Vascular occlusions (CRVO, BRVO)

The retina is the innermost layer lining the inside of the eye. The retina behaves as the film in a camera: when the light hits the retina it excites some specialized cells so-called photoreceptors; this excitation generates one message in our brains.

To function well, these cells require a very high level of oxygen and nutrients. The retina is probably the most oxygen demanding tissue of your whole body. The required oxygen is brought to the retina by the retinal blood vessels. These divide into a fine capillary network when they reach the retina, but each network only supplies a very small area of the retina. There is no other source of oxygen or nutrients than this specific set of capillaries.

Arteries carry blood with oxygen and nutrients to the capillaries and veins carry deoxygenated blood away from the capillaries, after the cells have consumed the oxygen and nutrients.

As occurs in other larger vessels elsewhere in the body, the arteries and veins in the retina are susceptible to clogging and when this happen, the normal circulation is interrupted and the retina can not function properly any longer. If nothing is done, the part of the retina not receiving oxygen and nutrients can die.

If this is your case, you might notice that your vision may be blurry to a varying degree, but the vision drop is usually not associated with pain, at least not initially.

Risk factors for retinal vascular occlusions are the same as for other vascular diseases such as stroke or heart attack: advanced age, smoking, hypertension, atherosclerosis and diabetes are some of the conditions that are most frequently related to an increased risk.

Avoiding these risk factors by avoiding smoking and exercise regularly can be good preventive measures.

Arterial occlusion

When the blockage affects an artery (carrying oxygenated blood and nutrients), it will cause ischemia (lack of oxygen) of the retinal area irrigated by the blocked artery. If the obstruction is not resolved soon, it will cause the death (infarction) of the affected area, and this is an irreversible loss.

Treatment options for this condition are limited, and unless something is done in the first few hours, the prognosis to recover your vision is very poor. This is an emergency and you should see an EyeMD (ophthalmologist) as soon as possible because your retinal damage will be reversible during the first 12 to 24 hours after the occlusion took place!.

If the blockage can be seen when you are examined, an operation can restore your sight if it is done early enough. After the acute episode has passed, your EyeMD and/or your GP should try to find out if there was a specific cause responsible for the arterial occlusion because finding this cause is the only way to prevent it from happening again in the retina or elsewhere (in your brain for example causing a stroke).

Venous occlusion

When the blockage affects a vein (carrying deoxygenated blood and metabolic wastes), it will hamper the normal venous drainage and increase the blood pressure above the occlusion. This increased pressure might cause bleeding (if the vein breaks) or edema (leakage of fluid).

Unlike arterial blockages, a vein obstruction may cause ischemia (lack of oxygen) of the surrounding tissues, or it may not. Nevertheless, as in the arterial occlusions, the retinal damage depends on the location of the occlusion: a central retinal vein occlusion (CRVO) affects the normal venous drainage of the entire retina while a branch vein occlusion (BRVO) affects only the portion of the retina that is drained by one particular branch.

The consequences of BRVO are similar to those seen in CRVO but tend to be less severe because a portion of the retina maintains its normal venous drainage.

The Eye Exam

During your visit, the ophthalmologist will perform a complete eye exam. After dilating the pupil, he will be able to look for the characteristic signs of the occlusion either arterial or venous, and determine how much damage has already taken place. To help him decide, he will probably do some additional examinations including:

• OCT (Optical Coherence Tomography): to assess the Macular Edema.
• FA (Fluorescein Angiography): to assess the retinal ischemia, the existence of neovascularization and the loss of fluid from the abnormal vessels.
• Microperimetry of the macula to determine the function of the retina.

Treating vascular occlusions

There is currently no treatment to prevent this condition other than treating all the risks factors mentioned earlier. However, we have medications that, when used soon enough will prevent complications and preserve your vision.

The presence of macular edema can compromise visual recovery but the most feared complication of vascular occlusions occurs when abnormal vessels (neovascularization) grow on the retina or in the front chamber of the eye. These abnormal vessels are very weak and prone to bleed inside the eye being responsible of eye diseases such as glaucoma or retinal detachment.

For the treatment of these complications we have nowadays the following alternatives:

• Laser therapy
• Intraocular injections of medications anti-VEGF or corticosteroids. The anti-VEGF (anti-vascular endothelial growth factor) such as Lucentis ®, Avastin ® or Eyelea ® are able to inhibit the neovascularisation. The corticosteroids such as Ozurdex ® are useful in cases of macular edema.
• Surgery may be necessary in some advanced cases with retinal fibrosis, retinal detachment, vitreous hemorrhage or neovascular glaucoma.