Introduction
Retinal artery obstruction (RAO) generally occurs in patients in their sixth decade of life. This condition typically presents with sudden, painless monocular vision loss that may be preceded by episodes of amaurosis fugax in the involved eye. The central retinal artery is the most common site of obstruction, followed by the branch and cilioretinal arteries, respectively. The primary ophthalmoscopic feature in RAO is ischemic whitening of the inner half of the retina. In central retinal artery obstruction (CRAO), the whitening involves the entire posterior pole, usually with a cherry red spot in the foveola (Slide 1, Slide 2, Slide 3, Slide 4, and Slide 5). Two factors are responsible for the cherry red spot in the foveolar area: enhanced visibility of the underlying choroid due to the thinness of the retina and decreased ischemic whitening secondary to a partial blood supply from the choriocapillaris. In branch retinal artery occlusion (BRAO) (Slide 6, Slide 7, and Slide 8) and cilioretinal artery obstruction, the whitening is more focal, confined to the area of the retina supplied by the artery. A significant complication of RAO is ocular neovascularization, which is usually confined to cases of CRAO or combined RAO and central retinal vein occlusion (CRVO).
Pathophysiology
Gass has outlined six mechanisms of RAO:
- embolism
- vascular narrowing
- vascular narrowing from extravascular disease
- thrombosis
- arterial spasm
- hypoperfusion
Disorders commonly associated with these mechanisms include hypertension, diabetes, atherosclerosis, and diseased cardiac valves. In patients younger than 30, other disorders are more significant, including migraine, coagulation abnormalities, congenital cardiac defects, trauma, and hemoglobinopathies.
The most common cause of RAO is embolism from one of three types of emboli: cholesterol emboli, calcific emboli, or platelet-fibrin emboli. Cholesterol emboli are yellow and refractile and often transient, disappearing in several days. A common source is the carotid artery; other sources include the aortic arch and ophthalmic artery. Calcific emboli are white and nonrefractile and can remain visible indefinitely. These emboli are usually derived from damaged cardiac valves. Platelet-fibrin emboli are long, gray, and sausage shaped and are often lodged in bifurcations. Similar to cholesterol emboli, platelet-fibrin emboli have a propensity to dissociate and move downstream into peripheral arterioles. A visible retinal arterial embolus is an important prognostic marker. These patients have increased mortality rates (primarily due to cardiac disease) compared with age-matched controls. Significant sources of exogenous embolic RAO are injected corticosteroid suspensions and talc.
Central Retinal Artery Obstruction
The central retinal artery accounts for 58% of all cases of arterial obstruction. Patients with CRAO usually present with vision in the counting fingers to light perception range and a large relative afferent pupillary defect. Up to 25% of eyes will have better vision due to a patent cilioretinal artery supplying all or part of the papillomacular bundle.
While the diagnosis of CRAO can usually be established with a history and ophthalmoscopic examination, fluorescein angiography can provide confirming evidence. The most important angiographic feature of CRAO is delayed arteriovenous transit time (> 11 seconds) followed by delayed arterial filling. Patchy or delayed choroidal filling (> 5 seconds) that persists for a minute or longer can also have diagnostic significance. It points to the presence of ophthalmic artery obstruction that may account for up to 5% of cases of CRAO. Other characteristic findings of ophthalmic artery obstruction include no light perception vision, an absent cherry red spot, leakage of dye at the level of the retinal pigment epithelium (RPE) and, several weeks after the initial insult, the development of RPE changes in the posterior pole and periphery. If a patient presents with these findings and neurological signs such as contralateral hemiplegia, acute carotid artery obstruction also must be considered.
The management of CRAO in the acute phase is unclear. No randomized controlled clinical trials have established the efficacy of any treatment. Although there are reports of significant visual improvement after treatment of CRAO, these results may simply reflect the natural history of the disease. Another complicating factor is that many patients may present too late for treatment to be effective: primate studies have demonstrated that retinal infarction occurs after the central retinal artery has been completely obstructed for 1½ hours, though CRAO in humans rarely involves complete arterial obstruction. Despite these factors, most clinicians advocate treatment if the patient presents within 24 hours of the visual loss. Common treatments include ocular massage, anterior chamber paracentesis, intravenous acetazolamide, and an inhaled mixture of 95% oxygen/5% carbon dioxide. The use of vasodilators such as papaverine hydrochloride and tolazoline hydrochloride (administered systemically or by retrobulbar injection) or sublingual nitroglycerin has also been described. Recent reports have focused upon local intra-arterial fibrinolysis for CRAO; this technique involves the administration of a thrombolytic agent into the ophthalmic artery. Because of the potential for serious complications with this technique, randomized controlled trials will be required to elucidate its role in the management of CRAO.
Giant cell arteritis is the cause of 1% to 2% of cases of CRAO. An erythrocyte sedimentation rate should be drawn in all patients older than 55 years of age to screen for this disease. If giant cell arteritis is suspected, patients should be treated with an aggressive systemic corticosteroid regimen to protect the fellow eye.
Patients with CRAO should also undergo a physical evaluation, as systemic abnormalities can be uncovered in 90% of cases. This should include carotid artery duplex scanning to rule out ipsilateral carotid artery disease. Echocardiography in patients with significant cardioembolic risk factors may also be necessary. Younger patients may need a more extensive evaluation.
The visual prognosis in eyes with CRAO is usually guarded, though up to 35% may regain visual acuity of 20/100 or better. Eyes with CRAO must be monitored carefully for the development of ocular neovascularization: 18% will develop iris neovascularization and 3% disc neovascularization. Panretinal photocoagulation can induce regression of the iris neovascularization in two-thirds of cases.
Branch Retinal Artery Obstruction
Branch retinal artery obstruction accounts for 38% of all cases of arterial obstruction (Slides 6 through 8). Most cases involve the temporal vessels, though involvement of the nasal vessels may be underreported due to lack of symptoms. The visual prognosis is better than CRAO: 80% of eyes will regain visual acuity of 20/40 or better. There is no proven treatment for BRAO, though the work-up is essentially the same as for CRAO. A BRAO involving an arterial bifurcation is more likely to be embolic in nature. The risk of ocular neovascularization with BRAO is low, though it may be higher in diabetic patients.
A cilioretinal artery is present in 20% of eyes. Unlike the major retinal arterioles, the cilioretinal artery receives its blood supply from the choroidal circulation. Cilioretinal artery obstruction accounts for 5% of all arterial obstructions and, as described by Brown and colleagues, usually presents clinically in one of three situations: as an isolated arterial occlusion and in association with a CRVO or anterior ischemic optic neuropathy (AION). The association with CRVO may be related to a mismatch between a low perfusion pressure in the cilioretinal artery and an increased hydrostatic pressure in the occluded venous system. Posterior ciliary artery insufficiency may link cilioretinal artery obstruction and AION.
Although there is no proven treatment for cilioretinal artery obstruction, the management of these patients may differ depending upon the clinical presentation. Cases of isolated cilioretinal artery obstruction should receive the same systemic evaluation as patients with CRAO or BRAO. If AION is present, giant cell arteritis must be ruled out. Cases with concomitant CRVO (see below) should be managed according to the recommendations of the Central Vein Occlusion Study.
The isolated cilioretinal artery obstruction has the best visual prognosis, with 90% of eyes obtaining 20/40 or better visual acuity; cases with AION have the poorest visual prognosis, with most eyes ending up worse than 20/200.
Combined BRAO and CRVO
Duker and colleagues have divided cases of combined BRAO and CRVO into two groups: younger patients without systemic disease and older patients in the vasculopathic age range.
In the younger age group, where patients often present with prominent disc edema and relative afferent pupillary defects, inflammation has been proposed as the primary etiological factor. Mechanisms include primary optic nerve inflammation with secondary compression of the retinal vasculature or a primary vasculitis that impacts both the optic nerve and retinal vasculature. While there is no proven treatment available, these eyes have a good visual prognosis, with most regaining visual acuity of better than 20/40.
In the older age group, generalized atherosclerotic disease is considered the primary risk factor. This could lead to primary venous obstruction with secondary disc edema and retinal arterial compression. Alternatively, a primary branch arterial obstruction of sufficient magnitude could lead to secondary venous stasis and thrombosis. The management of these patients includes careful observation for ocular neovascularization and prompt PRP per the CVOS guidelines. Eyes with ischemia and ocular neovascularization generally have poor visual prognosis; most are left with visual acuity less than 20/400.
Combined CRAO and CRVO
Patients with combined CRAO and CRVO usually present with vision in the hand-motions range and ophthalmoscopic features of both entities. Retrobulbar injection may account for up to 25% of cases. Because more than 80% of these eyes will develop anterior segment neovascularization and neovascular glaucoma, early panretinal photocoagulation has been recommended by some investigators.
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