Central Retinal Vein Occlusion

Functional Tests

Visual Acuity: In our study we found that a visual acuity of better than 20/200 (6/60) was seen in 58% of the non-ischemic, as compared to only 1.7% in the ischemic type of CRVO. A visual acuity of better than 20/400 (6/120) was seen in 81% of the patients of the non-ischemic, as compared to about 7% of the ischemic type of CRVO only. A visual acuity of 20/400 or worse was seen in only 19% of the non-ischemic, whereas it was seen in 93% of the ischemic type of CRVO. Thus, our studies showed that if an eye with CRVO has 20/400 or worse vision, then there is about a 90% chance that that eye has ischemic CRVO.

Peripheral Visual Fields: We find that peripheral visual fields, recorded with a Goldmann perimeter, give us very useful information in making this differentiation. Visual fields plotted with an automated perimeter do not help. We use, in the Goldmann perimeter, 3 isopters: I-2e, I-4e, and V-4e of the Goldmann perimeter. We find that in non-ischemic CRVO, the peripheral visual fields with V-4e and I-4e are perfectly normal, and they may even be normal with I-2e, or the I-2e is still seen in spite of the presence of peripheral visual field defects with this isopter. In ischemic CRVO, however, the eye may or may not see V-4e, or may or may not see I-4e, and usually does not see the I-2e target at all (Figure 1). Thus, in our study, in non-ischemic CRVO, all 3 targets were seen in 71% of the eyes, and I-4e and V-4e were seen in the remaining 29%, with no eye being unable to see any target or only V-4e. In contrast to that, in ischemic CRVO, only 8% of the eyes could see all the 3 targets, 63% saw I-4e and V-4e, and 18% only V-4e, and 10% could not see any target at all. Thus, if an eye can see I-2e or has normal visual fields with I-2e, that is definitely in favor of being a non-ischemic CRVO. If an eye can see only V-4e or no target at all, that eye definitely has ischemic CRVO.

Visual Fields

Figure 1. Non-ischemic (left) and Ischemic (right) CRVO I-2e=Red I-4e=Blue III-4e=Brown V-4e=Yellow

Relative Afferent Pupillary Defect (RAPD): The next test which helps us is the relative afferent pupillary defect. This is performed by a "swinging flashlight test". In a person with two normal eyes, if we shine the light on one eye, the pupil of that eye constricts immediately; then if we swing the light to the other eye, that pupil also constricts immediately. However, if one eye has retinal or optic nerve disorders, then if we first shine the light on the normal eye that pupil constricts, and then shine the light on the bad eye, instead of constricting the pupil dilates immediately in that eye; That is what is called a relative afferent pupillary defect. We can quantify the degree of relative afferent pupillary defect by using photographic neutral density filters (Figure 2). They come in 0.3, 0.6, 0.9 and 1.2 log units of transmission density, each 0.3 reducing the light by half. If we know that one eye has a relative afferent pupillary defect and the other eye is normal, then this filter is placed in front of the normal eye, not the bad eye. We start with 0.3 log unit filter in front of the normal eye. If on doing the test, the pupil in the bad eye still dilates, then we go to the next filter, 0.6 log unit. If it still dilates we go to the next filter, 0.9 log unit, and after that to the 1.2 log unit filter. (In fact, we can combine these filters to get higher log units.) So we keep doing that until the pupil in the bad eye starts to constrict instead of immediately dilating. That gives us the degree of the relative afferent pupillary defect.

Figure 2. Neutral density filters for quantifying afferent pupillary defect

In our study we found that in non-ischemic CRVO, 97% of the eyes had a relative afferent pupillary defect of <0.6 log units. In contrast to that, in ischemic CRVO, 94% of the eyes had a relative afferent pupillary defect of >0.9 log units and 91% of the eyes >1.2 log units (Figure 3). Our studies also showed that all eyes with ocular neovascularization had relative afferent pupillary defects of >1.2 log units.

Figure 3. RAPD in ischemic vs. non-ischemic CRVO

Electroretinography: We found that the electroretinography (ERG) parameter with the best sensitivity and specificity to differentiate ischemic from non-ischemic CRVO was amplitude of the b-wave (in both photopic and scotopic ERG). If the amplitude of b-wave is <60% of the normal, then there is an 80% chances that we are dealing with an ischemic CRVO. Or, if the amplitude of the b-wave is reduced by one or more standard deviation of the equipment, then we have a high chance of that being an ischemic CRVO. The advantage of ERG is that we can do this test even if the other eye is not perfectly normal, or in patients with only one eye. Relative afferent pupillary defect can only be tested when there is a perfectly normal second eye.