180 3 Acquired Retinal Diseases
There are two important questions to consider about eyes with a central retinal vein occlusion (CRVO): Can the waveform of the ERG be used to classify the CRVO as ischemic and nonis- chemic types? Can ERGs be used to predict the development of neovascular glaucoma?
It has been reported that the b/a ratios of the ERGs of the affected eyes of patients with a CRVO who developed neovascularization of the iris (NVI) were significantly lower than the
ratios of the fellow eyes and lower than the ratios in the affected eyes of individuals who did not develop NVI [1, 2]. However, there are also cases where the ERG of a patient with an ini- tially normal b/a ratio changes so the ratio becomes <1.0 or vice versa (Fig. 3.13). This sug- gests that the initial findings may change during the course of the disease. Even though such case variance is present, the b/a ratio is an important index for evaluating the prognosis of CRVO.
3.2 Retinal Circulatory Disturbances
3.2.1 Central Retinal Vein Occlusion
Fig. 3.13. A 39-year-old woman had a central retinal vein occlusion (CRVO) in her right eye. Note the nonischemic pattern in the fluorescein angiogram (left, A) and ERG (right, A) at her initial visit. One month later, new hemorrhages appeared in the retina, the fluorescein angiogram shows a nonperfused area (left, B), and the waveform of the ERG has become negative (right, B). Two months later the ERG b-wave is further depressed (right, C)
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In cases of complete central retinal artery occlusion (CRAO), the ERG consists of a normal or supernormal a-wave and a markedly reduced b-wave [2]. However, this is not the case in most patients [3]. Platelet-fibrin and cholesterol are the major types of emboli, and both are soft and consequently quickly become fragmented and cast into the distal radicles of the retinal circulation. As a result, by the time the patient is examined fluorescein angiogra- phy often fails to detect complete obstruction of the central retinal artery.
The ERGs elicited by a single bright flash (mixed rod and cone ERG), the visual fields, and visual acuities of five patients with CRAO are shown in Fig. 3.14. Despite the extremely con- stricted visual fields and poor visual acuity, the ERGs in the affected eyes are well preserved.
Although the b/a ratio in the affected eyes is lower than that of fellow eye, none of the ERGs from the affected eyes has a negative configura- tion. These results suggest that the function of the retinal layer related to the ERG can recover to some degree following recovery of the retinal
3.2.2 Central Retinal Artery Occlusion
Fig. 3.14. Mixed rod–cone (bright flash) ERGs, visual fields, and visual acuity (right) obtained from the affected eye and normal fellow eye of five patients with central retinal artery occlusion (CRAO). Despite the severe decrease of the subjective visual func- tions, the ERGs are relatively well preserved
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182 3 Acquired Retinal Diseases
circulation, as demonstrated by fluorescein angiography. However, because the ganglion cells are more vulnerable to ischemia, they suffer irreversible damage soon after the occlu- sion. Thus, the marked deterioration of the sub- jective visual function in patients with CRAO may result largely from the damage to ganglion cells.
The fundus appearance of ophthalmic arte- rial occlusion is similar to that of CRAO in the acute stage. However, when the ophthalmic artery is occluded, not only central retinal
artery circulation but also choroidal circulation is disturbed, resulting in marked deterioration of the a-wave [3]. This finding is important for differentiating CRAO from ophthalmic arterial occlusion.
Electrooculography (EOG) of CRAO shows a depressed light rise with a low base value. This finding indicates that the mid-retinal cells nec- essary for detecting the EOG light rise are compromised, producing an abnormal light peak/dark trough (L/D) ratio [3, 4].
References
1. Karpe G, Uchermann A (1955) The clinical elec- troretinogram. IV. The electroretinogram in circu- latory disturbances of the retina. Acta Ophthalmol 33:493–516
2. Sabates R, Hirose T, McMeel JW (1983) Elec- troretinography in the prognosis and classification of central retinal vein occlusion. Arch Ophthalmol 101:232–235
3. Miyake Y (1974) Electro-oculographic change in retinal arterial occlusion and its analysis. Acta Soc Ophthalmol Jpn 78:311–323
4. Gouras P, Carr RE (1965) Light-induced DC responses of monkey retina before and after central retinal arterial interruption. Invest Ophthalmol 4:310–317
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