Stephen D. Klyce

Unilateral Keratoconus: Incidence and Quantitative Topographic Analysis

OBJECTIVEnThis study was designed to determine the incidence of unilaterality in a population of patients with clinical keratoconus and to compare quantitative descriptors of topography between affected corneas from patients with unilateral keratoconus and corneas of patients with bilateral disease.nnnDESIGNnRetrospective clinical study with new evaluation of some patients.nnnPARTICIPANTSnOne hundred sixty-four patients from the University of Texas South-western Medical Center and Wills Eye Hospital who were diagnosed to have moderate-to-advanced keratoconus on the basis of characteristic topographic patterns associated with corneal thinning.nnnINTERVENTIONnCorneal topography was evaluated in both eyes of each patient.nnnMAIN OUTCOME MEASURESnQuantitative descriptors of corneal topography were compared between 5 affected corneas of patients with unilateral keratoconus (combined cases from University of Texas Southwestern, LSU Eye Center, and Wills Eye Hospital) and 15 corneas of patients with moderate-to-advanced bilateral keratoconus (1 cornea from each patient). Indices selected for statistical analysis were the Keratoconus Predication Index (KPI), Surface Asymmetry Index (SAI), and Surface Regularity Index (SRI).nnnRESULTSnThree (1.83%) of 164 patients with moderate-to-advanced keratoconus had no topographic evidence of keratoconus in the opposite eye. There were no statistically significant differences in KPI, SRI, or SAI values between the affected corneas of patients with unilateral and bilateral keratoconus. Over a period of 4 years of observation, signs of keratoconus in the previously normal eye developed in a patient with unilateral keratoconus.nnnCONCLUSIONSnThe authors found no differences in the quantitative descriptors of corneal topography between corneas with keratoconus from unilateral and bilateral cases. The authors results suggest that the incidence of unilateral keratoconus is very low. Patients initially diagnosed with unilateral keratoconus, if observed for a sufficient period, commonly had signs of keratoconus develop in the opposite eye. The possibility that all cases of unilateral keratoconus may eventually become bilateral cannot be excluded. Therefore, refractive surgical procedures should not be performed on apparently normal corneas when keratoconus is detected in the opposite eye.

Quantification and Mathematical Analysis of Photokeratoscopic Images

The need for accurate and precise methods of evaluating the contour of the anterior corneal surface has paralleled the rapid development of keratorefractive surgery. Thus, the suboptimal refractive predictability of procedures such as penetrating keratoplasty, astigmatic keratotomy, and radial keratotomy has provided impetus for the development of techniques for measuring and presenting corneal topography that are essential for understanding changes induced by surgical manipulations of the corneal surface.

Imaging, Reconstruction, And Display Of Corneal Topography

The cornea is the major refractive element in the eye; even minor surface distortions can produce a significant reduction in visual acuity. Standard clinical methods used to evaluate corneal shape include keratometry, which assumes the cornea is ellipsoidal in shape, and photokeratoscopy, which images a series of concentric light rings on the corneal surface. These methods fail to document many of the corneal distortions that can degrade visual acuity. Algorithms have been developed to reconstruct the three dimensional shape of the cornea from keratoscope images, and to present these data in the clinically useful display of color-coded contour maps of corneal surface power. This approach has been implemented on a new generation video keratoscope system (Computed Anatomy, Inc.) with rapid automatic digitization of the image rings by a rule-based approach. The system has found clinical use in the early diagnosis of corneal shape anomalies such as keratoconus and contact lens-induced corneal warpage, in the evaluation of cataract and corneal transplant procedures, and in the assessment of corneal refractive surgical procedures. Currently, ray tracing techniques are being used to correlate corneal surface topography with potential visual acuity in an effort to more fully understand the tolerances of corneal shape consistent with good vision and to help determine the site of dysfunction in the visually impaired.