amount and axis of TA should be compared with MA. Q-value of both surfaces is within the normal range [1 , 0], but on the upper limit indicating a mild hyperprolate cornea. TL is < 500 m. Difference in thickness between the TL and pachy apex is < 10 m. The TL is < 500 m in absolute value. Angle kappa is not significant; x-coordinate is < 200 m in absolute value. ACV is 240 mm3, which is very high; ACA is wide 39.2; and ACD is 3.73 mm, which is quite deep.

The anterior sagittal map: SB, no SRAX, I-S = 1 D, which is normal.

The anterior sagittal map: although S-I is normal (1 D), it is an abnormal pattern, which can be considered either superior steep or junctional with significant SRAX.

The anterior elevation map shows normal shape in BFS mode and normal values within the central 5-mm zone in BFTE mode.

The posterior elevation map shows tongue-like extension in BFS mode and abnormal values (> 15 m) within the central 5-mm zone in BFTE mode.

The pachymetry map: concentric pattern, I-S is normal (< 30 m), and there is no horizontal or vertical displacement in the TL.

The pachymetry map: thin cornea, abnormal I-S (> 30 m), and there is an abnormal inferior displacement of the TL (690 m).

Ectatic Corneal Disorders

Topographic & Tomographic Features of Ectatic Corneal DisordersApart from keratoglobus, ectatic corneal disorders consist of four major entities: KC, PMD, FFKC and Pellucid-like Keratoconus(PLK). These disorders can be classified morphologically and tomographically.

Morphologic Classification

There are three morphologic patterns of a cone .a. Nipple cone .b. Oval cone .c. Globus cone .These patterns are better described by the tangential map. In mild cases, cone morphology may be indeterminate .

Tomographic Classifications

Tomographically, an abnormal cornea can be classified according to the elevation maps, pachymetry map or curvature map. Table summarizes tomographic classification. When more than one of the following criteria is found, any of the above-mentioned patterns is considered as frank KC, FFKC, early stage KC, or at least a case of suspicion according to the severity and amount of the following criteria.

Tomographic Classifications

On the sagittal map:a. Steep K 49 D.b. K-max 49 D.c. Difference in K-max between the two eyes 2 D.d. K-maxSteep K 1 D.e. SRAX > 22. f. S-I 2.5 D.g. I-S 1.5 D.

Tomographic Classifications

On the elevation maps:a. Skewed or irregular hourglass, isolated island or tongue-like extension.b. Plus values > 12 m on the anterior elevation map (within central 5-mm zone). c. Plus values > 15 m on the posterior elevation map (within central 5-mm zone).

Tomographic Classifications

On the pachymetry map:a. Dome-shaped, Bell-shaped or Globus.b. S-I > 30 m.c. TL < 470 m, or < 500 m in case of abnormalities.d. Thickness @ pachy apexthickness @ TL > 10 m.e. Y coordinate value of the TL > 500 m.f. Difference in thickness between the two eyes @ the TL > 30 m.

Tomographic Classifications

On thickness profiles:a. Average > 1.1.b. Quick slope.c. S-shape.d. Flat shape.e. Inverted.

Forme Fruste KeratoconusForme Fruste Keratoconus (FFKC) is a subclinical disease and is not a variant of KC. Although clinicians use many other terms such as mild KC, early KC and subclinical KC, their exact meanings and applications are less certain. These terms are not universally accepted. The diagnosis of KC is clinically aided by tomography, while the diagnosis of FFKC is only by tomography or even beyond tomography by measuring corneal biomechanics or other considerations.

Forme Fruste KeratoconusRecently, there are two definitions of this disease:1. In a patient with a normal cornea in one eye and KC in the other, the normal cornea can be considered FFKC. If both eyes are normal but the patient has a first- or second-degree relative with KC, that patient may be considered to have FFKC. In other words, it is the potential to develop ectasia after keratorefractive surgery.2. Another definition of FFKC is a cornea with abnormal tomography that is not distinct enough to be classified clearly as one of the ectatic disorders

Pellucid Marginal Degeneration andPellucid-like KeratoconusPellucid marginal degeneration (PMD) is an ectatic corneal disorder characterized by peripheral inferior corneal thinning observed with slitlamp biomicroscopy and Scheimpflug image. Pellucid-like keratoconus (PLK) is a different entity; it is KC that has some features of PMD as will be discussed bellow.Differentiation between these two entities is important for the right plan of treatment. PMD and PLK can be differentiated by their tomographic features on the curvature, elevation and pachymetry maps.

Pellucid Marginal Degeneration andPellucid-like Keratoconus Curvature map: The anterior sagittal curvature map shows a claw pattern . This is seen in both PMD and PLK. Elevation maps: There are two important things related to each other that can be identified on the elevation maps, peripheral location of the cone and the "kissing birds" sign. Neither of these two signs is a hallmark of PMD or PLK.

Pellucid Marginal Degeneration andPellucid-like KeratoconusPachymetry map: In PMD, the pachymetry map shows a characteristic pattern known as "bell-shape" pattern . This pattern is a hallmark of PMD and absent in PLK . Pachymetry profiles: In KC, PLK and in PMD, the curve deviates from the normal range rapidly and usually before the 6-mm zone . The S-shape is an indicator of ectatic corneal disorders . In advanced cases of PMD, the curve usually follows an inverted course .

QUICK 12 POINT READING ON PENTACAM

pentacam

BFS>8 ANT ; >18 POST

DECISION

PRKNO RISK DO PRK OR LASIK

K READINGS

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K READINGS effect on flap properties

Some typical presentations

KeratoconusKeratoconus is a bilateral, noninflammatory degeneration of the eye characterized by paracentral corneal ectasia and steepening, high keratometry values, and often against the- rule astigmatism. The thinning is reported to originate in the stroma of the cornea.

KeratoconusEarly keratoconus presents on topography as inferior or central steepening, with or without a difference compared with the fellow eye. Mild astigmatism may be present. Several indices have been developed to differentiate these eyes from normal eyes, such as the inferior-superior (I-S) difference, but a high false-positive rate exists. Although these indices may be helpful, the diagnosis remains largely dependent on pattern recognition on the part of the clinician.

Keratoconuskeratoconus is a disease that progresses anteriorly through the cornea, starting at the posterior surface and progressing toward the anterior surface. Therefore, the anterior elevation map appearance is similar to that of the posterior surface, but this is seen later in the disease process. WE consider anterior elevation to be abnormal if it is > +4 m at the thinnest point or> +6 m at the anterior apex.

KeratoconusKeratoconus is a disease process that is first evident in the posterior corneal surface; it then moves anteriorly through the corneal surface until it is evident anteriorly. Therefore, posterior elevation maps are useful in detecting the earliest cases of subclinical keratoconus. As with anterior elevation maps, we have chosen to use a best fit sphere as the reference surface . WE consider posterior elevation of > +19 m at the thinnest point or > +6 m at the posterior apex to be abnormal .

KeratoconusEyes with keratoconus, or subclinical keratoconus, typically have thinner corneas than normal eyes. Keratoconic eyes also have a more progressive increase in corneal thickness from the center to the periphery. In other words, there is a more rapid increase in thickness when moving from the center to the periphery in eyes with keratoconus than there is in normal eyes. Furthermore, the thinnest point of a keratoconic eye typically is inferior to the center of the cornea, which is known as inferior displacement. The pachymetric map on the Pentacam is useful to detect these differences in eyes with possible keratoconus .

KeratoconusThe Pentacam offers the ability to view measured tomographic data in many different formats, depending on the needs of the clinician. The four-map view is the standard view of the Pentacam. The four-map refractive view presents 4 maps that are most useful to clinicians screening patients for refractive surgery. This view is useful because it shows the traditional axial power map, anterior elevation map, posterior elevation map, and pachymetric map. Each map provides valuable data regarding the health and structure of the cornea. When viewed together as a group, a tremendous amount of data are available to the clinician on one page.

Pellucid Marginal DegenerationSimilar in findings to keratoconus, pellucid marginal degeneration is characterized by inferior peripheral thinning of cornea due to an idiopathic, noninflammatory condition. Generally thought to be a distinct condition from keratoconus, many experts now consider PMD to be the same process as keratoconus but occurring in a different area of the cornea, thus generating a unique topographic appearance .

Pellucid Marginal DegenerationAlthough high amounts of against-the-rule astigmatism are usually found, typically there is normal central corneal thickness and an intact central epithelium and, thus, lack of corneal scarring. Corneal topography remains the gold standard for diagnosis. Although the crab-claw appearance may be present in patients topography maps for both keratoconus & PMD additional elevation maps and locations of corneal thinning can differentiate the 2 degenerations.

Pellucid Marginal Degeneration Pellucid marginal degeneration typically has a 1- to 2-mm wide band, or strip, of thinning that is more peripherally located, whereas keratoconic patients usually have an inferior temporal spot or area of thinning in a cone shape.

PRK or LASIKEyes with a history of prior excimer laser surgery, either PRK or LASIK, have similar appearances. In eyes with prior myopic treatment, there is central flattening relative to the periphery. This results in flatter simulated K readings on the axial power maps. The opposite is true of eyes with prior hyperopic treatment because tissue is removed from the periphery to induce a relative steepening centrally. Anterior elevation maps are particularly important in eyes with prior excimer laser surgery, as excimer laser procedures remove tissue from the anterior corneal surface .

Post Refractive Surgery EctasiaEctasia is described as progressive thinning and steepening of the cornea, resulting in irregular astigmatism and loss of best-corrected visual acuity Topographically, post refractive surgery ectasia resembles keratoconus. The hallmarks are inferior steepening on the axial power map, as well as elevation abnormalities noted first posteriorly and later anteriorly. However, the presence of central flattening from myopic excimer laser surgery complicates topographic interpretation.

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