Corneal Topography In Clinical Practice
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Developments in corneal topography allow for increasingly precise, detailed analysis of the corneal surface. This test is becoming indispensable in the treatment of complex corneas: keratoconus, corneal transplants, orthokeratology, etc. In refractive surgery, the combined analysis of the anterior and posterior cornea has permitted better screening for forme fruste keratoconus at risk for post-LASIK ectasia. Topography also assists in the calculation of premium intraocular lenses. Topography is an indispensable test for analyzing and following corneal disease.
Corneal topography is a non-invasive medical imaging technique for mapping the surface curvature of the cornea, the outer structure of the eye. This procedure may be carried out with a Pentacam, which uses a rotating camera to create a 3D image of the anterior of the eye. This second edition has been fully updated to provide the latest developments in corneal topography and tomography using the Pentacam machine. Beginning with an introduction, the following sections describe the fundamentals of corneal topography and use of the Pentacam with different ophthalmic disorders. With nearly 250 high quality, colour images and illustrations, this concise guide is especially useful to graduate and postgraduate students in learning how to read and interpret corneal topography.
In this article, we will review what corneal topography and tomography are, why they are useful, and how to interpret a normal Pentacam scan. We will also review 5 clinical uses for topography that will prepare you well for cornea clinic.
It is laid out in six sections beginning with a short introduction, followed by a concise chapter on the basic anatomy of the eye with an overview of several instruments that can be employed to measure corneal topography. The fundamentals of corneal topography as applied in the Pentacam system are briefly presented with a description of the various functions available from the software and how they are displayed. Wavefront analysis of ocular refractive surfaces and densitometry is mentioned. Detecting keratoconus, recognising its progression if considering corneal cross-linking treatment and performing topographic screenings prior to refractive surgery are potentially hot topics, so the section on keratoconus and keratoectasia will prove especially useful for those practitioners new to the subject.
A key feature of the Pentacam system is its ability to provide an accurate map of the entire anterior and posterior surfaces of the cornea from limbus to limbus. Several examples of typical topographical patterns are depicted, and a section is dedicated to the common causes of artefacts and their interpretation. The final section deals with those topographical patterns characteristic of irregular astigmatism and corneal topography pertinent to cataract surgery, where choice of incision and intraocular lens (IOL) type can be tailored to individual patients.
Experienced corneal specialists are likely to find the text too basic, but junior colleagues, ophthalmic nurses, newly-qualified optometrists involved in corneal clinics and biometrists should find a refreshing insight provided by the clear descriptions and notes on the clinical interpretation of the illustrations.
The Pentacam (Oculus Optikgerate, Wetzlar, Germany) is an anterior segment analyzer that implements the Scheimpflug principle in photography to capture slit images and generate a variety of data in a non-contact fashion. The system is equipped with a rotating Scheimpflug camera, and a light source that emits UV-free blue light with a wavelength of 475 nm. All projected slits overlap at the central cornea to increase the accuracy of central data. A single acquisition provides users with color maps of the corneal topography and pachymetry, and elevation maps of the anterior and posterior corneal surfaces.5
The main focus was given to screen patients for refractive surgery, the interpretation of elevation maps, corneal thickness maps and its progression analysis. Drs. Belin & Khachikian present the topic in an orderly, systematic method ,that starts with the basics and progresses to understanding complex cases. The book is extensively illustrated with generous use of color. This masterpiece should serve as a basic text for any refractive surgeon interested in understanding modern topography.
The morphological characterization of the cornea using corneal topographers is a widespread clinical practice that is essential for the diagnosis of keratoconus. The current state of this non-invasive exploratory technique has evolved with the possibility of achieving a great number of measuring points of both anterior and posterior corneal surfaces, which is possible due to the development of new and advanced measurement devices. All these data are later used to extract a series of topographic valuation indices that permit to offer the most exact and reliable clinical diagnosis. This paper describes the technologies in which current corneal topographers are based on, being possible to define the main morphological characteristics that the keratoconus pathology produces on corneal surface. Finally, the main valuation indices, which are provided by the corneal topographers and used for the diagnosis of keratoconus, are also defined.
The use of previously described corneal topography technologies involves the generation of errors during the acquisition process and data handling, and can be distinguished between intrinsic and extrinsic errors.
Recent studies show that such errors can be reduced by new corneal topographers without a significant influence in data acquisition and processing [35, 42], so ophthalmologists accept the existence of these errors, which are considered negligible when we consider the clinical advantage that these topographers provide for the diagnosis of ectatic corneal diseases.
Another source of error is the execution of the scanning process together with other clinical examination procedures. Recent research works have confirmed a significant variation in corneal parameters after instillation of different anesthetic eye drops. It has also been demonstrated that the dramatic reduction of the intrasession repeatability of measurements in this context, so this should be considered in order to avoid inappropriate clinical decision making .
The manifestation of this protrusion in the surface of the anterior shape generates a structural weakness in the cornea, which also implies a change in the morphology of the posterior corneal surface, showing an increased curvature when comparing with a healthy cornea, even during incipient stages . There are several studies that have characterized and clinically evaluated the geometric correlation between anterior and posterior surfaces, both in healthy corneas [56, 57] and corneas with keratoconus disease . Other studies have evaluated the curvature and the elevation of the posterior surface of the cornea in eyes with keratoconus to quantify this increase and assess whether these changes can be used as clinical tools for the diagnosis of subclinical keratoconus [55, 59].
Corneal topographers provide different maps to represent the measurements that characterize the surface of a keratoconic cornea. The ophthalmologist has to decide what scale should be considered in each case in order to get the best information from a clinical point of view : the absolute scale gives the entire diopter range that the topographer can measure on a color scale, so it loses sensitivity to small changes whereas the relative scale adjusts the dioptric measurement range for each cornea, so it is sensitive to small changes and is more appropriate for a customized analysis of corneal morphology [7, 9, 52, 60].
Elevation maps. These maps do not represent data directly measured by the corneal topographer, but are obtained by comparing the reconstruction of the anterior or posterior corneal surface to the best fitted surface, typically a sphere, a toroid, a revolution ellipsoid or a non-revolution ellipsoid. The difference between both surfaces is provided by altimetry data that correspond to the elevation maps (Fig. 7) [14, 63, 64]. In addition, typical dimensions of the reference surface are around 8 mm in diameter, so scenarios that may influence the data acquisition process, such as shadows generated by eyelashes, are avoided . These maps present several advantages: i) data are presented quantitatively in μm, so they are highly accurate and have a high sensitivity to small changes that can occur in the corneal morphology as a result of keratoconus , ii) topographers allow the selection of the best suited surface to perform the elevation map, which results in a sensitivity increase of the clinical diagnosis. This map provides, for both corneal surfaces, the elevation of the corneal apex, the elevation of the minimum thickness point  and the elevation of the center of the central region . As the posterior surface is not altered by the excimer laser photoablation or by the generation of the corneal flap, nor is adulterated by the hyperplastic effect of corneal epithelium, data from the posterior surface could be very useful for the clinical diagnosis of keratoconus [9, 13, 47, 65, 66].
The main use of corneal topography is the generation of indices that allows quantification of the level of irregularity of the corneal morphology, at a local or general level. From these data, there are clinical studies that have tried to determine cut-off values to distinguish between normal and pathological corneas as well as to define several severity degrees of keratoconus. However, the main problem with these indices lies in the fact that each index has a high degree of specificity for the corneal topographer for which it has been developed, and cannot be directly extrapolated to other corneal topographers. In scientific literature, there are numerous indices used for the diagnosis of keratoconus, which are known as univariate or multivariate detection systems. Depending on the analysis approach, it could be a single index or a combination of indices that allows the interpretation of the main topographic characteristics of the keratoconus disease. 59ce067264