Topography disparity can even provide a useful parameter for diagnosing the subclinical form of the disease, said David P. Piñero, PhD, and colleagues from the Medimar International Hospital, Alicante, Spain.
Alicante, Spain-Clinicians can use topography disparity and ocular residual astigmatism to diagnose keratoconus, researchers said.
Topography disparity can even provide a useful parameter for diagnosing the subclinical form of the disease, said David P. Piñero, PhD, and colleagues from the Medimar International Hospital, Alicante, Spain. They published their finding in Graefe’s Archive for Clinical and Experimental Ophthalmology.
The magnitude of the ocular residual astigmatism increases significantly in keratoconus. This measurement is calculated as the vectorial difference between refractive astigmatism (calculated to the corneal plane) and corneal astigmatism.
It results from the combination of the toric components of the crystalline lens and the posterior corneal surface with perceptual physiology. Perceptual physiology is the perceptual preference for a specific object orientation that may contribute to differences between corneal and manifest refractive astigmatism.
Topography disparity characterises the irregular component of anterior corneal astigmatism. It is calculated as the vectorial difference between the regular astigmatism of the superior and inferior hemidivisions of the cornea.
To see how well these two parameters could help diagnose keratoconus, Dr. Piñero and his colleagues retrospectively analysed 43 eyes with keratoconus in 27 patients ranging in age from 17 to 72 years old. They compared them with 11 eyes with subclinical keratoconus ranging in age from 11 to 54 years old, and with 101 normal eyes in 101 patients ranging in age from 15 to 64 years. There were no significant differences between the groups in age or spherical equivalence.
All the patients had examinations from 2012 to 2013 in the Ocular Surface and Cornea United, Department of Ophthalmology, Medimar International Hospital. Diagnosis of keratoconus depended on the identification of corneal topography revealing an asymmetric bowtie pattern with or without skewed axes and at least one keratoconus sign on slit lamp examination, such as stromal thinning, conical protrusions of the cornea at the Apex, Fleischer rings, Vogt striae, or anterior stromal scars.
Diagnosis of subclinical keratoconus depended on corneal topography showing an abnormal localised steepening or an asymmetric bow tie pattern combined with a normal-appearing cornea at slit lamp biomiscropy and at least one of the following: keratometric curvature greater than 47 D, oblique cylinder greater than 1.5 D, central corneal thickness less than 500 μm, or clinical keratoconus in the fellow eye.
The researchers measured ocular residual astigmatism and topography disparity using iAssort software (iAssort, Cheltenham, Australia) to analyse data from a Pentacam tomographer (Oculus, Wetzlar, Germany).
They found larger vector magnitudes of ocular residual astigmatism and a large level of scattering in the keratoconus group. The mean magnitude of ocular residual astigmatism in this group was 3.23 D. In the subclinical keratoconus group, it was 1.16 D. And in the healthy group it was 0.79 D. The differences were statistically significant (P < 0.001).
The researchers found larger values and more scattering of topography disparity in the keratoconus group, as well. The mean magnitude of the topography disparity in this group was 9.04 D and the median was 7.57 D. In the subclinical keratoconus group the mean was 2.69 D, and the median was 1.41 D. In the healthy group, the median was 0.89 D, and the mean was 0.81. The differences among the groups were statistically significant (P < 0.001).
Analysing the receiver operator curve, the researchers found “great diagnostic ability” for both measurements to detect keratoconus. For ocular residual astigmatism with a cut-off point of 1.21 D, they calculated a sensitivity of 83.7% and specificity of 87.1%. For topography disparity, with a cut-off point of 1.64 D, they found a sensitivity of 93.3% and a specificity of 92.1%. This is as good or better than the accuracy reported for other tomographic or biomechanical parameters, the authors wrote.
The contribution of the posterior corneal surface appeared to account for most of the difference in ocular residual astigmatism between healthy eyes as compared with the keratoconus and subclinical keratoconus groups.
However, “the magnitude of the ocular residual astigmatism in clinical keratoconus was also influenced significantly by the perceptual process,” wrote Dr. Piñero and colleagues. They explained that the interaction of some corneal higher-order aberrations, such as coma or secondary astigmatism, with refractive astigmatism may have played a role in the perception of the distortion of the image.
But ocular residual astigmatism and topography disparity were not equally accurate in detecting subclinical keratoconus. For ocular residual astigmatism, the researchers found a sensitivity of 60.0% and a specificity of 84.2%. For topography disparity, sensitivity was 80.0% and specificity was 80.2%.
The researchers speculated that a larger sample size might result in a higher sensitivity calculation for ocular residual astigmatism. They acknowledged the differences in the sizes of the groups in the sample as a weakness of the study.
The results also showed a trend toward higher levels of topography disparity in thinner keratoconus corneas. Researchers speculated that the mechanical weakening of the keratoconus cornea leads to a more significant susceptibility of the cornea to be deformed by the effect of IOP.
David P. Piñero, PhD
Dr. Piñero did not indicate any proprietary interest in the subject matter.