What are the retinal veins?
The retina is a very thin sheet of nerve tissue that lines the inside of the back of the eye. The macula, the central part of the retina, is responsible for our central vision, allowing us to read, drive and recognize faces. Much like other parts of the body, the retina and macula have both arteries and veins. The arteries transport oxygenated blood to the retina from the heart, and the veins transport blood without oxygen from the retina back to the heart. Several small veins drain blood into the larger Branch Retinal Veins, and those four branch retinal veins then come together to form the single Central Retinal Vein, the major vein of the retina. Proper functioning of the retinal vessels is crucial for appropriate delivery of oxygen to the retinal tissue.
What is a retinal vein occlusion?
A retinal vein occlusion (RVO) is a blockage of blood flow in one of the retinal veins. It can occur in either a branch retinal vein (branch retinal vein occlusion, BRVO) or in the central retinal vein (central retinal vein occlusion, CRVO). The occlusion causes bleeding in either one section of the retina (BRVO) or throughout the retina (CRVO) and can also produce swelling of the macula (macular edema). The region of the retina in which the blockage occurs will be deprived of oxygen (ischemia).
In severe vein occlusions, this oxygen deprivation can occur in the macula (macular ischemia). If a significant amount of ischemia occurs, new blood vessels may begin to grow on top of the retina (neovascularization).
Retinal vein occlusion is a major cause of vision loss in the United States and is more common in patients 65 years of age and older. BRVO is slightly more common than CRVO, and once either occurs in one eye, there is approximately a 10% chance of it developing in the other eye within the next three years.
How does RVO affect vision?
A retinal vein occlusion will usually be sudden, painless and occur in only one eye at a time. Patients will notice blurring either in their central vision, in one corner of their vision or throughout their entire field of vision, depending on the location of the blockage and the resulting macular edema or ischemia, if present.
What are the complications of RVO?
Macular edema is the most common complication and most frequent reason for vision loss in RVO. In severe vein occlusions, usually of the central vein variety, new blood vessels will begin to grow on top of the retinal tissue (neovascularization). The neovascularization can result in a vitreous hemorrhage (bleeding into the clear gel that fills the back of the eye) or retinal detachment (separation of the retina from the wall of the eye). Patients affected by a vitreous hemorrhage will usually notice a sudden increase in dark floaters. With a retinal detachment, a shadow or blind spot will present itself in a corner of the vision. Sometimes, the neovascularization occurs in the front of the eye (on the iris), causing the eye pressure to rise to a dangerously high level (neovascular glaucoma) and often resulting in the eye becoming red and painful. If untreated, neovascular glaucoma can lead to blindness.
What causes RVO?
The most common causes of RVO include:
- Diabetes mellitus
- Hypercoagulability (blood clots too easily)*
- Vasculitis (inflammation of the blood vessels)*
* More common in younger patients
How is RVO diagnosed?
The standard test to confirm the diagnosis is a fluorescein angiogram (FA). A dye called fluorescein is injected into a vein in the arm and a photograph is taken of the dye flowing through the blood vessels, allowing a retina specialist to verify the location and severity of the blocked vein. To detect and/or confirm macular edema, the standard test is Optical Coherence Tomography (OCT). OCT is completely non-invasive, and uses a weak laser to scan the macular tissue which allows for an accurate measurement of the thickness of the macula. Both of these tests are safely and quickly performed in the office.
What is the treatment for RVO?
Since there is no cure for either BRVO or CRVO, the focus of management is on the following:
- The evaluation and treatment of underlying risk factors (e.g., diabetes or hypertension), leading to improved treatment of systemic problems and prevention of heart attack and stroke.
- The evaluation and treatment of complications such as macular edema and neovascularization, leading to preservation of vision.
Laser photocoagulation: Laser has been shown to be effective in improving macular edema in BRVO but not CRVO. A usually painless laser beam is directed through the pupil toward the affected region of the macula. Complications are uncommon, but can include the creation of a blind spot (scotoma).
Anti-VEGF injections: In both BRVO and CRVO, abnormally high levels of vascular endothelial growth factor (VEGF) occur, promoting both macular edema and neovascularization. Anti-VEGF medications that counteract VEGF are injected directly into the vitreous through the sclera (white of the eye). The eye is numbed before each injection and the doctor closely monitors each patient's condition following the injection.
Two medications currently being used, Lucentis® (ranibizumab) and Avastin® (bevacizumab), have both been successful in reducing the edema and improving vision in both BRVO and CRVO. A third medication, Eylea® (aflibercept), is effective for treating edema secondary to CRVO.
A common treatment approach is to start with six monthly injections and continue as needed over the next several months. Complications are very uncommon, but can include a) infection (endophthalmitis) b) retinal detachment and c) vitreous hemorrhage, all of which are related to the injection itself and not specifically to the medications. Although only Lucentis and Eylea are approved by the FDA for treatment, Avastin is also commonly used in this manner, and all are thought to be effective and equally safe for the eye.
Steroids: Steroids are another type of medication that can decrease macular edema. Similar to Lucentis, Eylea and Avastin, the steroid is injected directly into the vitreous through the sclera. Two of the most commonly used steroids are preservative-free Triamcinolone Acetonide Suspension and Ozurdex (a dexamethasone implant). The primary difference between them is that the complete dose of triamcinolone is given at one time while small doses of dexamethasone are administered over time, although the treatment is not given more than once every three months. The complications from steroid injections are similar to those of the anti-VEGF injections, but can also include the progression of cataract and the development of glaucoma.
Laser photocoagulation: Laser is the best way to control and decrease neovascularization in either BRVO or CRVO. A stronger laser is applied to the affected region of the retina. Mild peripheral vision loss can sometimes occur.
If the retina specialist is unable to perform the laser treatment due to either a vitreous hemorrhage or retinal detachment, a vitrectomy will be recommended. A vitrectomy is an outpatient surgical procedure performed in an operating room that involves removal of the vitreous from the inside back of the eye through three openings made in the sclera using a microscope and several small instruments. As is the case with anti-VEGF or steroid injections, there is a risk of infection, cataract, retinal detachment or permanent vision loss.
As with any treatment, it is critical that patients have a thorough discussion with their retina specialist regarding all of the treatment options, including the risks, benefits and alternatives, before deciding the appropriate course of action.