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The eye is a singularly revealing window to the human vascular system. Nowhere else in the body can clinicians so readily view the local effects of systemic vascular disease. You can use that ability not only to diagnose retinal vascular disease, but also to detect signs of underlying pathology. This article, the second in a series of retinal "atlases," describes the retinal vasculopathies you will see most commonly.
Diabetic Retinopathy
This is the most common retinal vascular disease, accounting for 10 percent of new cases of blindness each year, and is the leading cause of blindness for those age 20-74.1,2 Yet, half of all patients with high-risk retinopathy go undiagnosed.2
Patients may have either juvenile-onset (type-1) diabetes, or adult-onset (type-2) diabetes. The former causes a higher incidence and greater severity of retinopathy. The retinopathy begins when the llary basement membrane thickens, leading to vessel closure. There's a dropout of pericytes, the cells that keep normal capillaries tight. The capillary walls weaken, causing microaneurysms that then rupture, with leakage of serum proteins, lipids and blood.
Blood accumulates as dot and blot hemorrhages in the retina's inner nuclear and outer plexiform layers, and flame-shaped hemorrhages form in the nerve fiber layer. Serum lipids aggregate as exudates, and a build-up of serous fluid causes retinal edema. This can lead to clinically significant macular edema (CSME).
Venous beading results, and further capillary closure leads to focal hypoxia and cotton wool spots. More widespread hypoxia causes a release of a vascular endothelial growth factor, stimulating neovascularization either from the disc or the capillaries of the retina or iris.
Neovascularization of the disc or elsewhere on the retina poses risk for vitreal hemorrhage and subsequent tractional retinal detachment. Iris neovascularization may cause neovascular glaucoma with secondary angle closure.
Intraretinal microvascular abnormalities may be a precursor to retinal neovascularization. Diabetic retinopathy is either proliferative (with neovascularization) or non-proliferative (without ovascularization).3,4 Diabetic maculopathy is considered a separate entity, consisting of dot and blot hemorrhages, hard exudates and CSME.5
Diabetic maculopathy calls for laser treatment only in cases involving CSME. The surgeon applies argon laser photocoagulation to all leaking microaneurysms more than 500µm from the center of the foveal avascular zone. If fluorescein angiography reveals that microaneurysms are the cause of CSME, they receive focal photocoagulation. If diffuse leakage causes CSME, the surgeon applies photocoagulative burns in a grid pattern.6-8 The aim is to preserve the remaining vision.
Cases of high-risk proliferative retinopathy-when vision may drop to 5/200 or worse-as well as iris neovascularization warrant panretinal photocoagulation. Argon laser burns destroy areas of hypoxic retina from just beyond the vascular arcades of the posterior pole to the far periphery.4,9 The eye's oxygen demand decreases, and the neovascularization regresses. Prompt treatment of high-risk proliferative retinopathy or iris neovascularization reduces the risk of severe vision loss by 50 percent.4
Hypertensive Retinopathy
Systemic hypertensive vascular changes can affect the retinal vessels.10 Hypertensive retinopathy initially involves narrowing of the retinal arterioles. In prolonged systemic hypertension, the retinal vessels become arteriolosclerotic. Fibrosis in the vessel wall and perivascular tissue causes further narrowing.
As the sclerotic process advances, the arterioles change color, first exhibiting increased luster, then changing to copper, then silver. Ultimately the arterioles take on a cord-like appearance with no apparent bloodstream. They'll also show focal constriction representing sclerosis from prolonged diastolic pressures of 110mm Hg or higher.11
You'll also see arteriovenous crossing constrictions. As the sclerotic arteriole crosses a weak-walled venule, it compresses that venule. This impedes drainage and can contribute to a branch vein occlusion.
Later stages of hypertensive retinopathy will reveal flame-shaped hemorrhages in the nerve fiber layer. Arteriolar occlusion with subsequent cotton wool spots will also occur within the nerve fiber layer.
Macular edema may develop in hypertensive retinopathy, but tends to be self-limiting, and is amenable to photocoagulation. Edematous residue in the form of a macular exudative star signals advanced, long-standing hypertension. Disc edema often accompanies malignant hypertension. We don't fully understand its cause. In some cases it may be due to increased intracranial pressure. Edema of the disc and surrounding retina, often with an accompanying macular star, is a medical emergency.
Managing hypertensive retinopathy involves little direct intervention. If there's persistent macular edema, refer the patient for photocoagulation. Also refer to a primary care physician to manage the hypertension.
Retinal Vein Occlusion
Retinal vein occlusions manifest in four forms: central retinal vein occlusion (CRVO), hemi-central retinal vein occlusion (HRVO), branch retinal vein occlusion (BRVO) and papillophlebitis.
CRVO and HRVO develop in similar ways: The central retinal vein compresses while passing through the lamina cribrosa, blocking drainage from the retina. The cause of this blockage is thought to be thrombosis due to compression from a sclerotic central retinal artery, altered blood flow or a combination of these factors. BRVO develops when a sclerotic retinal arteriole compresses a thin-walled venule, blocking drainage.12 Papillophlebitis is essentially a CRVO in a young, healthy adult; the difference is that papillophlebitis is thought to develop from an inflammatory process.13,14
CRVO appears as dilated, tortuous veins in all four retinal quadrants, deep retinal dot and blot hemorrhages, superficial NFL hemorrhages, and macular and disc edema. There may be neovascularization of the disc, retina or iris. Retinal exudates and collateral vessels on the optic disc are also common.15,16 Papillophlebitis and HRVO share this appearance, though the latter involves only half the retina.13-17 BRVO affects only one quadrant,15,16 and the hemorrhaging tends to be triangular with its apex at an A-V crossing.
In these vein occlusions the major vessels themselves do not leak. Rather, the leakage comes from the thin-walled retinal capillaries draining into the major vessels. A profound occlusion can destroy the integrity of the retinal capillaries. The subsequent ischemia puts the patient at risk for neovascularization. In ischemic CRVO, patients are likely to develop iris neovascularization and neovascular glaucoma.18 In ischemic HRVO and BRVO, patients tend to manifest neovascularization on the optic disc and retina; this raises the risk of vitreal hemorrhage and tractional retinal detachment.19,20 Papillophlebitis rarely develops any noticeable ischemia. In rare case where there is marked ischemia, the condition behaves much like ischemic CRVO.13,14
Macular edema is the most common cause of vision reduction in retinal vein occlusions. In BRVO or HRVO, macular edema is amenable to argon laser photocoagulation, but in many cases it resolves without treatment. Clinicians usually wait at least three months before taking this step. If the edema persists beyond 18 months, however, it will permanently disrupt the RPE with irreversible vision loss, and photocoagulation will bring no benefit.21,22
As for macular edema in CRVO, doctors used to treat it with argon laser photocoagulation. Research shows this treatment doesn't improve vision in these patients.23
Ischemic vein occlusions with neovascularization require panretinal photocoagulation to prevent severe vision loss. Clinicians once used prophylactic laser treatment to prevent neovascularization. Recent research, however, indicates you should monitor for neovascularization, and only then refer for treatment.24
The best way to assess ischemia in retinal vein occlusions is fluorescein angiography. However, wait several weeks after the occlusion appears to allow the hemorrhages to clear. Visual acuity and pupil testing can lend insight. If there's an APD and vision worse than 20/200, the occlusion is likely ischemic.25
Visual acuity reduction of 20/200 or worse may be due to macular edema and hemorrhage. Otherwise, the most likely cause is macular infarction and destruction of the perifoveal capillary network. Angiography will show an enlarged foveal avascular zone and hypofluorescence beyond the macular region. Visual loss in these cases is irreversible.
Patients with retinal vein occlusions have a higher incidence of systemic disease, particularly vascular conditions. Refer to a primary care physician for evaluation.26-28
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