Acknowledgments

There are three types of retinal vein occlusions (RVO): branch retinal vein occlusion (BRVO), central retinal vein occlusion (CRVO), and hemicentral retinal vein occlusion (HCRVO). The causes of all three types of retinal vein occlusions are multifactorial, involve diverse anatomic conÞgurations and physiologic pathways, and may differ in each instance of occlusion.

There are both predisposing and precipitating factors for RVO.1 For example, a predisposing factor might be atherosclerotic disease of the central retinal artery or primary open-angle glaucoma.2 A precipitating factor might be acute dehydration and elevated serum viscosity associated with a viral infection. The consequences of RVO are also variable. They depend on the relevant anatomy and can involve the clotting cascade, vascular physiology, and the biochemistry of the bloodÐretina barrier.

This chapter focuses on aspects of ocular anatomy pertinent to the understanding of RVO, and on the pathologic anatomy of RVO. The next chapter focuses on relevant ocular physiology and the pathophysiology of RVO. For clarity, an arbitrary distinction between anatomy and physiology will not be made. For example, a natural place to present BernoulliÕs principle, a physiologic concept, will be in the present chapter along with the discussion of the anatomic aspect of the central retinal vein as it narrows in its passage through the lamina cribrosa. Likewise, the biochemical pathways involving homocysteine metabolism are most relevant to the pathology of atherosclerosis, a pathologic process covered in

this chapter. On the other hand, the absence of sympathetic innervation of retinal vessels is more appropriately discussed along with the physiologic concept of autoregulation in the next chapter.

Commonly used abbreviations in this chapter are collected in Table 1.1 for reference. In addition, each abbreviation will be introduced with its spelled out term at the Þrst use of the term in the chapter.

1.1 Anatomy and Histology

At the gross anatomic level, the axial length of the eye and the diameter of the scleral outlet have been hypothesized to be relevant to the pathogenesis of CRVO.3 The axial length of eyes with CRVO is signiÞcantly shorter than fellow eyes and eyes of healthy controls suggesting that a short eye increases risk for CRVO (Table 1.2).3,7,9 Whether axial length is shorter in eyes with BRVO is less certain (Table 1.2). Three of Þve studies report shorter mean lengths for eyes with BRVO compared to fellow eyes. Five of six studies report shorter mean lengths for eyes with BRVO compared to eyes from healthy con- trols.3,5-9 A study measuring the scleral outlet of eyes with CRVO and comparing it to controls has not been done, but a smaller diameter sclera outlet has been hypothesized to increase risk for CRVO.3 The hypothesized mechanism is that

Table 1.1 Abbreviations used in anatomy and pathologic anatomy of retinal vein occlusions

mMicron

SSuperior

TTemporal

Table 1.2 Axial length of the eye in retinal vein occlusion

shorter eyes are associated with crowding of the central retinal vein (CRV) and central retinal artery (CRA) as they pass through the lamina cribrosa, leading to increased venous blood ßow turbulence.7,9

Histologically, the retina is a multilayered sheet of neuronal, glial, and vascular tissue that lines the inside posterior two-thirds of the eye. It is bounded posteriorly by the retinal pigment epithelium, BruchÕs membrane, and choroid and anteriorly by the vitreous humor. The macula is a circular area of diameter 5.5 mm with a center located 17¡, or 4.0Ð5.0 mm, temporal, and 0.53Ð 0.8 mm inferior to the center of the optic disc (Fig. 1.1).11-13 Among its distinguishing features are the high density of cones, ³2 layers of ganglion cells, and xanthophyll pigment within bipolar and ganglion cells.11,12 The central 1.5-mm circular area of the macula is the fovea, denoted by a gently curved depression in the retinal surface. Within the fovea is a roughly circular avascular area, the foveal avascular zone, approximately 400Ð500 m in diameter which contains only cones, present at a density of approximately 140,000/mm2.11

A cross section through the retina just outside the area centralis shows ten layers (Figs. 1.2 and 1.3). Proceeding from the vitreous to the choroid are the internal limiting membrane, nerve Þber layer, ganglion cell layer, inner plexiform layer, inner nuclear layer, outer plexiform layer, outer