Longitudinal study of the normal eyes in unilateral keratoconus patients.

PURPOSE To determine the rate at which clinically normal eyes in unilateral keratoconus (KC) patients progress to KC and to identify the risk factors that might predict the development of KC in a longitudinal study. DESIGN Prospective observational study. PARTICIPANTS We recruited 778 patients with KC and 252 normal controls in Los Angeles, California. One hundred sixteen of 778 patients (14.9%) were diagnosed with clinically unilateral KC at baseline. METHODS Both eyes of these unilateral KC patients were examined at baseline, and 85 patients were followed up with clinical evaluation and videokeratography for a period ranging from 6 months to 8 years. MAIN OUTCOME MEASURES Progression to clinical KC from previously normal fellow eyes. Quantitative and qualitative videokeratography variables, contact lens wear, and demographic variables were analyzed as potential predictive factors for this progression. RESULTS During the follow-up period, 30 of 85 (35.3%) clinically normal fellow eyes had KC develop, and 25 of these 30 (83.3%) had KC develop within the first 6 years after the initial KC diagnosis. By use of the time period from the patients' first diagnosis of unilateral KC to the end of the follow-up period (range, 6 months-41 years), the median time (estimated from the survival analysis) to the development of KC was 16.7 years (95% confidence interval, 11.34, 28.91). Fellow eyes with higher inferior-superior dioptric asymmetry value (I-S) or KC percentage index (logKISA) values had higher risk for KC developing (I-S, risk ratio [RR] = 1.348, P = 0.022; log(KISA), RR = 4.245, P = 0.003). Asymmetric patterns also showed an increased risk for KC developing (P = 0.03), especially the asymmetric bowtie with skewed radial axes (AB/SRAX) pattern. CONCLUSIONS Approximately 50% of clinically normal fellow eyes will progress to KC within 16 years. The greatest risk is during the first 6 years of the onset. Quantitative indices (I-S and KISA values) and qualitative patterns (AB/SRAX) might predict this progression.