Hemi-CRVO is a variant of central retinal vein occlusion (CRVO). To understand that, one has to consider the development of the central retinal vein. Mann [11] has given a definitive account of this. The hyaloid artery enters the optic stalk at an early age and runs through it to reach the cavity of the optic cup. At first it has no accompanying vein. During the third month of intrauterine life, two venous channels, one on either side of the artery, can be recognized. These run in the optic nerve parallel to the artery and are united to each other by numerous anastomoses. As the two venous channels travel toward the ocular end of the optic nerve, they approach one another and finally coalesce at a variable distance behind the optic disc, forming the main trunk of the central retinal vein in the optic nerve. One of the two venous channels in the anterior part of the optic disc usually disappears before birth, leaving the central retinal vein as one trunk. However, in a certain proportion of cases, the embryonic pattern may persist. In my anatomical studies on the central retinal artery in man [12], in three of six eyes, histological serial sectioning and reconstruction demonstrated (quite incidentally) the existence of two venous channels, lying on either side of the central retinal artery (Fig. 25.1). The two venous trunks were seen in the optic nerve head and retrolaminar part of the nerve only, and then they joined to form the classic central retinal vein. The two venous trunks in the anterior part of the optic nerve were connected to one another by an anastomotic channel, as described by Mann [11] during fetal life. A schematic representation of the two intraneural trunks of the central retinal vein is shown in Fig. 25.2. The length of the intraneural part of these trunks can vary widely, as suggested by the three eyes with this anomaly in my study [12]. In my original study of 41 eyes with hemi-CRVO [1], the arrangement of retinal vessels on the optic disc provided additional evidence of such an anomaly. This is extremely variable; generally the two veins are from the upper and lower halves of the retina, but in some the two veins have a different distribution – one covering more than half of the retina and consequently the second one a much smaller area, as little as one quarter of the retina, so that in such an eye, the smaller vein looks like a “branch retinal vein.” In such a case, occlusion may involve only the smaller vein, and that would earlier have been diagnosed as “branch retinal vein occlusion.” Angiography is usually most helpful in defining the pattern of the superior and inferior retinal veins on the optic disc and their separate entry into the optic disc (see below). Chopdar [13] in a clinical study reported a 20.5 % incidence of dual trunks of the central retinal vein in humans.

In eyes with two central retinal veins in the anterior part of the optic nerve, the superior vein drains the upper half of the retina and the inferior the lower half (Fig. 25.2). Venous occlusion can involve one of the two venous channels and consequently result in fundus changes in the corresponding half of the retina, and the pattern of such a venous occlusion is similar to that seen in CRVO and not the usual branch retinal vein occlusion. My clinical and experimental studies on CRVO (see Chap. 24) have shown that CRVO consists of two distinct clinical entities: nonischemic and ischemic. My studies on hemi-CRVO have also shown that it is also of nonischemic and ischemic types, as is evident from the discussion below.