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    Topography disparity effectively detects subclinical keratoconus

    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.

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