Retinal Vein Occlusions
How the Eye Works
Light enters the eye and is focused by the clear dome over the eye called the cornea, and the lens, which lies behind the iris, the colored part of our eye. It is focused onto the retina, which lines the back of the eye like wallpaper. The retina is light sensitive and converts light signals into electric/ chemical signals that are understood by the brain. The center part of retina is called the macula. The macula is responsible for our ability to read, see fine detail, see people's faces, and appreciate color. Arteries deliver “fresh” blood to the retina and veins take the “used” blood away.
Retinal vein occlusions are very common causes of visual loss involving the retina. Retinal vein occlusions destroy the macula and the retina by causing the blood vessels to leak, close up or grow where they do not belong. Retinal vein occlusions start by preventing blood from leaving the retina. There are 2 forms: 1) Branch Retinal Vein Occlusion (BRVO), and 2) Central Retinal Vein Occlusion (CRVO). In a BRVO only a small branch of the retinal veins are blocked. In a CRVO, the main outflow of blood flowing into the “central” vein is blocked. Blockage of a retinal vein causes swelling or “edema” in the retina. The small retinal arteries can close in response to this process leading to further visual loss from poor blood flow called “ischemia.” Ischemia can lead to the growth of abnormal blood vessels; a process called “neovascularization.” Neovascularization can cause further visual loss from bleeding inside the eye or glaucoma, high pressure in the eye that may result in blindness.
The most common risk factors include aging, high blood pressure, diabetes, smoking and glaucoma. With age, patients develop blood vessel blockages from progressive narrowing of the inside of the blood vessels. Rarer risk factors include blood clotting disorders and inflammatory and infectious conditions which cause vasculitis (inflammation of blood vessels). Medical testing is ordered only if other symptoms accompany the BRVO or CRVO or other diseases are suspected. Men and women are affected equally, most after age 50. The disease tends to effect only one eye, but 10% of patients can have disease in both eyes. Anti-coagulants such as Heparin, Coumadin and aspirin have not been shown to be of value in preventing branch vein occlusion or managing its complications. Because anti-coagulants may be associated with systemic complications, they are prescribed only in specific clinical circumstances, for example for patients with known clotting abnormalities.
Branch Retinal Vein Occlusion (BRVO)
BRVO is the most common cause of retinal vascular occlusive disease. A patient may note no change in their vision, slight distortion or profound visual loss. Segmental swelling and blocked blood vessels are typical of a BRVO. A fluorescein angiogram, a study that looks at the circulation of the eye is used to evaluate macular edema and ischemia. This helps to determine if laser treatment is needed. Laser photocoagulation helps seal the leaking capillaries to promote resolution of the macular edema to prevent further visual loss and hopefully improve vision. If the fluorescein angiogram reveals macular ischemia, vision improvement is unlikely and laser treatment is not recommended. Ocular Coherence Tomography, OCT, is also employed to measure the retinal thickness and obtain microscopic images of the area of concern. OCT helps the doctor “track” the course of the disease and give actual measurements of the retinal swelling. Patients are typically observed for at least 3 months to monitor resolution of the swelling and intraretinal bleeding before laser is advocated. In some cases, surgery to separate the artery compressing and causing a blockage in the vein can be offered to treat a BRVO. Injections of steroids into the eye may also help patients with non-resolving macular edema, swelling in the central retina that is causing visual loss. Neovascularization develops in 40% of patients with large areas of ischemia, usually in the first 6 to 12 months. Without laser treatment 60% or more of these patients will develop visual loss from bleeding in the eye. Laser treatment can help prevent this complication. The fluorescein angiogram identifies which patients may benefit from this prophylactic treatment.
Macular pucker is a late problem of BRVO, which can lead to loss of vision. Scar tissue called an epiretinal membrane, forms on the macula and distorts the retina. This results in distorted vision, metamorphopsia, which is not improved with laser treatment. Severe neovascularization can cause retinal detachment from pulling by these vessels on the retina (traction detachment). Retinal surgery, called a vitrectomy, can treat these complications.
Central Retinal Vein Occlusion (CRVO)
CRVO can be non-ischemic or ischemic. In non-ischemic there is significant macular edema, swelling in the central retina that is causing visual loss. Ocular Coherence Tomography, OCT, is also employed to measure the retinal thickness and obtain microscopic images of the area of concern. OCT helps the doctor “track” the course of the disease and give actual measurements of the retinal swelling. This swelling can lead to central vision loss and may respond to focal laser treatment. Injections of steroids into the eye may also help patients with non-resolving macular edema. Wide areas of poor blood flow are found in the ischemic form of CRVO. This leads to neovascularization, growth of abnormal blood vessels, which can cause bleeding in the eye (vitreous hemorrhage) or a peculiar type of neovascularization that occurs in front of the eye on the iris (rubeosis irides). Rubeosis blocks the eye’s fluid outflow channels. This causes very high pressure known as neovascular glaucoma. This glaucoma may produce severe vision loss, eye pain and loss of the eye. Laser photocoagulation treatment is used to treat retinal neovascularization and rubeosis irides. Vitrectomy surgery is used to treat severe vitreous hemorrhage and neovascularization. Glaucoma and vitrectomy surgeries are used to treat severe neovascular glaucoma. A new surgery, Radial Nerve Sheathotomy may help to restore the retinal circulation and reverse the disease in some cases. Other surgeries and drug delivery systems are on the horizon to treat this devastating disease.