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Retinal vein occlusions occur when there is a blockage of veins carrying blood with needed oxygen and nutrients to the nerve cells in the retina. A blockage in the retina’s main vein is referred to as a central retinal vein occlusion (CRVO), while a blockage in a smaller vein is called a branch retinal vein occlusion (BRVO).
Most BRVOs occur at an arteriovenous crossing—an intersection between a retinal artery and vein. These vessels share a common sheath (connective tissue), so when the artery loses flexibility, as with atherosclerosis (hardening of the arteries), the vein is compressed. The narrowed vein experiences turbulent blood flow that promotes clotting, leading to a blockage or occlusion. This obstruction blocks blood drainage and may lead to fluid leakage in the center of vision (macular edema) and ischemia—poor perfusion (flow) in the blood vessels supplying the macula.
CRVO develops from a blood clot or reduced blood flow in the central retinal vein that drains the retina. And we have learned that a large number of conditions may increase the risk of blood clots. Some eye doctors advise testing for them. However, it is not certain how these health conditions are related to CRVO—and some of them, if diagnosed, have no agreed-to or necessary recommended treatment. Many eye doctors do not advise testing for a CRVO in one eye, but do recommend a visit with a family doctor to be sure there is no diabetes or high blood pressure.
CRVO that occurs in both eyes at the same time can be related to systemic disease; in these cases, a tendency toward abnormal blood clotting is definitely more common and medical testing to detect so-called “hypercoagulable states” is indicated. While some eye doctors coordinate such testing, most refer patients to their family doctors, internists, or hematologists (physicians specializing in diseases of the blood) for testing.
The common risk factors for BRVO are:
Most patients with CRVO develop it in one eye. And, although diabetes and high blood pressure are risk factors for CRVO, its specific cause is still unknown.
BRVO causes a sudden, painless loss of vision. If the affected area is not in the center of the eye, BRVO can go unnoticed with no symptoms. In rare cases of an undetected vein occlusion, visual floaters from a vitreous hemorrhage (blood vessels leaking into the vitreous gel of the eye) can be the main symptom; this is caused by development of abnormal new blood vessels (neovascularization) in the retina.
Mild CRVO may show no symptoms. However:
Most often, BRVO is diagnosed by an eye exam that shows retinal hemorrhage (blood vessels leaking into the retina), thickened and twisted blood vessels, and retinal edema
(swelling with fluid). Two types of retinal imaging tests aid the diagnosis of BRVO: Fluorescein angiography (FA) and Optical coherence tomography (OCT). FA provides images of fluid leaking from damaged or abnormal retinal vessels, demonstrating:
FA is very valuable for detecting BRVO and the flow of the blood vessels. Once BRVO has been found, OCT is used to provide a better assessment of whether macular edema is present, and if so, how severe it is.
CRVO is typically a clinical diagnosis—that is, one based on medical signs and patient-reported symptoms. When a retina specialist looks into the eye, there is a characteristic pattern of retinal hemorrhages (bleeding) and a diagnosis is made. Common conditions that can take on an appearance of CRVO include diabetic retinopathy (retina disease) and retinopathy related to low blood counts, such as anemia and thrombocytopenia (a deficiency of blood platelets). Swelling of the center of the retina, called macular edema is common, and to detect this and measure the amount of swelling, an optical coherence tomography (OCT) image is often obtained. To help distinguish CRVO from conditions that may mimic it, and to assess closure of small blood vessels, or to search for or confirm growth of new abnormal vessels, FA imaging may be performed.
Treatment for BRVO begins with identifying underlying risk factors and treating them. Risk factors are assessed using several methods:
Eye treatment is aimed at treating retinal complications rather than at trying to relieve the blockage itself. Macular edema, the main reason for visual loss from BRVO, is often treated with intraocular (in-the-eye) injections of anti-VEGF drugs designed to stop the growth of abnormal new blood vessels in the eye and decrease leakage. Local anesthetic eye drops are given before the injections to numb the eye and minimize discomfort. There are currently 3 anti-VEGF drugs. In several large clinical studies, all 3 of these anti-VEGF drugs have demonstrated good results, with over 50% of patients enjoying significant visual improvement. The use of these drugs may require frequent retreatment, but injection schedules are determined on a case-by-case basis. Laser treatment may be used along with anti-VEGF therapy in hard-to-treat cases. Laser therapy for macular edema involves applying light laser pulses to the macula in a grid pattern. In a large multi-center clinical trial, after 3 years of follow up, this treatment showed improvement of vision in approximately two-thirds of patients.
Intraocular injections of steroids are another potential treatment for eyes that don’t respond to anti-VEGF drugs. A clinical trial that evaluated steroid treatment using a slow-releasing steroid implanted in the eye, showed that approximately 30% of BRVO patients enjoyed significant visual improvement following treatment. While intraocular steroids can have some side effects such as an increase in eye pressure and cataract progression, in most cases, these side effects can be controlled.
Overall, BRVO carries a generally good prognosis. In fact, some BRVO patients don’t require treatment at all, either because the blockage did not involve the macula, or because they have not experienced a decrease in vision. Over 60% of patients, treated and untreated, maintain vision better than 20/40 after 1 year.
CRVO has a better prognosis in young people. In older patients who receive no treatment, about one-third improve on their own, about one-third wax and wane and stay about the same, and about one-third get worse. If there is macular edema, it may improve on its own. In patients with CRVO, vascular endothelial growth factor (VEGF) is elevated; this leads to swelling as well as new vessels that are prone to bleeding. The most common treatment, based on results from powerful randomized clinical trials, involves periodic injections into the eye of an anti-VEGF drug to reduce the new blood vessel growth and swelling. Although anti-VEGF drugs reduce the swelling, they are not a cure. As the drug leaves the eye and moves into the bloodstream, the effect in the eye wears off, so re-injection is often needed. A rare lucky patient needs only one injection, but the norm is a series of periodic injections over the course of a few years.
Another option for treating macular edema from CRVO is with an injection of intraocular steroid. This could be either a liquid steroid or a small steroid pellet. The steroid injections typically last several months, but can cause elevated intraocular pressure requiring eye drops or increased rate of cataract formation.
CRVO comes in 2 types: Ischemic (pronounced is KEY mick) and Non-ischemic CRVO.
Patients with ischemic CRVO have worse vision with less chance for improvement. They have a tendency for the eye to cause new blood vessels to grow—and in the front of the eye, these new vessels can clog the outflow of normal eye fluids. The eye pressure goes up and glaucoma develops. In the back of the eye, new blood vessels may cause bleeding. When there is ischemic CRVO with new vessels, anti-VEGF injections lead to prompt, but often temporary, control of the new vessels. Laser treatment tends to offer a more permanent effect. In some cases, both treatments are used.
Non-ischemic CRVO can worsen and become ischemic, so when CRVO is diagnosed, monthly checkups are initially recommended. It’s important to note that early detection of macular edema or abnormal blood vessels is important; most patients can avoid severe vision loss if treatment is begun before substantial damage develops in the eye.
Retinal neovascularization is a potentially serious complication of BRVO in which an inadequate blood supply (ischemia) causes abnormal new blood vessels to grow on the surface of the retina. This growth can further decrease vision by causing vitreous hemorrhage that causes floaters and loss of vision, retinal detachment, and glaucoma. When neovascularization develops, scatter laser photocoagulation therapy is used to create burns in the area of the vein occlusion (blockage). The aim is to try to lower the oxygen demand of the retina and thus stop the abnormal blood vessels from growing. Patients receive an anesthetic to numb the eye and make the treatment more comfortable. Scatter photocoagulation has been shown to reduce neovascularization related complications from 60% to 30%. Because only a few patients develop abnormal new blood vessels in the retina, not many need scatter photocoagulation treatment.
Source: American Society of Retina Specialists