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5 Epidemiology of Retinal Vein Occlusions |
a prospective, clinic-based study using a novel classification system based on site of the occlusion and the presence or absence of optic disk edema (see Chap. 4). Mean age was significantly higher in optic cup-sited RVOs (69.0 ± SD13) than in arteriovenous RVOs (64.4 ± SD9.8) (P < 0.05). Smoking was significantly more prevalent in the arteriovenous RVOs (28.7%) than in the rest of the groups (10.8–18.4%) (P < 0.05). Male gender was significantly more prevalent in the optic nerve head-sited RVOs with disk swelling (63.2%) than in the arteriovenous RVOs (48.7%) (P < 0.05).4 While this classification system is attractive for its underlying pathoanatomic rationale, it has been used only by the innovators in the 10 years since it was introduced. It is difficult to know how to integrate the observations of this study with those of studies using other classifications of RVO, in which, as an example, gender has not been found to be a risk factor.
In epidemiologic exposition, different methods of expression can convey the same idea. For example, all of the following statements mean the
same thing, and all formulations will be used in this chapter in the interest of readability:
•Gender is not a risk or protective factor for the prevalence of BRVO.
•Gender is not associated with the prevalence of BRVO.
•There is no statistically significant difference in the prevalence of BRVO between men and women.
Although case series and case-control studies have value for suggesting areas to explore in more robust population-based studies, they often imply associations that do not hold up under scrutiny.39 An example is the association of male gender with CRVO found in case series but not in populationbased studies. Case series give too little weight to conditions that produce fewer symptoms. For example, in a population-based prevalence study, the proportion of BRVOs and the overall prevalence of BRVOs will be higher than in a clinical case series because 16% of BRVOs occur outside the temporal vascular arcades and likely cause no symptoms for the affected patient.39
The Major Epidemiologic Studies of Retinal Vein Occlusion
The Blue Mountains Eye Study (BMES) was a population-based study of Australians from a community near Sydney. There were 3,654 participants of age 49 or greater comprising 82% of the eligible sample population. Initial examinations were done from 1992 to 1994 from which prevalences of RVOs were calculated. Follow-up examinations done 10 years later allowed 10-year incidence data to be calculated.46
The Beaver Dam Eye Study (BDES) was a population-based study of 4,926 residents of Beaver Dam, Wisconsin, from 1988 to 1990 (83% of the population) with follow-up examinations of 3,684 of 4,541 possible subjects (81%) in 1993–1995 and of 2,764 of 3,334 possible subjects (83%) in 1998–2000.30
Both studies examined predominantly white populations using similar protocols. At baseline, all subjects underwent a complete eye examination after pupil dilation. Fundus photographs were taken, and a diagnosis of RVO was based on grading-center evaluation of photographs and not on clinical examination.8 Blood pressure was measured, and laboratory testing was done for blood glucose, serum total cholesterol, and HDL cholesterol. Glaucoma was diagnosed with prespecified criteria.8