4.1Nosology of Retinal Vein Occlusions

Retinal vein occlusions (RVOs) are separated into three types – branch, central, and hemicentral. The assumption is that the different sites of venous occlusion have different susceptibilities to thrombosis based on anatomic and local factors.5 Although a three-part classification is precise, in many studies, RVOs are pooled because it is difficult to collect large numbers of cases without pooling. Unfortunately, methods of pooling vary across studies. Many studies pool hemicentral RVO (HCRVO) and BRVO.9,16,32,42 Probably more pool HCRVO and CRVO, on the evidence that these two entities are pathogenetically similar.1,15,23,24,38 Additional support for the practice is the observation that there are no important differences in the prevalence of associated systemic and ocular conditions between CRVO and HCRVO (see Chap. 6).1,24 In contrast, clear differences exist in the prevalence of predisposing conditions between BRVO and either CRVO or HCRVO.1 For example, prevalences of an elevated erythrocyte sedimentation rate, intraocular pressure (IOP), and positive purified protein derivative skin testing were statistically significantly higher in CRVO than BRVO, whereas hypertension and hyperopia were more prevalent in BRVO than CRVO.1 Moreover, many CRVOs evolve into a clinical picture of HCRVO and vice versa, but BRVOs never evolve into a clinical picture of CRVO or HCRVO.15 Only a few studies pool

certain HCRVOs (the ones where the occlusion can be identified at the optic cup) with certain BRVOs (the ones where the occlusion can be identified at the optic cup), and other HCRVOs (the ones with occlusion within the optic nerve) with CRVOs.5 Although pooling of CRVO and HCRVO has a stronger rationale, there is no trend in time that pooling of BRVO and CRVO is decreasing. Moreover, difficulties determining where the occlusion is in HCRVO means that at a practical level, misclassification of HCRVO is common.45

Commonly used abbreviations in this chapter are collected in Table 4.1 for reference. In addition, each abbreviation will be introduced with its spelledout term at the first use of the term in the chapter.

Methods of pooling in studies may justifiably differ depending on the goals of the research. If the goal is to learn about the consequences of venous occlusion, then pooling BRVO with HCRVO has a rationale.42 If the goal is to discover predisposing anatomical risk factors, then pooling HCRVO and CRVO has a rationale.21 The author’s preference is that each type of occlusion be considered individually, but because so many reports pool subtypes of RVO, it is not feasible to overlook the practice in a textbook covering the subject. The approach we will take is to identify when pooling occurs so that appropriate reservation by the reader can be applied to the results reported.

Historically, classifications of RVO have been based on etiology, arterial characteristics, location of the occlusion, degree of ischemia, presence/ absence of macular or optic disc edema, and presence/absence of neovascularization (Table 4.2).36