Central Retinal Artery Occlusion (CRAO)

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CENTRAL RETINAL ARTERY OCCLUSION (CRAO)
Overview

A central retinal artery occlusion (CRAO) occurs suddenly and is an ophthalmic emergency that requires immediate evaluation. It is a sudden obstruction of retinal blood flow usually due to an embolus which causes occlusion of the central retinal artery which supplies blood and oxygen to the retina. This occlusion often leads to complete (or almost complete) visual loss in the affected eye.

A CRAO may also occur secondary to inflammation and thickening of the artery (giant cell arteritis), thrombus formation and embolization, vasculitis causing retinal vasculature inflammation, traumatic vessel wall damage, or spasm. The lack of oxygen delivery to the retina during the blockage often results in severe vision loss in the area of ischemic retina and permanent visual loss within 90 minutes of the onset of the occlusion to normal blood flow.

A CRAO originates in either or both the ophthalmic and/or central retinal arteries which are the primary blood vessels supplying the eye. The main artery that supplies the eye and surrounding structures is the ophthalmic artery (a branch of the carotid artery). The central retinal artery is the first branch of the ophthalmic artery, and it supplies nerve fibers in the optic nerve as well as the inner layers of the retina.

After entering the eye through the optic nerve, the central retinal artery divides into superior and inferior branch retinal arteries. In 85-90% of patients the central retinal artery supplies all of the blood to the retina.

The cilioretinal artery is found in 10-15% of patients. This is an extra artery that brings blood from a branch of the short posterior ciliary arteries and the choroid (blood vessels below the retina) to the edge of the optic nerve and supplies some portion of the center of the retina (macula). The blood flow through the cilioretinal artery remains intact in patients with CRAO and becomes the only source of oxygen to the retina.

The only patients with a CRAO that retain good central vision is if the patient has a cilioretinal artery supplying blood and oxygen to the center of the retina (macula).

Symptoms

The development of a Central Retinal Artery Occlusion (CRAO) causes symptoms such as sudden, painless, vision loss that occurs over seconds. Visual acuity may vary depending on whether the ophthalmic artery (more severe) or the central retinal artery (slightly less severe) is obstructed or if a cilioretinal artery is present (only chance of preservation of some central vision).

Visual loss may have been preceded by transient loss of vision in the past (amaurosis fugax) in the case of embolic sources.

Complete vision loss to no light perception should raise suspicion of an ophthalmic artery occlusion.

Patients with central retinal artery occlusion (CRAO) typically retain light perception or hand motions vision and complain of visual loss over the entire field of vision.

Risk Factors

The risk factors and demographics of retinal artery occlusion are similar to systemic stroke and include several modifiable risk factors:

Older age
Smoking
Hypertension
Obesity
Diabetes
Hyperlipidemia
Cardiovascular disease
Coagulopathy
Male gender

Physical Exam

Relative afferent pupillary defect (may be present in central retinal artery occlusion or ophthalmic artery occlusion).
Decreased brightness acuity
Hollenhorst plaque (a cholesterol plaque that breaks off from the heart, aorta, subclavian or carotid arteries and flows to and blocks the central retinal artery. The plaque is usually visualized at the optic nerve head.
Cherry red spot (retina swelling) - due to edema (swelling) of the nerve fiber layer (inner retina) as it becomes ischemic. This finding may take hours to develop, and the edema is associated with a worse visual prognosis. Over the course of about a month, the inner retina becomes atrophic as the swelling resolves.
Boxcarring of retinal vessels - segmental and sluggish blood flow through retinal arteries.
Ophthaldynomometry - poor perfusion of the arterial tree can be demonstrated by the ability to induce retinal pulsations in the central retinal artery by digital pressure on the eyeball.

Testing

Optical coherence tomography (OCT) reveals hyper-reflectivity of the inner retina in acute stages. The amount of retinal edema is related to visual prognosis
Fluorescein angiography (FA)
- Delay in the filling of the retinal arteries and a delayed arteriovenous transit time in the affected areas.
- Flow of blood in retinal arteries is very sluggish. Fluorescein is noted to travel very slowly to the peripheral retina along the branches of retinal arteries.
- Delayed choroidal filling should point to an ophthalmic or carotid artery obstruction.
- Over time, the vessels re-open and flow reverts to normal, despite the persistence of retinal vessel narrowing.
Electroretinography (ERG)
- Shows a characteristic diminution of the b-wave. This is due to inner retinal ischemia. The ERG may be normal in some cases (despite poor visual acuity) if the blood flow renormalizes.

Laboratory Tests

The following blood test should be obtained for patients older than 50 presenting with a CRAO:

Erythrocyte Sedimentation Rate (ESR)
C-reactive protein (CRP)
Complete Blood Count (CBC) with platelets should be obtained in patients over the age of 50 who have symptoms of giant cell arteritis.

Patients younger than 50 should have a hypercoagulable workup including:

Anti-Phospholipid Antibody Syndrome
Inflammatory disorders
Hypercoagulable states

In older individuals, atherosclerosis and emboli are possible causes of the ischemia and the workup includes:

Echocardiogram - should be performed to determine cardiac function and abnormalities of the valves.
Electrocardiograms (EKG) - may reveal a rhythm defect.
Carotid Doppler - Cartotid artery stenosis should be evaluated with carotid ultrasound looking for calcification or plaque in the carotid artery.

Medical Therapy

It is critical to initiate intervention within 90 minutes of onset of visual loss in patients with CRAO. The treatment includes the following:

Anterior Chamber Paracentesis (removing fluid from the eye to lower the intraocular pressure). Theoretically, the paracentesis lowers the intraocular pressure and may allow the embolus to move further down the vessel and away from the central retina.
Digital massage of the eye (to increase and rapidly decrease the eye pressure to try to move the embolus from the area of occlusion). Theoretically, the digital massage lowers the intraocular pressure and may allow the embolus to move further down the vessel and away from the central retina.
Pressure Lowering Eyedrops (to lower the intraocular pressure and may allow the embolus to move further down the vessel and away from the central retina.
Breathing into a paper bag - Increasing carbon dioxide concentration in the blood has also been proposed to induce vasodilation. Dilation of the retinal arteries may allow the embolus to move further down the vessel and away from the central retina.
Hyperbaric Oxygen - Patient may be given an oxygen mask to try to increase oxygen perfusion through the choroidal circulation. A mixture of 95% oxygen and 5% carbon dioxide (Hyperbaric treatment) has also been proposed to increase blood flow.

If there is a suspicion of Giant Cell Arteritis (aka Temporal Arteritis) the treatment includes the following:

Corticosteroids
Laboratory Testing - including a Erythrocyte Sedimentation Rate (ESR), C-Reactive Protein (CRP) and a CBC looking for low platelet count. Sedimentation rate along with C-reactive protein have a 15% false negative rate as well. This means that 15% of blood tests will be negative even if the patient actually has GCA.
Temporal artery biopsy - Temporal artery biopsies although important, have an 85% false negative rate, This means that 85% of biopsies will be negative even if the patient actually has GCA.

Complications

Abnormal blood vessel growth (neovascularization) of the iris, retina or angle are common complications after central retinal artery occlusion. This can raise the intraocular pressure and cause pain and further vision loss in the affected eye. Findings of neovascularization may be delayed in patients treated with hyperbaric oxygen. This may be treated with anti-VEGF agents or may require surgery if it progresses to vitreous hemorrhage or uncontrolled glaucoma.