Assessing computerized tomography and higher‐order aberration in the diagnosis of manifest and subclinical keratoconus

Although we have increased our knowledge of keratoconus (KC) substantially since its earliest detailed description 150 years ago, and the quantum of knowledge has grown exponentially over the last 50 years, the pathogenesis of the disease remains enigmatic and diagnosis of its earliest manifestations remain problematic. We do know that KC is a non-inflammatory progressive corneal thinning disorder that commonly results in corneal ectasia, irregular astigmatism, corneal scarring and decreased visual acuity. Typically KC is slowly progressive, with onset usually during puberty to early adulthood, and although one eye may be affected initially, ultimately both eyes are usually affected to some degree. KC has a reported prevalence from 8.8 to 54.5 per 100 000. This wide variation is in part because of the differing diagnostic criteria used between studies as well as the prevalence of other factors, such as genetic predisposition and differing exposure to environmental or local cofactors. The clinical diagnosis of established KC in eyes with biomicroscopic, keratometric and retinoscopic signs is not difficult. However, diagnosis of forme fruste KC or subclinical KC is more challenging. The terms ‘forme fruste KC’ and ‘subclinical KC’ were introduced to indicate a very early preclinical stage or nonprogressive form of the disease in eyes that do not show the classical signs on clinical examination. The unequivocal diagnosis of subclinical KC, even with computerized videokeratography (CVK), can be difficult because threshold criteria remain to be uniformly defined. Rapid advances in Placido disk-based CVK over the last quarter century have led to a variety of quantitative indices that have been found to be highly sensitive and specific in aiding the diagnosis of KC. Rabinowitz and McDonnell (1989) reported one of the first widely accepted numerical methods to differentiate between the normal and keratoconic cornea based on CVK. Their criteria included a central corneal power of greater than 47.2 D, an inferior–superior (I–S) value of greater than 1.4 D, a difference in the central corneal power between fellow eyes of more than 1.0 D and a (non-orthogonal) degree of skewing of the steepest radial axis more than 21 degrees. Although Placido disk-based CVK remains a very useful tool in the diagnosis of KC, it has a principle limitation in that it only obtains data regarding the anterior surface of the cornea. The advent of Orbscan slit-scanning topography (Orbtek, Inc., Salt Lake City, UT, USA) and more recently the Pentacam (Oculus, Wetzlar, Germany) meant that posterior corneal curvature and wide-field corneal pachymetry could also be studied in KC. Several studies have emphasized the importance of posterior elevation in detecting the earliest forms of the disease. Lim et al. (2007) recently confirmed a significant difference in the mean posterior corneal elevation (46 17 mm) in a KC-suspect group compared with normal eyes, and although the KC-suspect group exhibited corneal pachymetry within the normal range (mean 504 40 mm) this was significantly thinner than normal controls. In addition to tomographic differences, it has been highlighted that measurement and analysis of corneal generated wavefront aberrations may also be useful in the detection of KC. Maeda et al. (2002) and Shah et al. (2003) have reported the higher-order aberrations of KC eyes to be significantly different than those of normal eyes with dominance of coma-like aberrations over spherical-like aberrations. Indeed, Pantanelli et al. (2007) demonstrated that higher-order aberration is around 5.5 times greater in KC eyes and vertical coma is the dominant aberration. Lim et al. (2007) have also illustrated that KC, and KC-suspect, eyes had significantly greater total higher-order Root-Mean-Square values (3rd–5th order) than normal eyes, with a mean of 1.73 mm in KC, 0.94 mm in KC-suspect eyes and 0.49 mm in normal eyes. It has been suggested by Jafri et al. (2007), among others, that a combination of higher-order aberrations and corneal topographic assessments may be best in distinguishing early KC from normal eyes. A product of vertical coma and I–S ratio may best separate normal, KC-suspect and KC eyes. Mahmoud et al. (2008) have recently reported an additional indice for the detection of keratoconic patterns in corneal topography maps derived from differing devices. The Cone Location and Magnitude Index is calculated on the available axial and tangential curvature data. The Cone Location and Magnitude Index calculated from axial data provided a sensitivity and specificity of 92% and 100%, respectively, in discriminating KC from normal eyes. Clinical methods to accurately track the progression of KC are extremely important because of the variable nature of its clinical course. In the Collaborative Longitudinal Evaluation of Keratoconus Study, subjects exhibited a mean 8-year increase in corneal curvature of 1.60 D in the flat corneal meridian (measured with manual keratometry); however, 24% of eyes demonstrated an increase of 3.00 D or more. Other topographic parameters suggest the progression of the disease over a 2-year period can be observed by a change in the apex location that is also associated with increase in the apex elevation (mean: 0.056 mm at baseline vs. 0.069 mm at 24 months) and keratometry (minimum values of 48.2 D and 54.1 D, respectively). Although the apex in KC is generally inferior and bilateral in expression, unilateral superior apex KC has also been reported. KC is frequently an isolated disease, but many systemic and environmental associations have been reported, including atopy, Clinical and Experimental Ophthalmology 2008; 36: 807–809 doi: 10.1111/j.1442-9071.2009. 01919.x