Indices and screening tests are important to detect early keratoconus and corneal topography is certainly the reference examination that should be systematically performed as part of the preoperative assessment prior to refractive surgery. Iatrogenic corneal ectasia is the most dreaded complication after LASIK. Its most prominent risk factor is the presence of an undiagnosed condition evocative of subclinical keratoconus.
NB : In the view of the author of this page, what is called « keratoconus » encompasses a broad spectrum of conditions all characterized by the presence of a corneal deformation. This deformation is caused primarily by eye rubbing. The various ways in intensity, duration, frequency in which (often allergic) patients rub their eyes explain the broad spectrum of corneal topographic changes, from minor to pronounced, from on eye to the other eye (the lack of intereye symmetry reflects the asymmetry in eye rubbing from one side to the other). The larger the deformation, the weaker the cornea has become. Performing LASIK on a weakened cornea can increase the corneal wall weakening, and in turn aggravate its deformation.
The minor topographic changes are causing diagnostic challenges, which will be referred here as « detection of subclinical keratoconus ».
Introduction
The detection of subclinical keratoconus is one of the most important step in the preoperative evaluation of a refractive surgery candidate to avoid iatrogenic ectasia. This complication induces similar clinical features to those of keratoconus. Topographically, it usually corresponds to progressive increase of the curvature in the inferior paracentral zone, inducing a decline in best corrected visual acuity. Based on a large series of cases reported in the literature, Randleman et al. (1) proposed a score that can be used to predict the risk of ectasia (Ectasia Risk Score) in order to prevent the development of post-LASIK corneal ectasia. This score takes into account the preoperative topographic appearance, the preoperative central corneal thickness, the residual posterior wall, the patient’s age, and the planned correction.
Among these various parameters, the presence of undiagnosed early keratoconus is the main risk factor for post-LASIK ectasia (2).
Many topographic keratoconus screening tests have been developed to help the clinician to more accurately identify the earliest forms of this disease. These tests are designed to identify topographic criteria allowing even earlier diagnosis of keratoconus at a subclinical stage. Although advanced forms of keratoconus generally do not raise any diagnostic difficulties, no subjective method of topographic mapping is currently available to reliably detect or exclude the presence of early keratoconus, which can be attributed to the following elements:
- the quality of the examination (patient movements or blinking that interfere with interpretation of the examination)
- defective perception or analysis by the clinician
- stage of keratoconus: the early topographic changes of the disease are difficult to detect and characterize: automated screening indices are designed to provide an objective aid to the detection of early subclinical forms.
It should also be stressed that some corneal anomalies can have the same topographic expression as early keratoconus (corneal warpage), and that some healthy corneas can also present isolated topographic variations, with an identical appearance to those observed during early keratoconus (e.g.: inferior steepening of the cornea, skewed radial axes, etc.). Regardless of the criteria adopted, it is impossible at the present time to design an objective method able to totally eliminate incorrectly classified corneal topographies (false-negative and false-positive). However, the role of topographic screening tests is to reduce to a minimum the number of incorrectly classified corneal topographies.
For an isolated index (or a group of indices), and regardless of the cut-off value adopted, a screening method based on a cut-off value is unable to discriminate between two populations (green: reference population, red: population with subclinical keratoconus), which, by definition, present similar characteristics. Comparison of the distributions of the values of the index in each population generally reveals a normal distribution and an overlap between the two populations. However, the cut-off value can be adjusted in order to minimize the false-positive and false-negative rates.
This page deals with the topographic indices and screening tests that have been proposed to facilitate early identification of subclinical keratoconus. The terms “test” and “index” are sometimes used interchangeably in the context of screening.
According to a more rigorous definition, a test is an operation designed to provide a positive or negative result or a diagnostic probability. A test is based on the use of one or several numerical indices, which are then combined to produce a qualitative verdict, or a global numerical score. The verdict depends on the value of the score with respect to the defined cut-off value of the test. A test can also consist of a neural network, or a decision tree, into which are entered the values of various numerical indices. A test generally provides a qualitative or semiquantitative result (diagnostic probability of a particular disease).
This type of test is generally called an index, named after the person who designed the test (e.g. Rabinowitz’s indices), which contributes to the implicit confusion between “test” and “index”. The reader must remember that whenever he or she compares the value of an index to a defined cut-off, he/she performs a test!
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