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ischemia is a continuous variable, for purposes of classification, a cutpoint of 5 disc areas of capillary nonperfusion is frequently chosen to define ischemic from nonischemic.4 This cutpoint arose because of the report that retinal and disc neovascularization were not observed in an eye following BRVO unless ³5 disc areas of capillary nonperfusion was present.45 Another definition of an ischemic BRVO is that it manifests a broken perifoveal capillary ring at the border of the foveal avascular zone and that there be any area of capillary nonperfusion, with no lower limit in area, within 1 DD of the center of the macula.10 Definitions vary from study to study and make
a
comparisons across studies employing different definitions difficult.13
4.3 Central Retinal Vein Occlusion
CRVO is most commonly subclassified as nonischemic or ischemic.26 Synonyms for nonischemic CRVO include partial CRVO, incomplete CRVO, perfused CRVO, and venous stasis retinopathy. Synonyms for ischemic CRVO include complete CRVO, total CRVO, nonperfused CRVO, and hemorrhagic CRVO.3,18,38
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Fig. 4.5 Images typical of a nonischemic central retinal vein occlusion. The patient was a 78-year-old man with hypertension who complained of acute onset of right eye blurring. Visual acuity of the right eye was 20/25. (a) Monochromatic photograph of the fundus shows dilated retinal veins, intraretinal hemorrhages, and absence of cotton wool spots. The disc is edematous (orange arrow). (b) Frame from the mid-phase fluorescein angiogram shows good capillary perfusion. Capillary perfusion is
excellent throughout this single frame. (c) Scanning laser ophthalmoscopic image of the right fundus showing the horizontal orientation of the spectral domain optical coherence tomography (SD-OCT) line scan in (d). (d) SD-OCT image shows a foveal cyst (green arrow). (e) Magnified image of the fluorescein angiogram frame in (b) showing an intact perifoveal capillary border (yellow arrow)
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Classification into ischemic or nonischemic CRVO is not standardized. The most accurate, but most complicated, system of classification involves synthesizing data from six examination modalities – visual acuity, Goldmann visual field testing, pupillary testing for a relative afferent pupillary defect, electroretinography (ERG), ophthalmoscopy, and fundus FA.25,26 The quality of the information gained is excellent; however, few ophthalmologists in clinical practice use the system because it is based on rarely available ERG, labor-intensive quantitation of the relative afferent pupillary defect (RAPD), and unreimbursed Goldmann perimetry. It is an unpractical standard.
Many other definitions of ischemia for CRVOs exist. When the ERG is used as part of the definition, there are many subdefinitions used to categorize CRVOs as nonischemic or ischemic (see Chap. 9).28 Other definitions do not involve
the ERG, but add clinical course.38 Others exclude the ERG and visual field, but include other modalities.41 Other studies classify CRVO as ischemic or nonischemic based on the fluorescein angiogram alone with various cutpoints chosen to distinguish ischemic from nonischemic cases (Fig. 4.5).27,28,46,49 Because of the nonstandardized terminology, it is not possible to compare statements made about effects of ischemia across studies with confidence. At best, broad trends can be identified.
In some cases, there is too much hemorrhage to be able to assess the area of capillary nonperfusion and apply any definition of ischemia. The CVOS showed that this clinical picture is tantamount to ischemia, because 83% of such eyes either went on to develop iris and/or angle neovascularization or were later shown to be ischemic when enough blood cleared to allow FA to be interpreted.47
Differences in Philosophy on How Far to Pursue Classification Relative to Ischemia
In reading the literature on CRVO, it becomes apparent that there are two schools of thought regarding how much effort to put into classification of a CRVO as nonischemic or ischemic. On one extreme, Hayreh advocates a great expenditure of resources and effort.21 Using 6 variables at the time of presentation, he is confident that he can classify eyes as nonischemic or ischemic. He states that ocular neovascularization “is a complication of only ischemic CRVO.” Others expend less effort and fewer resources in the classification effort, do not worry too much if an eye is misclassified, and in effect simply follow all eyes with CRVO closely until the risk of ocular neovascularization is low.48
Controversy has arisen from these philosophical differences. Hayreh has challenged the validity of the results of the CVOS based on the criticism that it incorrectly classified eyes and pooled results of nonischemic CRVO with those of ischemic CRVO, blurring differences in outcomes that he contends are stark when the eyes are correctly classified using more than just FA.22
Central Retinal Vein Occlusion with Nonischemic and Ischemic Hemispheres
It is a little known fact that an eye may appear to have a CRVO but actually consist of two simultaneous hemi-CRVOs which may differ in their degree of ischemia. The following case report illustrates such a case (Fig. 4.6).
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Fig. 4.6 Images of an eye with what would be classified as an ischemic CRVO. Less obvious, however, is that this eye possesses two hemicentral retinal veins, both of which are occluded, but with different grades of occlusion. The superior hemicentral retinal vein has a more complete occlusion with a thrombus presumably more anterior within the optic nerve. This produces an ischemic picture in the superior hemiretina. The inferior hemicentral retinal vein has a lesser grade of occlusion with a thrombus presumably reflecting a thrombosis more posterior within the optic nerve. This produces a pattern of nonischemic vein occlusion inferiorly. (a) The red free photograph shows different gradations of ischemic whitening. Just superonasal to the fovea is a zone of more opaque whitening and superotemporal to the fovea a zone of less opaque whitening. Scattered around the disc, but more densely arranged in the superior fundus, are cotton wool spots. (b) Magnified red free photograph of the optic disc. The two first-order superior branch retinal veins join within the optic nerve to form the superior hemicentral retinal vein (yellow oval). The inferior hemicentral retinal vein is denoted by the blue arrow. (c) Frame from the mid-phase of the fluorescein angiogram shows that this CRVO is composed of two hemicentral retinal vein occlusions, the superior being ischemic and the inferior being nonischemic. The green arrow denotes a large area of capillary nonperfusion superiorly. The turquoise arrow shows good capillary perfusion in the distribution of the inferior hemicentral retinal vein. (d) Magnified view of the juncture of the two first-order branch veins to form the superior hemicentral retinal vein (purple arrow) during the mid-phase of the fluorescein angiogram. The frame demonstrates an
asymmetry of obstruction within the superior hemicentral retinal vein. The first-order superotemporal branch retinal vein experiences a lesser grade occlusion than the firstorder superonasal branch retinal vein as witnessed by the asymmetry of fluorescein return in the two branches. Some laminar venous return is present in the superotemporal branch (yellow arrow), but at this time in the angiogram, there is no fluorescein venous return in the superonasal branch (orange arrow). (e) Frame from the late phase of the fluorescein angiogram shows that fluorescein leakage is much less in the zone of capillary nonperfusion (compare the area denoted by the pink arrow to the area denoted by the maroon arrow). (f) A horizontal OCT line scan through the macula. (g) A vertical OCT line scan through the macula. Note that the amount of retinal thickening and subretinal fluid does not colocalize with the area of fluorescein leakage on the fluorescein angiogram. The retina is as thick in the superior macula where fluorescein leakage is less as in the inferior macula where fluorescein leakage is greater. It is informative to compare the distribution of the whitening with the distribution of the capillary nonperfusion in the early frame of the fluorescein angiogram (a). Areas of the fundus more remote from the disc, for example, superotemporal to the macula are not as whitened (tan arrow in a) as others (red arrow in a) but do have capillary nonperfusion on the FA (compare these areas in c). The areas more remote from the center of the macula have a thinner nerve fiber layer to opacify with hypoxia, which may account for the gradations of whitening. Note the difference in number of intraretinal hemorrhages in the nonperfused zone compared to the perfused zone in the inferior hemimacula (a)
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Fig. 4.6 (continued)
Besides classifications based on ischemia, some studies classify CRVOs by the amounts of disc edema, venous engorgement, peripapillary hemorrhage, and retinal hemorrhage.20 These studies usually employ reference photographs for grading, but data on reproducibility are not provided.
Because of the inherent subjectivity of these systems, it is not possible for the clinician to generalize the results of such studies to clinical practice.
The classification of HCRVO is beset by the same issues as for CRVO, and the previous discussion applies.