Centre For Eye Health
Keratoconus is classified as a progressive, non-inflammatory disorder, involving corneal thinning and steepening, and is usually bilateral in nature. It has always been important to assess for risk factors and early signs of keratoconus in patients considering refractive surgery. Given the advent of crosslinking in particular, early diagnosis of keratoconus, especially in rapid progressors has become increasingly important; however, the literature on methods for early diagnosis as well as what constitutes progression is at times equivocal.
This article provides a brief overview of current knowledge in the field, focusing on early diagnostic methods as well as techniques for determining progression, while the accompanying chairside reference aids in differentially diagnosing keratoconus from other mimicking conditions.
DIAGNOSIS OF EARLY KERATOCONUS
Keratoconus has been linked to atopy and repeated knuckle rubbing of the eye.1
Pre-Descemet’s and posterior polymorphous corneal dystrophies have also been associated with keratoconic steepening,2 while a number of eye and systemic conditions, such as Leber’s congenital amaurosis as well as Down, Turner, Marfan and Ehlers-Danlos syndromes have an increased prevalence.3
Ethnicity can also guide diagnosis; persons of Asian, Arabian, Maori and Pacific Islander descent are more commonly affected.3
Functional markers of keratoconus are limited in the early stages but can include blurred vision, which is typically more evident in scotopic conditions, as well as decreased contrast sensitivity, glare, haloes, ghosting and monocular diplopia.4
Structural markers utilising standard optometric clinical equipment are characterised by an increase in astigmatism, scissors reflex on retinoscopy, oil droplet reflex on ophthalmoscopy, prominent corneal nerves, and abnormal or distorted keratometry readings.5 On the contrary, Vogt striae, Fleischer’s ring and Munson’s sign as well as reduced corneal thickness are not typically present in early stages.5
Structural markers utilising imaging
Numerous imaging devices have been shown to detect changes in corneal properties with the aim of differentially diagnosing early keratoconus from an asymmetric normal eye; however, there are no specific, agreed-on instrumental values that define keratoconus. As a result, the initial diagnosis is often based on a combination of these results or alternatively a change in values signalling progression.
A wide variety of corneal topographers are currently commercially available with a correspondingly large number of techniques and analyses aimed at the early diagnosis of keratoconus and the ability to quantify change. Elevation and sagittal maps are the most commonly utilised features, typically exhibiting an elevation of the anterior and posterior surfaces or an asymmetric ‘bow tie’ appearance, respectively6 (Figure 1). It is typically more prominent on the posterior surface in early keratoconus. While this elevation is most commonly evident inferior to the visual axis, it can occur centrally and in other locations.
A wide variety of indices, such as the ‘KISA% index,’ are based on topographically-derived values and have been proposed in the literature to determine the likelihood of a cornea being keratoconic. To date, all of these markers have shown limited sensitivity and specificity.
Figure 1. Topography of an early keratoconic eye: asymmetric corneal steepening is shown in the inferior anterior (A) and posterior (B) surfaces, and is more prominent in the posterior surface maps. An asymmetric bow-tie pattern is demonstrated on the axial curvature map (C) and minimum corneal thickness is located inferotemporal to the visual axis (D).
As well as reduced values compared to expected norms, corneal thickness maps often show a decentration of the thinnest point inferior to the visual axis.7 Epithelium thickness measurements, now commercially available with anterior optical coherence topographers (OCT) have been shown to demonstrate a relative thinning over the apex of an impending cone prior to notable stromal thinning.8
As early keratoconic corneas are typically asymmetric with an elevation inferior to the visual axis, higher order aberrations are often elevated above expected normal values and are predominated by coma.9
The Ocular Response Analyser, and more recently the Corvis, have been used to assess corneal resistance. Research has found the corneal resistance factor to be significantly lower in patients with early keratoconus.7
A large number of parameters have been proposed to determine whether keratoconus is progressive, including changes in each of the detection methods mentioned above. However, for most, longitudinal studies investigating the sensitivity and specificity and/or a definition of the degree of change signifying progression are still outstanding.
Proposed specific change criteria in the literature include: epithelial thinning by greater than or equal to 2%, an increase in Kmax of greater than or equal to 1.00 D, increased manifest cylinder of greater than or equal to 1.00 D over 24 months and a change in radius of the back surface optic zone of a best fitting RGP lens greater than or equal to 0.1 mm.10
In a recently published paper, 36 corneal specialists collaborated in an effort to standardise classification and management of keratoconus with regards to three topics: definition/diagnosis, non-surgical management and surgical treatment of keratoconus. Progressive keratoconus was defined as consistent change in two or more of the following variables with the value of the change more than the test-retest variation of the instruments:
• steepening of the anterior corneal surface
• steepening of the posterior corneal surface
• corneal thinning6
It is important to note that while values for instrument variability have been shown to be small in normal eyes, variability markedly increases in pathological states.11,12
In summary, it is important to take into account historical risk factors as well as functional and structural markers when assessing a patient for early keratoconus and potential progression. Wavefront aberrometry, epithelial thickness measurements and corneal resistance measurements can be used as adjunctive measures to topography and traditional clinical techniques.
The author thanks Michael Yapp, Angelica Ly, Elizabeth Wong and Dr Barbara Zangerl for their input into this article.
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3. Mas Tur V et al. A review of keratoconus: Diagnosis, pathophysiology, and genetics. Surv Ophthalmol 2017.
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11. Wang Q et al. A comprehensive assessment of the precision and agreement of anterior corneal power measurements obtained using 8 different devices. PLoS One 2012; 7: 9: e45607.
12. Flynn TH et al. Differential precision of corneal Pentacam HR measurements in early and advanced keratoconus. Br J Ophthalmol 2016; 100: 9: 1183–1187.