Ординатура / Офтальмология / Английские материалы / Electrophysiology of Vision_Lam_2005

Other ocular vascular disorders:

Hypertensive retinopathy

Idiopathic polypoidal choroidal vasculopathy

VASCULAR OCCLUSIONS

The ophthalmic artery is a branch of the internal carotid artery and gives rise to the ciliary arteries and the central retinal artery. The ciliary arteries, in turn, give rise to the choroidal arteries and the choriocapillaris, a network of capillaries adjacent to the Bruch’s membrane and the retinal pigment epithelium. The choriocapillaris supplies the retinal pigment epithelium and the outer layers of the retina including the photoreceptor layer, outer plexiform layer, and the outer portion of the inner nuclear layer (see Chapter 1). The central retinal artery, which is visible at the optic nerve head, provides circulation to the inner layers of the retina including the nerve fiber layer, ganglion cell layer, inner plexiform layer, and the inner portion of the inner nuclear layer. The venous drainage from these inner retinal layers is provided by the central retinal vein. The ERG components such as the b-wave and oscillatory potentials have their origins in the inner retinal layers and are more likely to be selectively impaired when the retinal circulation provided by the central retinal artery and vein is disrupted. In contrast, the ERG a-wave, mostly a photoreceptor response, is impaired when

Figure 14.1 (Facing page) Fundus photograph, visual field and multifocal ERG of a 53-year-old woman who had complete amaurosis fugax of the right eye lasting 5 min followed by almost complete recovery except for a persistent inferior area of blurred vision. Visual acuity was 20=20 in each eye and fundus appearance was normal. Visual field showed a consistent inferior defect. Multifocal ERG revealed impaired responses (circled) corresponding to the inferior visual field defect due to ischemic retinal damage. Further work-up with echocardiogram revealed patent foramen ovale and pulmonary hypertension as the cause of her embolic events.

choroidal circulation is compromised but is relatively spared if the retinal circulation is reduced.

Ophthalmic Artery Occlusion

Disruption of blood flow of the ophthalmic artery may occur due to atherosclerosis, thrombosis, or embolism. In addition, the ophthalmic artery is a branch of the internal carotid artery, and any significant impediment of carotid artery blood flow may produce hypoperfusion of the ophthalmic artery. Ophthalmic artery hypoperfusion, in turn, results in hypoperfusion of not only the central retinal artery supplying the inner retinal layers but also the ciliary arteries providing circulation to the choroid and the outer retinal layers. The extent of ERG findings in ophthalmic artery occlusion correlates with the degree of retinal ischemia. All components of the full-field ERG including scotopic and photopic a- and b-waves are impaired in ophthalmic artery occlusion (1). In contrast to central retinal artery or vein occlusion, a selective decrease in b-wave amplitude due to ischemia of the inner retinal layers does not typically occur in ophthalmic artery occlusion. The EOG light-peak to dark-trough ratios are usually reduced in ophthalmic artery occlusion because of ischemia to the retinal pigment epithelium and photoreceptor cells (1). The VEP findings parallel ERG responses but may be further impaired due to ischemic optic neuropathy.

Central Retinal Artery Occlusion

Clinical features of central retinal artery occlusion (CRAO) include diffuse retinal edema and the appearance of a ‘‘cherry-red spot’’ at the fovea. Retinal edema does not occur at the fovea where there is no ganglion cell layer and the retina is the thinnest so that the color of the choroidal circulation stands out against the surrounding edematous opaque retina. Fluorescein angiography shows absent or markedly reduced filling of the retinal arteries. Thrombosis or embolus of the central retinal artery at the laminar cribosa is often the cause of CRAO. Risk factors for CRAO include atherosclerosis, embolism from carotid intravascular plaques, diseased

cardiac valves, or intracardiac deposits, and vasculitic disorders such as giant cell arteritis. With time, the central retinal artery opens or recanalizes and flow is at least partially restored, but the ischemic damage is irreversible. Visual prognosis is unfavorable with final visual acuity of 20=200 or worse in most patients. Hayreh et al. found irreversible damage to the retina of rhesus monkeys after 107 min of complete mechanical clamping of the central retinal artery but the retina recovered well after 97 min (2). Treatments include reduction of intraocular pressure to increase ocular blood perfusion by ocular massage and aqueous removal with anterior chamber paracentesis. Other treatments include inhalation of supplemental oxygen or carbogen (a mixture of increased concentration of carbon dioxide and oxygen), hyperbaric oxygen treatment, and intravenous thrombolytic medications.

The diagnosis of CRAO is by retinal appearance with fluorescein angiography support if needed. However, in a patient with a remote history of CRAO, retinal appearance may be nearly normal, and ERG is helpful in detecting retinal dysfunction. In addition, ERG can differentiate a CRAO from an ophthalmic artery occlusion. Disruption of the retinal arterial circulation from CRAO produces ischemia of the inner retinal layers whereas ophthalmic artery occlusion results in hypoperfusion of the entire retina.

Standard full-field ERG in CRAO typically reveals a selective reduction of b-wave amplitude due to ischemia of the inner retinal layers (Fig. 14.2) (3–6). The a-wave, which receives contribution from photoreceptor activity, is relatively spared in CRAO but some impairment may occur. Of interest, the photopic negative response (PhNR) of the full-field ERG is severely reduced in CRAO (7). The PhNR is a negative response occurring after the b-wave and is a measure of retinal ganglion cell function. The reduction of PhNR in CRAO implicates that the damage also involves the retinal ganglion cells and their axons in the optic nerve.

In general, patients with CRAO have the following on standard full-field ERG: (1) scotopic rod flash response— marked reduced and prolonged b-wave, (2) scotopic combined rod–cone bright flash response—normal or reduced a-wave

Figure 14.2 Full-field ERG responses of a 77-year-old patient with CRAO of the right eye. All responses are impaired in the affected eye with a relative selective reduction of b-wave such that the b- to a-wave amplitude ratio is less than 1 for the scotopic combined rod–cone response (‘‘negative ERG pattern’’).

with normal or prolonged implicit time and a marked selective decrease in b-wave amplitude with prolonged implicit time, producing a reduced b-wave=a-wave amplitude ratio of less than 1 (negative ERG pattern), (3) oscillatory potentials—severely reduced, (4) photopic cone flash response—normal or reduced a-wave with normal or prolonged implicit time and a decrease in b-wave amplitude with prolonged implicit time, producing a reduced b-wave=a-wave amplitude ratio, and (5) photopic 30-Hz cone flicker response—reduced and prolonged b-wave.

Yotsukara and Adachi-Usami (6) documented fullfield ERG improvement with scotopic combined rod–cone

bright-flash response in 8 of 15 CRAO patients treated with ocular massage, intravenous urokinase, and hyperbaric oxygen. Delayed b-wave implicit times and reduced b-wave=a- wave amplitude ratios were noted in the affected eyes as compared with the unaffected eyes. Improvements of the ERG parameters correlated with visual improvement.

Branch Retinal Artery Occlusion

Branch retinal artery occlusion (BRAO) caused by embolus or thrombosis results in regional retinal edema. Intravascular embolic plaque may be visible, and fluorescein angiography typically shows reduced perfusion in the distribution of the retinal artery. Risk factors for BRAO include atherosclerosis, hematologic disorders, vasculitis, and embolism from carotid artery or diseased cardiac valves. With time, the retinal arteriole opens or recanalizes with at least partial restoration of the blood flow, but the ischemic damage is irreversible. Visual prognosis is related to macular involvement and the size of the involved retina. The diagnosis of BRAO is by retinal appearance with support from fluorescein angiography if needed. The effect of BRAO on full-field ERG is related to the size of the involved retina, and the full-field ERG may be normal or mildly impaired with a selective decrease in b- wave amplitude similar to CRAO. Multifocal ERG is more likely than full-field ERG to detect regional retinal dysfunction from BRAO and may be potentially useful in patients with a remote history of branch retinal vein occlusion (BRVO) when retinal signs are minimal (Fig. 14.3). Of interest, Hasagawa et al. (8) found that the second-order multifocal ERG response was more impaired than the first-order responses in the retinal region affected by BRAO in five patients. The authors concluded that the reduction in firstand second-order multifocal ERG responses was due to inner retinal dysfunction.

Central Retinal Vein Occlusion

Clinical features of central retinal vein occlusion (CRVO) include dilated tortuous retinal veins, scattered intraretinal

Figure 14.3 Visual field and multifocal ERG of a 69-year-old woman with a superior BRAO producing an inferior visual field defect. The area of markedly impaired multifocal ERG responses (circled) corresponds well to the visual field defect.

and nerve fiber layer hemorrhages, retinal edema, and optic disc edema. Physiologic mechanism of CRVO is likely related to atherosclerosis of the central retinal artery, which impinges on the adjacent central retinal vein and results in

reduced venous flow with or without thrombosis. Risk factors for CRVO include hypertension, diabetes mellitus, and glaucoma.

Two types of CRVO are recognized, non-ischemic, and ischemic. Non-ischemic CRVO, also called as venous stasis retinopathy, is less severe and characterized by mild dilation and tortuosity of retinal veins with scattered retinal hemorrhages. Fluorescein angiography shows prolonged retinal venous filling with minimal areas of non-perfusion. Development of neovascularization of the anterior segment as a reaction to retinal ischemia is rare. Visual prognosis is generally favorable in non-ischemic CRVO, but some patients diagnosed initially with non-ischemic CRVO may progress to ischemic CRVO.

In contrast to non-ischemic CRVO, ischemic CRVO also called as hemorrhagic retinopathy is associated with marked retinal venous dilation and tortuosity, hemorrhages, edema, nerve fiber infarcts (cotton–wool spots) and prominent areas of capillary non-perfusion on fluorescein angiography. The risk for development of neovascularization of the anterior segment (iris and=or angle) and neovascular glaucoma is highest during the first four months after CRVO.

Neovascularization is treated with panretinal laser photocoagulation or, in severe cases, with cryotherapy or laser destruction to the ciliary body. In the past, macular edema was treated with grid laser photocoagulation but results from the Central Vein Occlusion Study (9), a multicenter randomized clinical trial, do not support this treatment. Visual prognosis is poor in ischemic CRVO.

The diagnosis of CRVO is primarily by retinal appearance with support from fluorescein angiography if needed. However, in a patient with a remote history of CRVO, the retinal signs may be minimal, consisting of only mild venous tortuosity, and ERG is helpful in diagnosing retinal ischemia.

The extent of ERG findings in CRVO correlates with the degree of retinal ischemia, and the full-field ERG may be minimally to severely impaired. In those with mild nonischemic CRVO, the ERG responses may show amplitude loss

Figure 14.5 Examples of initial scotopic combined rod–cone fullfield ERG responses in CRVO in patients with and without subsequent iris neovascularization. Note the greater impairment of the ERG response and the relatively selective impairment of the b-wave in the patient with subsequent iris neovascularization. Several ERG parameters have been demonstrated as predictors of iris neovascularization, including b-wave amplitude, b-wave implicit time, and b- to a-wave amplitude ratio of the scotopic combined rod–cone response as well as b-wave implicit time of the photopic 30-Hz flicker cone response. (From Ref. 22) with permission from the American Medical Association.)

100% sensitivity and specificity, the Central Vein Occlusion Study (13) recommends frequent follow-up examinations in CRVO patients, which theoretically has the potential to detect neovascularization promptly in all patients. However, patient return rate may not be 100%, and less developed health care systems may not have the resources to provide adequate number of follow-up examinations to all CRVO patients.