CORNEAL TOPOGRAPHY

Suraj ChhetriB. Optometry

16th batch

The word topography derived from Greek word : ‘topos’ (place) and ‘graphein’ (to draw)

Corneal topography corresponds to the graphic representation of the geometrical properties of corneal surface

Tomography

Derived from Greek word : Tomos (slice/ section) and grapho (to write )

Topographic reprsentation of nepal Saggital section of human head

Dimensions of cornea

Anterior SurfaceDh- 11.5mmDv- 10.6mm

Posterior surfaceDh=Dv- 11.5mm

Center- 0.52mmPeriphery- 0.67mmLimbus- 1.2mm

Human corneal surface is aspheric.Central optic zone radii of curvatureAnterior- 7.8mmPosterior- 6.8mm

Diameter Thickness Radii of curvature

OPTICS OF CORNEA Cornea is the most powerful refractive element of the eye

Contributes about 43D(70%) of refractive power of eye

Even a minor modification on its surface can lead to a significant alteration of the images formed on the retina

The most critical element to preserving corneal optics is the status of the corneal surface and tear film

The swelling properties of the cornea( Mild epithelial edema) can produce the symptoms of halos around bright lights or Sattler’s veil,…

While moderate stromal edema can also decrease visual acuity( VA) primarily through light scatter, although this does not become significant until swelling of 70 % is achieved.

Causes of irregular corneal surface

Corneal pathologies Ectatic degenerative diseases

Basement Membrane DystrophyBullous KeratopathyInfectious KeratitisTraumaCorneal ulcer

KeratoconusPellucid marginal degenerationTerriens marginal degenerationKeratoglobus

Corneal topographers have emerged as a powerful tool with which to assess the etiology of factors that degrade vision by

producing irregularities on the corneal surface that lead to optical aberrations

Any distortion in the corneal surface leads to reduced quality retinal image.

• Direct examination of the corneal surface with the biomicroscope does not provide enough resolution to detect vision-reducing irregular astigmatism

• Although retinoscopy provides a greater sensitivity to irregular astigmatism, the distortion seen in the retinal reflex (e.g., scissoring and distorted shadows) Does Not Always Indicate the nature or the location of the irregular astigmatism

• Keratometers and Ophthalmometers have proven to be useful when one limits the measurement of corneal power in spherocylindrical notation

• Assumes the normal pupil to be approximately 3 mm in diameter.

• Clinicians have long sought a device that the entire surface of the cornea

• Issues of corneal regularity, symmetry, and the general nature of the peripheral cornea are important for the understanding of corneal optics.

• Reflection techniques, such as the Placido disk, keratometry, photokeratoscopy, and corneal topography all arise from this principle

• However, it was not until the development of corneal topography that clinicians were provided with easily under-stood color-coded maps of corneal curvature as well as quantitative indices of irregular astigmatism that correlate with potential visual acuity

Indications & uses• Preoperative and post operative assessment of refractive

patient

• Preoperative and post operative assessment of penetrating keratoplasty

• Irregular astigmatism

• Corneal dystrophy and bullous keratopathy

• Keratoconus (diagnostic and follow-up)

• Follow-up of corneal ulceration and abscess

• Contact lens fitting

• Evaluation of tear film quality

• To study unexplained low visual acuity after any surgical procedure:( trabeculectomy, extracapsular lens extraction,….)

• Post traumatic corneal scarring

• Reference instrument for IOL-implants to see the corneal difference before and after surgery

Background

• In 1619 Father Christopher Scheiner realized that one could estimate

corneal curvature by comparing the reflection of a window on the corneal surface to that on a series of different sized marbles.

The Placido disk, introduced in 1880 by Antonio Placido

Consists of a circular target of alternating white and black rings or mires with a central aperture through which one can view its virtual image

Example of a Placido disk pattern. It includes a series of spaced circular segments centered around a central point.

Distortion of the Placido mires images reflected on a surface S with spherical (a) and toroidal (b) surface geometry.

These days, a resurgence of interest in corneal topography has occurred, mainly due to

The increasing use of keratorefractive procedures and contact lens fitting in patients with corneal surface abnormalities

Topography:Technique

Keratoscopy

Placido- disc

Photo-keratoscop

y

Video-keratoscop

y

Keratometry Rastersterography

Interferometry

Topography: principle

Optical principle and techniques

Keratometry

r= (-2d h’)/h Where, r = radius of curvature d = distance between object and 1st Purkinje image h’= image height h = mire separation

Keratometer mires

Keratoscopy– Is a general term that refers to the evaluation of

topographic abnormalities of the corneal surface by direct observation of the images of mires reflected from the surface of the cornea

Videokeratoscopic mires are closer together in the axis of steep curvature (arrow),and farther apart in the flat axis (arrowhead) in this post-penetrating keratoplasty patient.Major axes are not orthogonal. (Courtesy of John E. Sutphin, MD.)

Small degrees of abnormalities of corneal shape go undetected.

Could not be used in cornea with epithelial defects and stromal ulcers because cornea cannot reflect the target

Placido disc is used only as a gross method of qualitative assessment of the corneal surface

Disadvantages

Photokeratoscopy

When a photographic film camera is attached to a keratoscope, it is a photokeratoscope

In the technique, the keratoscopic image is photographed and the size of the images on the photographic film can be changed to change the size of the corneal image

The image of most photokeratoscope rings covers the paracentral, overlapping into the central and peripheral zones but leaving the optically important central 2-3 mm as well as the peripheral cornea

Current photokeratoscopes ( eg., Nidek PKS-1000 or Keracorneascope) have 9-15 rings which cover 55-75% of the corneal surface

The corneal cylinders of up to 3 D can escape detection by use of photokeratoscopy

Videokeratoscopy

When a television camera is attached to a keratoscope, it is a videokeratoscope

With the advent of computers, the videokeratoscopy has been computerised

A portrayal of the video recording of the corneal surface is called a videokeratography

Videokeratoscopy

• Presently, the computer-assisted videokeratoscopy

corneal topography.

It covers approximately 95% of the corneal surface

Rastersterography

It uses a direct image on the corneal surface

It projects a caliberated grid pattern of horizontal and vertical lines (spacing of 0.2 mm is used) onto the fluorescein

Stained tear film, takes a photograph and uses computer algorithms to analyse the pictures

Rasterserography The accuracy of the system is

0.30 D for a diameter of 7 mm.

The advantage of this systemis that it includes all of the cornea, including a part of the sclera.

interferometry

It uses the technique of light wave interference.

This method has become obsolete and is of no concern to present-world study of corneal topography.

Computer Topography Systems

Presently the term corneal topography system (CTS), or videokeratography, implies computerised, video-assisted technique that provides detailed information about the shape of the corneal surface

The technique has an excellent accuracy and repeatability.

Most corneal topographers evaluate 8000-10000 specific points across the entire corneal surface.

• Basic unit of CTS primarily consists of – A projection device– Video Camera– Digital computer attached to a slit-lamp chin rest

Computer topography system

Different types of CTS1.Placido-disc topography systems

Makes use of 8-32 concentric rings on the cornea

Commercially available Placido-disc topography systems

Corneal Modeling System( CMS)Computerised Corneal Topographer EH 270EyeSys 2000 Corneal Analysis SystemTMS-1 Topographic Modeling System

2. Slit-imaging topography systems

CTS based on this technology uses scanning slits that step over the corneal surface to acquire topographic information

This is similar to the slit-lamp in principle. Two slits are used, positioned at 45 deg angles to the right and left of the instrument axis

Slit-imaging topography systems

Twenty slit images are captured from each direction with overlap in a 7-mm diameter central area

Total corneal coverage is up to 10 mm, depending on the individual corneal shape

All images are captured within approximately 1.5 seconds.

Measure all surfaces of anterior segment.

Advantage

Relatively long scanning time.Data regarding the accuracy

and reproducibility of Pentacam are not available

yet.

Disadvantage

Commercially available ‘slit-imaging’ topography systems

orbsacn pentacam

Orbscan Most recent of the commercially available topography

systems.

It combines a slit scanning system and Plasido’s disk

Measures elevation and curvature of anterior and posterior surfaces of cornea along with full pachymetry map.

It uses the principle of back-scattered reflection, unlike the specular reflection employed in keratometry.

Back-scatter reflection. This is used in Orbscan. This is omni-directional.

Specular reflection. This is used in keratometry and is

angle dependent.

Orbscan Orbscan uses slit-beams and back-scattered light to

triangulate surface shape. The derived mathematical surface is then ray traced using a basic keratometer model to produce simulated keratometer (Sim K)values

Orbscan Orbscan II, the improved model is an integration of the Slit

scanning technology into the Placido-disc system OR

Orbscan + Placido-disc = Orbscan II

HOW IS RECORDING MADE?

40 slit images (40 slits limbus to limbus)are acquired in two 0.7 second periods.

During acquisition, involuntary saccades typically move the eye by 50 microns.

Eye movement is measured from anterior reflections of stationary slit beam and other light sources.

Eye tracking data permit saccadic movements to be subtracted form the final topographic surface.

Orbscan If we were doing topography with a machine, which

does not have slit scan imaging facility, we would not be able to see the topography of the posterior surface of the cornea

If any abnormality in the posterior surface of the cornea, for example as in primary posterior corneal elevation this would not be diagnosed

Orbscan Then if we perform Lasik on such a patient we would

create an iatrogenic keratectesia. The orbscan helps us to detect the abnormalities on the posterior surface of the cornea.

Colour meanings in the spectral direction ( from blue to red)

Red = high, steep, sharp, shallow, thin, or focused Blue = low, level, flat, deep, thick, or aberrated.

General quad map of a primary posterior corneal elevation. Notice the upper right map has an abnormality whereas the upper left map is normal. This shows the anterior surface of the cornea is normal and the problem is in the posterior surface of the cornea.

Pentacam Images the anterior segment of the eye, using a rotating

Scheimpflug camera, and pictures in these dimensions of the anterior segment are shown by this rotating process

The images captured are then used to construct the anterior corneal surface (corneal topography), posterior corneal surface and anterior axis and anterior lens surfaces

The Scheimpflug is modification of slit- lamp camera.

Has modified geometry to improve depth of focus and include distortion correction algorithms.

The image in the ordinary camera. The main disadvantage is limited depth of focus because the picture plane, objective plane and the film plane are parallel.

Scheimflug intersection

The Scheimpflug camera. Higher depth of focus, sharp image but distorted. The picture plane, the objective plane and the film plane cut each other in one line or one point of intersection.

A two-dimensional cross-sectional image results. When performing A scan, cameras are used to capture the image.

One centrally located camera detects pupil size and orientation, and controls fixation.

The second rotates 180 degrees to capture 25 or 50 images of the anterior segment to the level of the iris, and through the pupil to evaluate the lens.

Pentacam 500 true elevation data points are generated per image to

yield up to 25,000 points for each surface.

Data points are captured for the center of the cornea, an area that placido disc topographers and slit scanning devices are unable to evaluate.(Based on an elevation map)

Formats for display of data on color maps

1. Corneal Power Map (Axial or Sagittal)– 24-colour representation of dioptric power at various

points on the cornea

What is the axial curvature then???

The sagittal (axial method). The curvature power of the measured surface in point “a” is calculated using a tangent line in this point, the normal in this point intersects the reference axis at point b, ab is the radius (r) of point “a”, finally the equation is applied to calculate the power (K) at point a.

Corneal Power Map (Axial or Sagittal)Axial curvature closely approximates the power of the central 1-2 mm of the cornea but fails to describe the true shape and power of the peripheral cornea.

For almost sphere cornea… the curvature is almost the same from the centre to the periphery.In fact, cornea are not always spherical.

This is not a good descriptor in corneal topography.

2. Tangential or Intanstaneous map– A better geographic representation of the cornea

than the axial/saggital map.

The tangential (local method). The principle depends on tangent circles rather than straight lines.

Please notice the difference…this is an axial map…

3. Elevation map

Elevation is not measured by Placido-based topographers, but certain assumptions allow the construction of elevation maps

Elevation of a point on the corneal surface displays the height of the point on the corneal surface relative to a spherical surface.

The reference surface is mostly considered to be a sphere, or, to be precise, a Best Fit Sphere ( BFS)

Calculates corneal elevation data from a reference ellipse/sphere

In case of topographical landmaps, elevations are measur-ed from a reference plane atsea level,

for example..the height of Mt Everest is 8848 m

( from sea level)

98900-6S.PPT

DEVIATION FROM REFERENCE SPHEREDEVIATION FROM REFERENCE SPHEREHEIGHT FROM REFERENCE PLANEHEIGHT FROM REFERENCE PLANE

DESCRIPTIONS OF CORNEAL TOPOGRAPHYDESCRIPTIONS OF CORNEAL TOPOGRAPHY

SAGITTAL RADIUSSAGITTAL RADIUS TANGENTIAL RADIUSTANGENTIAL RADIUS

hh dd

rSrS rTrT

TTSS

The BFS (green) is the closest sphere to the corneal surface. Once the sphere radius and location are positioned, elevation is plotted as the distance from the surface to the sphere.

The relationship between the reference body and thecornea. Corneal surface (yellow) has steep center and flat periphery in accordance to this particular reference body.

4. Refractive Power Map

Optical power maps (or ‘refractive maps’) are functional maps and should not be confounded with curvature maps.

Since the cornea suffers aberrations, the refraction varies along the meridians even if the curvature does not change….owing to spherical aberration..esp…positive spherical aberration.

Why????

These light rays near the center of the cornea have a small angle of incidence relative to the surface normal, and thus the refracted angle is also quite small. Contrarily, light rays in the paracentral and peripheral cornea have a larger angle of incidence relative to the surface normal, and thus the refracted angle is larger.

Color codes

Absolute

Normalized

Klyce/wilson

Maguire/waring

Adjustable

SCALES

Absolute scale

• Scale range: 9.0D to 101.5 D

• Average value 43D

• Central area divided into 11 graduations 1.5D interval

• Beyond 11 graduations interval of 5.0 D

Normalized scale

• Divided into scale of 11

• Displays the max and min refractive power of examined eye

Adjustable scale

• 26 graduations

• Max and min value can be selected by examiner

Klyce/Wilson scale

• Scale range: 28.0D to 65.5D

• Interval of 1.5D

Maguire/waring scale

• Scale range: 32.0D to 57.0D

• Interval of 1.0D

Topographic display in Pentacam

Statistical indices employed in videokeratoscopy

SimK: Simulated keratometry It is obtained from the greatest power observed on

the corneal surface from an average of rings 12-16 along every meridian

The power and axis orthogonal to the highest power are also reported as they are in traditional keratometry

Higher than normal values are often associated with keratoconus, penetrating keratoplasty, and the occasional steep normal. Lower than normal values occur with myopic refractive surgical corrections and the rare flat normal.

SimK2: Simulated keratometry.

– SimK2 is the power of the flat meridian orthogonal (90°) to SimK1.

MinK: Minimum Keratometry Value

Often the meridians of highest and lowest power are not orthogonal.

It is useful to know the meridian for which the actual minimum power occurs, particularly in the planning of astigmatic keratotomy.

This situation most often occurs with keratoconus, penetrating keratoplasty, and trauma, although it may be present after cataract surgery as well.

• SAI: Surface Asymmetry Index

– The SAI measures the difference in corneal powers at every ring (180° apart) over the entire corneal surface.

– The SAI is often higher than normal in keratoconus, penetrating keratoplasty, decentered myopic refractive surgical procedures, trauma, and contact lens warpage.

– Adequate spectacle correction is often not achieved when SAI is high.

• Cyl: Simulated keratometric cylinder

– The simulated keratometric cylinder of the corneal surface is obtained from the Sim K readings.

– Higher than normal values of Cyl are associated with several pathologies, trauma, and surgery.

• SDP: Standard Deviation of corneal Power

– The SDP is calculated from the distribution of all corneal powers in a videokeratography.

– SDP is often high for keratoconus corneas, transplants, and trauma - all situations in which there is a wide range of powers occurring in the measured topography.

CVP: Coefficient of Variation of corneal Power

The CVP is calculated from the Standard Deviation of corneal Powers (SDP) divided by the grand average of corneal powers.

High values are found in Keratoconus, corneal transplants.

• IAI: Irregular Astigmatism Index

The IAI is an area-compensated average summation of inter-ring power variations along every meridian for the entire corneal surface analyzed.

The IAI increases as local irregular astigmatism in the corneal surface increases. IAI is high in corneal transplants shortly after surgery; persistence often sub optimal best spectacle corrected vision.

EDP: Elevation/Depression Power

EDP calculates the average power of apparent islands and valleys for those areas of the cornea that are within the demarcated pupil.

SRI: Surface Regularity Index

The SRI is a correlate to potential visual acuity and is a measure of local fluctuations in central corneal power.

When SRI is elevated, the corneal surface ahead of the entrance pupil will be irregular, leading to a reduction in best spectacle-corrected visual acuity. High SRI values are found with dry eyes, contact lens wear, trauma, and penetrating keratoplasty

PVA: Potential Visual Acuity

Irregularities in corneal topography ahead of the entrance pupil reduce the visual potential of the eye.

The consequence of these irregularities are assessed by the calculation of the Surface Regularity Index which has been correlated to PVA in a published clinical study.

The PVA is given as the range of best spectacle-corrected Snellen visual acuity that might be expected from a functionally normal eye with the topographical characteristics of the analyzed cornea.

ACP : Average Corneal Power

The ACP is an area-corrected average of the corneal power ahead of the entrance pupil.

It is generally equal to the keratometric spherical equivalent except for decentered refractive surgical procedures.

Abnormal values occur for the same reasons as for keratometry

CEI: Corneal Eccentricity Index

– The CEI is a measure of corneal eccentricity, a global shape factor.

– A positive (normal) value is obtained for a prolate surface, a nil value for a sphere and a negative value is used to indicate an oblate surface.

– Out of range values include keratoconus (higher than normal) and negative values often found with contact lens wear and myopic refractive surgical corrections.

EDD: Elevation/Depression Diameter

EDD is the equivalent diameter of the area found to contain powers within the pupil 1 Diopter or more from the mode.

It is calculated from twice the square root of this area divided by Pi.

The units are mm.

LogMAR: Log of the Minimum Angle of Resolution

LogMAR is a standard unit for expressing high contrast (Snellen) visual acuity.

It relates directly to how well the eye can discriminate between two points of light.

KPI: Keratoconus Prediction Index

The Keratoconus Prediction Index is less than 0.225 for normals.

It is obtained by discriminant analysis of corneal statistical indexes.

This numerical estimator is used to indicate the presence of a keratoconus pattern in corneal topography1 1 (Invest Ophthalmol Vis Sci 35:2749-2757, 1994)

• KSI: Keratoconus Severity Index

– Keratoconus suspect is interpreted when KSI reaches 0.15

– Clinical keratoconus is triggered at a KSI of 0.30 2

2 (Invest Ophthalmol Vis Sci 38:2290-2299, 1997)

DSI: Differential Sector Index

The DSI reports the area-compensated greatest difference between any two (out of eight) sectors of a given cornea.

Here it shows a DSI of 10 D.

OSI: Opposite Sector Index

The OSI is higher than normal in keratoconus. It represents the maximum difference between an area-corrected corneal powers between any two opposite sectors of the cornea.

CSI: Center/Surround Index

– The CSI is the area-corrected difference in average corneal power between the central 3 millimeters of analyzed area and an annulus surrounding the central area from an inner radius of 1.5 mm to an outer radius of 3 mm.

– The CSI is higher than normal with centrally located keratoconus and refractive surgical correction of hyperopia. It is subnormal for myopic refractive surgical corrections.

Corneal Topogaphic patterns in NORMAL CORNEAS

The normal cornea flattens progessively from the centre to the periphery by 2-4 D, with the nasal area flattening more than the temporal area

Two corneas of an individual show mirror-image asymmetry.

Variations are common in nature.

Corneal Topogaphic patterns in NORMAL CORNEAS

• Depending on the corneal curvature, Rabinowitz et al. in 1996 described 10 different corneal topographic patterns in normal eyes.

Corneal Topogaphic patterns in NORMAL CORNEAS

Round: 22.6%

Symmetric bow-tie: 17.5%

Asymmetric bow-tie: 32.1%

Irregular:7.1%

Clinical Applications of Corneal Topography

Corneal Topography in Ectasias

Mires lie closest together in the inferiocentral region where the cornea is steepest and furthest apart superiorly where the cornea is flattest.

Keratoconus

Keratoconus is almost always bilateral, and one cornea is more involved than the other (in this case OD). Note that, while OS at first impression looks fairly normal, the ‘lazy eight’ astigmatic pattern is a common characteristic of keratoconus.

keratoconus

Keratoconus suspect is diagnosed in curvature maps when the central corneal power or Sim K is greater than 47.2 D or I-S value greater than 1.4 D.Keratoconus…48.7 D and I-S value > 1.9

I-S = difference between the powers in the superior and the inferior paracentral regions

Also diagnosis:Mean central thickness: < 500μ; thinnest point <470 μm; difference of >100 μm from thinnest point to 7 mm zone

Pellucid marginal degeneration

Pellucid marginal degeneration

The topographical findings in PMD include high against-the-rule corneal astigmatism and inferior mid-peripheral steepening at 4 and 8 o’clock. This pattern in creates a “kissing pigeon” or “butterfly wing-like” or “crab claw” pattern diagnostic of PMD.

Terriens degeneration

In Terriens degeneration when the thinning is restricted to the superior or inferior area of the peripheral cornea, there is relative steepening approximately 90 deg away. ATR Astigmatism..with vertical flattening….The diagnosis always requires a pachymetry map.

Pterygium

Asymmetrical nasal flattening of the right eye of a patient with pterygium. This is marked with-the-rule toricity.

Clinical Applications of Corneal Topography

• Photorefractive keratectomy (PRK) and Laser-assisted in-situ keratomileusis (LASIK)

– Information about the quality, diameter and centration of the ablated zone along with its stability over a course of time.

–Decentration---halos or glare effectIrregular ablation zone---decreased visual acuity

Videokeratography of the left eye of a myopic patient after laser in situ keratomileusis (LASIK). Upper left, preoperative eye. Follow-up: lower left, 2 weeks; upper right, 2 months; lower right, 6 months after surgery. This format is useful to study topographic stability.

References