Diagnostic Procedures in Ophthalmology PDF

46 Diagnostic Procedures in Ophthalmology FRANCISCO ARNALICH, DAVID PIÑERO, JORGE L ALIÓ Corneal 4 Topography The cornea is the most important...

46 Diagnostic Procedures in Ophthalmology FRANCISCO ARNALICH, DAVID PIÑERO, JORGE L ALIÓ Corneal 4 Topography The cornea is the most important refractive element of the human eye, providing approxi- mately two-thirds of optical power of the eye, accounting for about 43-44 diopters at the corneal apex. Because its surface is irregular and aspherical, it is not radially symmetric, and simple measurement techniques are inadequate. The great upsurge in refractive surgery led to a need for improved methods to analyze corneal surface and shape since refraction and kerato- metric data alone were insufficient to predict surgical outcomes. Understanding and quanti- fying corneal contour or shape has become essen- tial in planning modern surgical intervention Fig. 4.1: Helmholtz ophthalmometer for refractive surgery, as well as in corneal transplantation. It is also very valuable for Keratometer assessing optical performance of the eye. The different methods for evaluating the In 1854 Helmholtz described the first true keratometer, which he called an ophthalmometer anterior surface of the cornea, developed over (Fig. 4.1). With some minor improvements, it is several centuries, have, in the present era, led to the modern corneal topographers. still being used clinically for calculating refraction, intraocular lens power and contact lens fitting. History of Corneal Measurement This apparatus is based on the tendency of In 1619 Scheiner analyzed corneal curvature by the anterior corneal surface to behave like a matching the image of a window frame reflected convex mirror and reflect light. The projection onto a subject’s cornea with the image produced of four points, or mires, onto the cornea, creates by one of his calibrated spheres. a reflected image that can be converted into a  Corneal Topography 47 corneal radius, “r”, using a mathematical reflections of a series of illuminated concentric equation that considers distance from the mire rings (known as Placido’s rings) first time in to cornea (75 mm in the keratometer), image size 1880 (Fig. 4.2). In 1896 Gullstrand developed and mire size (64 mm in keratometer). The corneal a quantitative assessment of photokeratoscopy. radius can be transformed into dioptric power The keratoscope, like a keratometer, projects using the formula: an illuminated series of mires onto the anterior corneal surface, usually consisting of concentric DP= (index of refraction of the lens - 1)/ r rings. The distance between the concentric rings The standard keratometric index represents or mires gives the observer an idea of the corneal the combined refractive index of the anterior and shape. A steep cornea will crowd the mires, while posterior surfaces of the cornea, considers the a flat cornea will spread them out. Surface cornea as a single refractive surface, and is irregularity is seen as mire distortion. 1.3375. Thus, the equation can be simplified to: When a photographic camera is attached to DP= 337.5/ r the keratoscope, it becomes a photokeratoscope, Although keratometers are still common in which gives semi-quantitative and qualitative ophthalmology clinics, they do have specific information about the paracentral, midperipheral limitations that need to be considered in order and peripheral cornea. to avoid misleading conclusions. Based on the mathematical equation, it is 1. Most traditional keratometers measure the possible to calculate corneal power from object central 3 mm of the cornea, which only size. Still, photokeratoscopy gives limited accounts for 6% of the entire surface. information on the central area, which is not 2. It assumes that the cornea is a perfectly covered by the mires. sphero-cylindrical surface, which it is not. The cornea is aspheric in shape, flattening between the center and the periphery. Usually the central corneal curvature is fairly uniform, and this is the reason why it can be used to calculate corneal power in normal patients. However, this is not true in some pathogenic conditions like ectatic disorders or after refractive surgery. 3. The keratometer provides no information as to the shape of the cornea either inside or outside the contour of the mire. Several corneal shapes can all give the same keratometric value so this apparatus is of little use should it become necessary to Fig. 4.2: Placido’s rings reconstruct the whole corneal morphology. Keratoscopy and Photokeratoscopy Videokeratoscopy Goode presented the first keratoscope in 1847. Modern corneal topographers are based on Placido is credited to photograph the corneal videokeratoscopy. A video camera is attached 48 Diagnostic Procedures in Ophthalmology to the keratoscope, and the information is Q is asphericity, a parameter that is used to specify analyzed by a computer that displays a color- the type of conicoid. coded map of power distribution or corneal For a perfect sphere this parameter takes the curvature of the anterior corneal surface (Fig. value of zero (Q=0), for an ellipsoid with the major 4.3). It overcomes some of the limitations of other axis in the X-Y plane (oblate surface) the asph- methods, since it measures larger areas of the ericity is positive (Q>0), for an ellipsoid with cornea, with larger number of points thus the major axis in the Z axis (prolate surface) asphe- increasing resolution. Computer technology ricity is negative (-1 1.4 while those with clinical keratoconus aberrations of an eye. Even in a normal eye with had central corneal power > 47.8 D or I-S > 1.9. no subjective need for refraction, optical However, using only these simple aberrations can be detected. measurements for a diagnosis could create Since the cornea has the highest refractive specificity problems. To solve the specificity power, more than 70% of the eye’s refraction, problem, the new strategy must be able to detect it is the principal site of aberrations, although and consider the unique characteristics of the lens and the tear film may also contribute keratoconus maps, such as local abnormal to aberrations. elevations. The Keratoconus Prediction Index, developed by Maeda et al, is calculated from Fundamentals the Differential Sector Index (DSI), the Opposite Sector Index (OSI), the Center/Surround Index Measuring Total Wavefront Aberration (CSI), the SAI, the Irregular Astigmatism Index It is possible to express ideal image formation (IAI), and the percent Analyzed Area (AA). This by means of waves. An ideal optical system will method partially overcomes the specificity provide a spherical converging wave centered limitation. at the ideal point image. However, in practice, Maeda et al also developed the neural network the resulting wavefront, differs from this ideal model, based on artificial intelligence. It is a much wavefront. The deviation from this ideal more sophisticated method for classifying corneal wavefront is called wavefront aberration, and the topography and detecting different corneal more it differs from zero, the more the real image topographic abnormalities; it employs indexes differs from the ideal image and the worse the that were empirically found to capture specific image quality. Ocular wavefront sensing devices characteristics of the different corneal patho- use four main technologies to determine the logies, including keratoconus. Further modifica- resulting or output wave: tions in neural network approach developed by 1. The Shack-Hartmann method is the most Smolek and Klyce supposedly produce 100% widely used and is inspired by astronomy accuracy, specificity and sensitivity in technology. It consists of analyzing the wave diagnosing keratoconus. emerging from the eye after directing a small low energy laser beam. This reflected wave is divided by means of a series of small Corneal Aberrometry: Fundamentals lenses (lenslet array) which generates and Clinical Applications focused spots. The position of spots is Whenever a point object does not form a point recorded and compared to the ideal one image on the retina, as it should be in an ideal 2. The Tscherning technique uses typically a grid optical system, one encounters an optical that is projected onto the retina. The aberration. Although one may feel that he is distortion of the pattern is analyzed and measuring the total refractive error, when used to calculate the wavefront aberration refracting a patient, one is actually only of the eye. 64 Diagnostic Procedures in Ophthalmology 3. The ray tracing system is similar to the as keratorefractive procedures or penetrating Tscherning technique. However, instead of keratoplasty, since the anterior corneal surface a grid, a programmable laser serially projects is the only one modified. The corneal wavefront light beams that forms spots on the retina aberration, which is the component of the total at different locations. ocular wavefront aberration attributed to the 4. The spatially resolved refractometer evaluates cornea, can be derived from the corneal topograp- the wavefront profile using the subjective hic height data. Specifically, the calculation of patient response. This technology is not wavefront aberrations is performed by expanding practical for clinical use. the anterior corneal height data into a set of orthogonal Zernike polynomials (Fig. 4.19). Measuring Corneal Wavefront Aberration Zernike Polynomials It is known that 80% of all aberrations of the human eye occur in the corneal area and only For a quantitative description of the wavefront 20% of aberrations originate from the rest of the shape there is a need for a more sophisticated ocular structures. The effect of corneal aberrations analysis than conventional refraction, as the latter is especially important after corneal surgery such only divides the wavefront in two basic terms: Fig. 4.19: Corneal wavefront analysis derived from height topography data  Corneal Topography 65 frequency (m). When talking about first, second, third order aberrations we point to indicate the radial order (n). Each radial order involves n + 1 term. There are an infinite number of Zernike terms that can be used to fit an individual wavefront. However, for clinical practice, terms up to the 4th radial order are usually considered: 1. Zernike terms below third order can be measured and corrected by conventional optical means. The first order term, the prism, is not relevant to the wavefront as it represents tilt and is corrected using a prism. The second order terms represent low order aberrations that include defocus (spherical component of the wavefront) and astig- matism (cylinder component). Wavefront Fig. 4.20: Zernike polynomial expansion maps that measure only defocus and astigmatism can be perfectly corrected using sphere and cylinder. One can obtain more spectacles and contact lenses. information by breaking down the wavefront into terms which are clinically meaningful, besides 2. After the second radial order comes high order aberrations. These are not measured by the sphere and the cylinder. For this purpose, conventional refraction or auto refraction. a standard equation has been universally accepted by refractive surgeons and vision The aberrometer is the only method available that can quantify these complex kinds of scientists, known as Zernike polynomials. distortions. Zernike polynomials are equations which are used to fit the wavefront data in a three dimen- 3. Third order terms describe coma and trefoil defects. sional way. The wavefront function is decompo- 4. Fourth order terms represent tetrafoil, sed into terms that describe specific optical aberrations such as spherical aberration, coma, spherical aberration and secondary astig- matism components. etc. (Fig. 4.20). Each term in the polynomial has Because spherical and coma aberrations refer two variables, ρ (rho) and θ (theta), where ρ is the normalized distance of a specific point from to symmetrical systems and the eye is not rotationally symmetrical, the terms spherical-like the center of the pupil, and θ is the angle formed and coma-like aberrations are normally used between the imaginary line joining the pupillary center with the point of interest and the horizontal. (Fig. 4.21). According to that, we can imagine that Wavefront Maps aberrations are strongly influenced by pupil size, and, therefore, aberrometric measurements Wavefront map describes the optical path diffe- should be related to the diameter of the patient’s rence between the measured wavefront and the pupil. reference wavefront in microns at the pupil Zernike terms (Znm) are defined using a double entrance. The wavefront error is derived index notation: a radial order (n) and an angular mathematically from the reconstructed wavefront 66 Diagnostic Procedures in Ophthalmology Fig. 4.21: Spherical-like and coma-like wavefront aberration maps by one of the techniques described above. It is Optical and Image Quality plotted as a 2D or 3D map for qualitative analysis in a similar fashion to corneal topography maps. In order to evaluate the impact of aberrations on visual quality following quantitative para- In wavefront error maps, each color represents meters have been defined (Fig. 4.23): a specific degree of wavefront error in microns (Fig. 4.22) and like corneal topography maps, Peak to valley error (PV error): This is a simple it is necessary to consider the range and the measure of the distance from the lowest point interval of the scale. to the highest point on the wavefront and is not  Corneal Topography 67 Fig. 4.22: Corneal wavefront aberration maps that include all kind of aberrations including low and high order the best measurement of optical quality since results and it is linked to the RMS by the Maréchal it does not represent the extent of the defect. formula. Root mean square error (RMS error): This measure Point spread function (PSF): This is the spread is by far the most widely used. In a simple way, function observed on the retina when the object the RMS wavefront error is a statistical measure is a point in infinity. PSF measures how well of the deviation of the ocular or corneal wavefront one object point is imaged on the output plane from the ideal (Table 4.1). In other words, it (retina) through the optical system. In the eye, describes the overall aberration and indicates small pupils (approximately 1 mm) produce how bad individual aberrations are. diffraction-limited PSFs, because of the pupil border. In larger pupils, aberrations tend to be Strehl ratio: This represents the ratio of the the dominant source of degradation. maximum intensity of the actual image to the maximum intensity of the fully diffracted limited Modulation transfer function, Phase transfer function image, both being normalized to the same and Optical transfer function: Sinusoidal gratings integrated flux. This ratio measures optical greatly simplify the study of optical systems, excellence in terms of theoretical performance because irrespective of the amount of eye aberra- 68 Diagnostic Procedures in Ophthalmology Fig. 4.23: Visual quality summary obtained with the CSO topographer. It is possible to visualize the wavefront map (gray scale), Strehl ratio, PSF and MTF function TABLE 4.1: REFERENCE VALUES FOR CORNEAL ABERRATIONS IN THE NORMAL POPULATION Pupil Total Astigmatism Spherical Coma RMS Spherical- Coma- (mm) RMS RMS aberration like RMS like RMS 3 0.19 ± 0.07 0.14 ± 0.08 0.04 ± 0.03 0.05 ± 0.03 0.07 ± 0.02 0.09 ± 0.03 5 0.53 ± 0.21 0.43 ± 0.24 0.15 ± 0.05 0.14 ± 0.08 0.18 ± 0.05 0.20 ± 0.08 7 1.26 ± 0.43 0.92 ± 0.53 0.52 ± 0.17 0.42 ± 0.23 0.57 ± 0.16 0.52 ± 0.22 RMS: root mean square, Coma primary coma: terms Z3±1, Spherical aberration primary spherical aberration: term Z40 Spherical-like: terms fourth and sixth order, Coma-like: terms third and fifth order Reference: Vinciguerra P, Camesasca FI, Calossi A. Statistical analysis of physiological aberrations of the cornea. J Refract Surg 2003; 19 (Suppl): S265-9.  Corneal Topography 69 tions, sinsusoidal grating objects always produce Clinical Uses of Corneal Topography sinusoidal grating images. Consequently, there are only two ways that an optical system can Pathological Cornea affect the image of a grating, by reducing contrast Corneal topography is a very important tool in or by shifting the image sideways (phase-shift). the detection of corneal pathologies, especially The ability of an optical system to faithfully ectatic disorders. Screening for these anomalies transfer contrast and phase from the object to or their potential development is a critical point the image at a specific resolution are called in preoperative evaluation for refractive surgery. respectively the modulation transfer function (MTF) Keratorefractive procedures are contraindicated and the phase transfer function (PTF). The eye’s in these abnormal corneas. optical transfer function (OTF) is made up of the MTF and the PTF. A high-quality OTF is, therefore, represented by high MTF and low Keratoconus PTF. Keratoconus is characterized by a localized conical protrusion of the cornea associated with an area of corneal stromal thinning, especially Clinical Applications at the apex of the cone. The typical associated Aberrometers allow practitioners to gain a better topographic pattern is the presence of an inferior understanding of vision by measurement of high area of steepening (Fig. 4.25). In advanced cases, order aberrations. These aberrations reflect a the dioptric power at the apex is at or above refractive error that is beyond conventional 55 D. In a small group of patients, the topographic spheres and cylinders. There may be a large group alterations may be centered at the central cornea. of patients whose best corrected visual acuity In these cases there may be an asymmetric bow- (BCVA) may improve significantly on removal tie configuration, and normally the inferior loop of the optical aberrations and this new refractive is larger than the superior loop (Fig. 4.26). entity has been called aberropia. Reduced optical Keratoconic corneas have three common charac- quality of the eye produced by light scatter and teristics that are not present in normal corneas: optical aberrations may actually be the root cause 1. An area of increased corneal power surroun- of blurred vision associated with dry eye ded by concentric areas of decreasing power syndrome and tear film disruption. Measurement 2. An inferior-superior power asymmetry of these aberrations can also be helpful in 3. A skewing of the steepest radial axes above keratoconus, orthokeratology, post graft fitting, and below the horizontal meridian. irregular astigmatism or when refractive surgery Keratoconus suspects are problematic. They has reduced the patient’s optical quality. may signal impending development of a clinical Customized ablations are the future step in keratoconus, but they may also represent a laser technology that should address not only healthy cornea. The lack of ectasia in the fellow spherical and cylindrical refractive errors (low- cornea does not indicate that the keratoconus order aberrations), but also high-order aberra- suspect will not progress to true keratoconus. tions such as trefoil and coma (Fig. 4.24). Thus, In these cases the ideal management is close vision can be optimized to the limits determined follow-up of the signs of keratoconus in order by pupil size (diffraction) and retinal structure to check on their stability, and a thorough and function. analysis of the videokeratographic indexes. 70 Diagnostic Procedures in Ophthalmology A B Figs 4.24A and B: Customized ablation designed according to corneal aberration for the correction of aberrations induced by a decentered ablation. There is a large amount of coma: axial map A and customized ablation designed B with the ORK-CAM software (Schwind) Pellucid Marginal Degeneration “butterfly” appearance that results in a flattening of the vertical meridian and a marked against- Pellucid marginal degeneration is characterized the-rule irregular astigmatism (Fig. 4.27). by an inferior corneal thinning between 4 and 8 O’clock positions above a narrow band of clear thinned corneal stroma. The ectasia is extremely Keratoglobus peripheral and it presents a crescent-shaped Keratoglobus is a rare bilateral disorder in which morphology. This pattern has a classical the entire cornea is thinned out most markedly  Corneal Topography 71 Fig. 4.25: Keratoconus topography pattern near the corneal limbus, in contrast to the peripheral increase in corneal power (steepening) localized central or paracentral thinning of and a very asymmetrical bow- tie configuration. keratoconus. It is very difficult to obtain reliable and reproducible measurements in these cases Terrien’s Marginal Degeneration due to the high level of irregularity and the poor quality of the associated tear film. Reliable In Terrien´s marginal degeneration there is a topographic examinations show an arc of flattening over the areas of peripheral thinning. 72 Diagnostic Procedures in Ophthalmology Fig. 4.26: Keratoconus with an asymmetric bow-tie configuration When thinning is restricted to the superior and/ common feature, as this disorder involves more or inferior areas of the peripheral cornea, there frequently the superior and/or inferior peripheral is a relative steepening of the corneal surface cornea. If the area of thinning is small or if the approximately 90 degrees away from the disorder extends around the entire circumference midpoint of the thinned area. Therefore, high of the cornea, central cornea may remain against-the-rule or oblique astigmatism is a relatively spared with a spherical configuration.  Corneal Topography 73 Fig. 4.27: Pellucid marginal degeneration topography pattern Fig. 4.28: Corneal astigmatism induced by a pterygium 74 Diagnostic Procedures in Ophthalmology Pterygium reached at the periphery. The corneal effect of surgery could be determined by analyzing the Pterygium is a triangular encroachment of the difference map between the preoperative and conjunctiva onto the cornea usually near the postoperative measurements. medial canthus. When the lesion continues to grow out onto the cornea, it could lead to a high Postradial Keratotomy (RK) degree of astigmatism. When the growth of pterygium is about 2 mm or more, a flattening Radial keratotomy (RK) corrects myopia by of the cornea at the axis of the lesion occurs placing a series of radial incisions (nearly full (Fig. 4.28). This produces a marked with-the-rule corneal thickness) leaving a central clear zone astigmatism, even of more than 4 D. The evolution (optical zone). These incisions cause a flattening of the pathology and the surgical outcome could of the central cornea due to retraction of the most be monitored by changes in corneal topography. anterior collagen fibers and the outward pressure of the intraocular force. This area of flattening is surrounded at an approximately 7 mm zone Postoperative Cornea in by a bulging ring of steepening called the Refractive Surgery paracentral knee. This increases asphericity and Keratorefractive procedures attempt to alter the corneal irregularity. curvature of the central and mid-peripheral A very typical finding in these corneas is cornea, and usually have a minimal effect on a topographic pattern with a polygonal shape. the corneal periphery. The area in which the Depending on the number of incisions made, curvature is modified is called the optical zone. squares, hexagons or octagons can be seen. The This tends to be surrounded by a small zone angles of the polygons correspond closely to the of altered curvature before normal cornea is central ends of the incisions (Fig. 4.29). Fig. 4.29: Polygonal pattern in a postradial keratotomy cornea  Corneal Topography 75 Postastigmatic Keratotomy (AK) pulsing beam of ultraviolet light) to reshape the cornea. To correct myopia, the excimer laser Astigmatic keratotomy is a simple modification flattens the central cornea by removing tissue of the radial keratotomy that is used to correct in that area. However, the optical zone needs astigmatism. Rather than placing incisions to be steepened to correct hyperopia. This is radially on the cornea, incisions are strategically achieved by removing an annulus of tissue from placed on the steepest meridian. The incisions the mid-periphery of the cornea. induce a flattening in that meridian, but provoke The topographic pattern in myopic correc- steepening in the perpendicular meridian, in a tions shows a flattening of the central cornea, process called coupling. Coupling results from oblate profile (Fig. 4.30). The treatment zone is the presence of intact rings of collagen lamellae usually easily delineated by the close proximity that run circumferentially around the base of of adjacent contours at its edge. Hyperopic the cornea. With the surgery, these rings become corrections have a pattern of central steepening oval in the operated meridian and transmit forces surrounded by a ring of relative flattening at to the untouched meridian. The stigmatic change the edge of the treatment zone (more prolate achieved is the sum of the flattening in one profile) (Fig. 4.31). In astigmatic treatment, the meridian and the steepening in its perpendicular treatment zone is oval. meridian. Inadequate ablations during surgery can be detected postoperatively by analyzing the Postphotorefractive Keratotomy resulting corneal topography. Decentrations can Photorefractive keratotomy (PRK) is a procedure only be identified by a relatively asymmetric which uses a kind of laser (excimer laser, a cool localization of the treatment area (Fig. 4.32). Other Fig. 4.30: Topographic pattern after a myopic ablation 76 Diagnostic Procedures in Ophthalmology Fig. 4.31: Topographic pattern after a hyperopic ablation Fig. 4.32: Pattern of decentered myopic ablation  Corneal Topography 77 Fig. 4.33: Central island after myopic photoablation complicated patterns that may lead to severe making the corneal surface more irregular (Fig. vision disturbances are the presence of focal 4.35). irregularities or central islands (Fig. 4.33) produced by an inhomogeneous laser beam or Postlaser Thermal Keratoplasty an irregular process of corneal healing. In laser thermal keratoplasty (LTK), a Holmium laser is used to heat corneal stromal collagen Postlaser in situ Keratomileusis in a ring around the outside of the pupil. The heat causes the tissue to shrink, producing a Postlaser in situ keratomileusis (LASIK) is an zone of localized flattening centered on the spot, excimer laser procedure like PRK, but in this and a surrounding zone of steepening. This case tissue is ablated under a superficial corneal bulging effect of the central cornea makes it possi- flap in order to minimize the influence of the ble to correct hyperopia. The typical topographic epithelium. The topographic patterns for myopic pattern shows the central corneal steepening and and hyperopic corrections are the same as in a ring of flattening overlying the spots. PRK (Figs 4.30 and 4.31). Although the ablation is covered by a flap of corneal tissue, surface irregularities and central islands may still occur. Postintrastromal Corneal Rings Decentration may also occur in a LASIK ablation, Implantation depending on the patient’s ability to maintain Intrastromal rings are small segments or rings, eye fixation during surgery (Fig. 4.34). Epithelial made of a plastic-like substance, that are inserted in-growth at the periphery of the flap-stromal into the periphery of the cornea to correct small interface produces an area of steepening degrees of myopia or hyperopia. They act as surrounded by an area of marked flattening spacers and by changing the orientation of the 78 Diagnostic Procedures in Ophthalmology A B Figs 4.34A and B: Topographic patterns of LASIK decentered ablations after myopic treatment A and after hyperopic treatment B  Corneal Topography 79 A B Figs 4.35A and B: Topographic analysis in a post-LASIK cornea with an epithelial in-growth at the inferonasal area: placido rings image A, and axial map B collagen lamellae, depending on their shape and performed, the quality of the surgical procedure, position, flatten or steepen the central cornea. whether sutures are still in place in the cornea, Intrastromal rings could also be used to reduce and the time elapsed after the procedure. Sutures the corneal steepening and astigmatism usually induce a central bulge in the corneal associated with keratoconus (Fig. 4.36). graft and its removal results in a decrease of the astigmatic component. The prolate configu- ration after keratoplasty is the most frequent Postkeratoplasty pattern with a high degree of irregularity (Fig. Keratoplasty topographies exhibit a wide variety 4.37). There can be multiple regions of abnormally of patterns, depending on the type of keratoplasty high or low power, or both simultaneously in 80 Diagnostic Procedures in Ophthalmology Fig. 4.36: Management of keratoconus by intrastromal rings the map. Irregular astigmatism over the entrance occur mixed with one another: (i) peripheral pupil may be detrimental to optimum visual steepening, (ii) central flattening, (iii) furrow acuity in the keratoplasty patient. depression, and (iv) central molding or central irregularity (Fig. 4.38). Inferior corneal steepening (pseudokeratoconus) Contact Lens-induced Corneal is caused by a superiorly riding contact lens that Warpage or Molding flattens above the visual axis with an apparent Corneal warpage is characterized by topographic steepening below. The topographic image could changes in the cornea following contact lens wear appear similar to keratoconus, but both conditions (most frequently in wearers of hard or RGP lenses) are easily differentiated. In corneal warpage, the as a result of the mechanical pressure exerted shape indexes do not indicate any keratoconic by the lens. There are at least 4 different forms condition, and the flat K is not as steep as in of noticeable topography change that usually keratoconus.  Corneal Topography 81 Other Uses of Corneal Topography Corneal topography is a diagnostic tool, but it is also essential before all refractive procedures, to enable the surgeon to understand the refractive status of an individual eye, and plan the optimum refractive treatment. The corneal topography is also used for the following purposes: 1. To guide removal of tight sutures after corneal surgery (keratoplasty, cataract surgery, etc.) that are causing steepening of the cornea (Fig. 4.39). 2. To help in the designing the astigmatic Fig. 4.37: Topographic pattern after keratotomy. penetrating keratoplasty Fig. 4.38: Corneal warpage 82 Diagnostic Procedures in Ophthalmology Fig. 4.39: Superior corneal steepening caused by a tight suture 3. To guide contact lens fitting: Selection of 2. Bogan SJ, Waring GO, Ibrahim O, Drews C, Curtis the probe lens and design of the lens. L. Classification of normal corneal topography based on computer-assisted videokeratography. 4. To calculate the keratometry values for the Arch Ophthalmol 1990;108:945-9. calculation of the required power of an 3. Boyd BF, Agarwal A, Alio JL, Krueger RR, intraocular lens for implantation. This is Wilson SE. (Eds). Wavefront analysis, aberro- an important issue in corneas that have meters and corneal topography. Highlights of undergone refractive surgery, because it is Ophthalmology, 2003. 4. Cairns G, McGhee CNJ. Orbscan computerized more difficult to estimate the real keratometric topography: Attributes, applications, and values in order to avoid over or under limitations. J Cataract Refract Surg 2005;31:205- corrections. 20. 5. To evaluate the effect of a keratorefractive 5. Corbett M, O’Brart D, Rosen E, Stevenson R. procedure. Corneal topography: principles and applications. BMJ Publishing Group, 1999. 6. Corneal Topography. American Academy of Ophthalmology. Ophthalmology 1999;106:1628- Bibliography 38. 7. Courville CB, Smolek MK, Klyce SD. Contri- 1. Ambrosio R Jr, Klyce SD, Wilson SE. Corneal bution of ocular surface to visual optics. Exp topographic and pachymetric screening of Eye Res 2004;78:417-25. keratorefractive patients. J Refract Surg 2003;19: 8. Dabezies OH, Holladay JT. Measurement of 24-29. corneal curvature: keratometer (ophthalmo-  Corneal Topography 83 meter). In Contact Lenses: the CLAO Guide 18. 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