Table 1. Common complex traits documented in 100 consecutive patients. Ocular region or tissue Cornea Lens Multiple Vitreous Macula Retina Disease name (abbreviation) Anterior basement membrane dystrophy (ABMD, OMIM 121820) Fuchs endothelial corneal dystrophy (Fuchs) Cataract, age-related Pseudoexfoliation (PXE, OMIM 177650) Glaucoma, open or closed angle or syndrome predisposing to glaucoma except for PXE Posterior vitreous detachment (PVD, No OMIM number) Age-related macular degeneration (AMD) Vitreomacular traction (Epiretinal membrane and macular hole, VMT; No OMIM number) Occlusive retinal vascular disease (RVO; No OMIM number) Retinal tears (with or without detachment; No OMIM number) Evidence for hereditary component Minimal[57] Strong[56,58] Strong[59,60] Strong[3] Strong[51,61] Unknown Strong[1] Minimal[62–64] Minimal[65,66] Strong[67] 1 7 96 3 12 54 54 20 7 5 *Please refer to the Discussion for an explanation of these approximate population estimates for individuals age 60 or older. doi:10.1371/journal.pone.0002510.t001 then analyzes with logistic regression models with estimation of odds-ratios for each genetic variant (a case-control study). The analysis enabled an estimation of the information on secondary traits that would be ignored in most ocular case-control studies. Specifically, we tested the hypothesis that comprehensive pheno- typing of common ocular traits would enable discovery of genetic risks for secondary traits. The genotype data would be re-analyzed after re-coding cases and controls based on each secondary trait. The analysis showed that even this primitive ascertainment strategy, without any effort to increase the representation of less common traits, would have enabled detection of genetic risks for most of the traits (see Table 1 for observed frequencies of each trait). For example, at a significance level of 0.05, a modest genotypic effect of OR = 2.0 would have been detected in 6 of the 10 traits when the minor allele frequency was between 0.10 and 0.20 and 9 of the 10 traits when the minor allele frequency was 0.40 (Table 3). There was sufficient power to detect odds-ratios of 1.7, 1.7, 1.6, and 1.6 for the four most common traits glaucoma, PVD, AMD, and VMT, respectively. When the number of markers tested increases, as in the case of genome-wide association studies with a significance level of 0.00001, a modest genotypic effect of OR = 2.0 would have been detected in 4 out of the 10 traits when the minor allele frequency was 0.20 and 6 out of 10 traits when the minor allele frequency was 0.40 (Table 3). Discussion We show that the average patient age 60 or older has at least 3.5 distinct ocular diseases or traits, and that the majority of these diseases arise from common complex traits affecting the eye. Further, we show that some traits steadily increase with age, but that others do not appear to increase after the age of 60 years. Finally, we show that comprehensive documentation of secondary common complex traits affecting the aging eye would substantially increase the value of a genome-wide genetic association study. We chose to use clinic-based cases and controls from a subspecialty clinic, because this represents the most common method used to ascertain patients for genetic case-control studies of eye diseases. Such samples are easily collected during the course of routine patient care. Further, large numbers of affected subjects with broad representation of the spectrum of disease and its complications can be collected in this manner, which is typically not possible with population-based approaches due to financial constraints. It is important to realize the limitations of clinic-based, case-control studies. The frequencies of traits do not necessarily reflect the underlying population prevalence and they tend to over-estimate population parameters such as relative risks and attributable fractions. Further, replication is essential for all genetic association results, regardless of the study design [28]. Table 2. Density of complex ocular traits in 100 consecutive patients age 60 or older arranged by presenting diagnosis. Density Complex Ocular Traits Prevalence (%) in retina clinic patients Estimate for Prevalence (%) in North America* 5 10 50 5 5 50 15 10 2 3 Presenting Diagnosis Age-related macular degeneration Vitreomacular traction Diabetic retinopathy Retinal vascular occlusion (all BRVO) Other All patients doi:10.1371/journal.pone.0002510.t002 Number of Patients 47 22 8 4 19 100 3.7 (0.19) 3.7 (0.27) 2.6 (0.18) 4.8 (0.48) 2.9 (0.30) 3.5 (0.13) PLoS ONE | www.plosone.org 3 June 2008 | Volume 3 | Issue 6 | e2510 Average number (standard error) of distinct diagnosis for the 20 observed traits Average number (standard error) of the 10 complex traits in Table 1 Range of the 10 complex traits in Table 1 2.9 (0.12) 2.8 (0.17) 1.4 (0.18) 3.8 (0.48) 1.8 (0.22) 2.6 (0.10) 1–5 2–4 1–2 3–5 0–4 0–5