Fig. 1.27 Trypsin digest preparation of the retina in an area of previous BRVO. (a) The retinal capillary bed shows numerous areas of acellularity (asterisks). Precapillary arteriolar and postcapillary venular channels have beading and dilation (arrows). (b) Higher magniÞcation of the acellular capillaries (asterisk) and beaded arterioles and veins (arrows). (c) Retina from an

uninvolved region of retina shows normal cellularity of the capillaries with a 1:1 ratio of pericytes (arrows) to endothelial cells (arrowheads). (d) MagniÞed view of the capillary bed from a part of the retina within the region of the BRVO. Only a rare pericyte (arrow) can be seen (Reproduced with permission from Frangieh et al.121)

initial location to a more anterior location closer to the lamina cribrosa.126,127 Unfortunately, whereas there is no histopathologic evidence to refute such a narrative, there is also no histopathologic evidence to support this account of events.128 Available pathologic specimens of CRVO are not representative of the spectrum of eyes with CRVO; thus, the critical role played by collaterals in this narrative cannot be tested based on the available histopathologic material.47 In the 29 cases of CRVO examined histopathologically by Green et al., collateral vessels were found within the optic nerve in only 1

case, a lower proportion than is noted clinically and a warning against overgeneralization from the sample studied.124

1.4.4Hemicentral Retinal Vein Occlusion

CRVO and BRVO have clear differences in pathoanatomy and pathophysiology, but conceptions of the differences with HCRVO are inconsistent.

Fig. 1.29 Pathologic slide of an eye with a central retinal vein occlusion (CRVO) that occurred within 24 h of death. Based on visual acuity of hand motions at 2 ft, there is a high likelihood that this patient had an ischemic CRVO. The asterisk denotes a patent central retinal artery. Fresh thrombus is present in the central retinal vein at and just

posterior to the lamina cribrosa (arrowhead). The posterior extent of the lamina cribrosa is indicated by red lines. A light scattering of intraneural hemorrhage is present nasally (arrows) in this patient who had thrombocytopenia with a platelet count of 8,000 and acute myelogenous leukemia (Reproduced with permission from Green124)