Branch Vein Occlusion

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Chapter 53 Branch Vein Occlusion

Introduction

Branch retinal vein occlusion (BRVO) is a common cause of retinal vascular disease.1 The Beaver Dam Study estimated the 15-year cumulative incidence of retinal vein occlusions (RVO) at 2.3% in the population, with a majority of these (78%) being BRVO.2 BRVO affects males and females equally and occurs most frequently between the ages of 60 and 70. The pathologic interruption of venous flow in these eyes almost always occurs at a retinal arteriovenous intersection, where a retinal artery crosses over a retinal vein.37 Systemic vascular diseases such as hypertension and arteriosclerosis are risk factors for BRVO, probably because they lead to thickening of the retinal artery.3,7 Other risk factors for BRVO include diabetes, smoking, hyperlipidemia, glaucoma, and ocular inflammatory disease.8 Antiphospholipid antibodies, elevated plasma homocysteine levels, and low serum folate levels have also been associated with increased risk of vein occlusion.911 A decreased risk is present in those with higher serum levels of high-density lipoprotein and light to moderate alcohol consumption.8 Other studies have suggested an increased risk of BRVO in eyes with shorter axial lengths.1215 This section discusses the pathophysiology, clinical features, evaluation, and treatment of patients with BRVO.

Pathogenesis

Because BRVO mostly occurs at arteriovenous crossings,4,8,16 underlying arterial disease may play a causative role. In 99% of 106 eyes with BRVO, the artery was located anterior to the vein at the obstructed site.7,8 Histopathologically, the retinal artery and vein share a common adventitial sheath, and in some cases, a common medium.17 The lumen of the vein may be compressed up to 33% at the crossing site.7,18 The vitreous may also play a role in compression of susceptible arteriovenous crossing sites, as evidenced by studies demonstrating that eyes with decreased axial length and higher likelihood of vitreomacular attachment at the arteriovenous crossing are at increased risk of BRVO.13,19

Some have postulated that turbulent blood flow at the crossing site causes focal swelling of the endothelium and deeper vein wall tissue, leading to venous obstruction.17,19,20 Other reports have demonstrated actual venous thrombus formation at the point of occlusion.3,18 The resulting venous obstruction leads to elevation of venous pressure that may overload the collateral drainage capacity21 and lead to macular edema and ischemia by mechanisms that are still under investigation. Unrelieved venous pressure can also result in rupture of the vein wall with intraretinal hemorrhage.17 Vision loss from RVOs is typically due to macular ischemia, macular edema, or complications from neovascular disease.

Clinical features

Signs

Patients typically present with a wedge-shaped distribution of intraretinal hemorrhage that is less marked if the occlusion is perfused (or nonischemic), and more extensive if the occlusion is nonperfused (or ischemic) and associated with retinal capillary nonperfusion. The Branch Vein Occlusion Study Group defined ischemic BRVO as those with greater than a total of five disc diameters of nonperfusion on fluorescein angiography (FA).1 The location of the venous blockage determines the distribution of the intraretinal hemorrhage; if the venous obstruction is at the optic nerve head, two quadrants of the fundus may be involved, whereas if the occlusion is peripheral to the disc, one quadrant or less may be involved with the intraretinal hemorrhage. If the venous blockage is peripheral to tributary veins draining the macula, there may be no macular involvement and no decrease in visual acuity. The most common location for BRVOs is in the superotemporal quadrant.5,6,22 This favored location may be attributed to a larger number of arteriovenous crossings in the superotemporal quadrant.

Figure 53.1 demonstrates the typical acute appearance of a BRVO involving the superotemporal quadrant of the right eye. A narrowed branch retinal vein passing under a retinal artery can sometimes be identified proximal to the hemorrhage. Rarely, a patient may present initially with very little intraretinal hemorrhage, which then becomes more extensive in the succeeding weeks to months. In these instances, it is presumed that an incomplete block at the arteriovenous crossing has progressed to more complete occlusion.

Over time the intraretinal hemorrhage may completely resorb. Without the characteristic segmental distribution of intraretinal hemorrhage, the ophthalmoscopic diagnosis may be more difficult, but the segmental distribution of retinal vascular abnormalities that occurred during the acute phase will persist and be apparent on FA. In many cases, macular edema may also be detected by optical coherence tomography (OCT). Consequently, in the chronic phase of the disease, after intraretinal hemorrhage absorption, the diagnosis may depend on detecting a segmental distribution of retinal vascular abnormalities that may include capillary nonperfusion, dilation of capillaries, microaneurysms, telangectatic vessels, and collateral vessel formation (Fig. 53.2).

Nonocular signs such as systemic hypertension have been associated with BRVOs.2,8,23 Thus, systemic blood pressure may be elevated. In bilateral cases or cases involving young patients, systemic manifestations of infectious disease, inflammatory or autoimmune conditions, neoplasm, or hypercoagulable states may be present.

Complications

There are three common vision-limiting complications of BRVO: (1) macular edema; (2) macular ischemia; and (3) sequelae of neovascularization. It is important to appreciate the variability of these complications before considering the benefits of treatment.2426

During the acute phase, extensive intraretinal hemorrhages may obscure macular ischemia and macular leakage on FA. Under these circumstances it is impossible to evaluate the perfusion status by FA because the hemorrhage itself blocks the view of the vasculature. In addition, the hemorrhage in the foveal center may reduce visual acuity independently of any macular edema or ischemia. Since this reduction in visual acuity may recover completely if there is no other cause for the visual loss, such as macular edema or macular capillary nonperfusion, observation in these cases can be considered. When there is extensive foveal hemorrhage, OCT is an important ancillary test to look for macular edema. Although it may be difficult to provide a prognosis in the acute phase, it is helpful to recognize that about one-third to one-half of patients with BRVO have a return of vision to 20/40 or better without any therapy.

Retinal and iris neovascularization, vitreous hemorrhage, traction retinal detachment, and neovascular glaucoma are complications that manifest late in the course of the disease due to ischemia. With the exception of macular ischemia, these complications can largely be treated or prevented. Thus, it is important that patients with BRVO be closely followed. From the Branch Vein Occlusion Study, 31–41% of patients with ischemic BRVO (defined as >5 disc diameters of nonperfusion on FA) developed neovascularization or vitreous hemorrhage, compared with 11% of patients with nonischemic BRVO.1

Clinical evaluation

Clinical examination

A complete ophthalmic examination should be performed, paying particular attention to the history of glaucoma and signs of intraocular inflammation, since these are risk factors for BRVO. Careful examination of the iris and angle should be performed in appropriate cases to monitor for early signs of rubeosis or neovascular glaucoma. Initially, when the risk of macular edema and neovascularization is higher, patients should be followed every month. Once stable, and if visually significant macular edema and other complications are not present, follow-up can be extended.

Fluorescein angiography

To help verify the diagnosis and evaluate for complications, FA should be obtained to delineate the retinal vascular characteristics that may have prognostic significance: macular leakage and edema, macular ischemia, and large segments of capillary nonperfusion that may portend eventual neovascularization. FA is the only technique that will accurately define the capillary abnormalities in BRVO; it is therefore particularly important that high-quality angiography be obtained (Fig. 53.3).

The characteristic finding on FA is delayed filling of the occluded retinal vein. Varying amounts of capillary nonperfusion, blockage from intraretinal hemorrhages, microaneurysms, telangiectatic collateral vessels, and dye extravasation from macular edema or retinal neovascularization are other features encountered. In chronic cases, when the hemorrhages have resolved, microvascular changes on FA may provide the only clues of a previous BRVO.

When FA demonstrates macular leakage and edema with cystoid involvement of the fovea, but no capillary nonperfusion, it is presumed that the macular edema is the cause of vision loss. Under these circumstances, about one-third of patients will spontaneously regain some vision. However, patients who have had decreased vision for over 1 year as a result of macular edema are much less likely to regain vision spontaneously. When macular edema is present ophthalmoscopically within the first 6 months after a BRVO and there is little or no leakage on FA, macular ischemia may be the cause of the macular edema. In such circumstances, the edema almost always spontaneously resorbs in the first year after the occlusion, often with return of vision.27

Unfortunately, acute BRVOs with dense intraretinal hemorrhages may make FA interpretation challenging due to blockage of fluorescence by the hemorrhages. Thus, it is advisable to obtain FA only after the intraretinal hemorrhages have cleared significantly from the macula. Other diagnostic tests, such as OCT, can be obtained in the acute phase to aid in the diagnosis of macular edema.

Wide-field angiography

Wide-field angiography is not a commonly used imaging modality for patients with BRVO, but a recent study supports its utility. Ultrawide-field FA using the Optos C200MA revealed a correlation between nonperfusion in the peripheral retina with macular edema and retinal neovascularization.28 Future studies to determine if laser photocoagulation of the nonperfused peripheral retina decreases macular edema and neovascularization may alter the therapeutic paradigm. In patients with recalcitrant macular edema or retinal neovascularization, wide-field angiography may reveal peripheral areas of nonperfusion helping to guide targeted laser photocoagulation.

Diagnostic workup

Treatment options

Laser treatment

Branch Vein Occlusion Study for macular edema

The collaborative Branch Vein Occlusion Study (BVOS),38 a multicenter randomized clinical trial supported by the National Eye Institute, reported that argon laser photocoagulation may reduce visual loss from macular edema for those eyes that meet study eligibility criteria and are treated according to that protocol. Important eligibility criteria included fluorescein-proven perfused macular edema involving the foveal center, absorption of intraretinal hemorrhage from the foveal center, recent BRVO (usually 3–18 months’ duration), no diabetic retinopathy, and vision reduced to 20/40 or worse after best refraction.

In the BVOS,38 argon laser photocoagulation was applied in a grid pattern throughout the leaking area demonstrated by FA (Fig. 53.5). Coagulation extended no closer to the fovea than the edge of the capillary-free zone and no further into the periphery than the major vascular arcade. Recommended treatment parameters included a duration of 0.1 second, a 100-µm diameter spot size, and a power setting sufficient to produce a “medium” white burn. FA was repeated 2–4 months after the treatment, and additional photocoagulation was applied to residual areas of leakage if reduced visual acuity persisted. Improvement in visual acuity was assessed in several ways.38 When improvement was defined as reading two or more Snellen lines (beyond baseline) at two consecutive visits, treated eyes showed visual improvement more often than untreated eyes. After 3 years of follow-up, 63% of treated eyes gained two or more lines of vision, compared to 36% of untreated eyes. The average gain in visual acuity for treated eyes was one more Snellen line than in untreated eyes.

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