Central retinal artery occlusion refers to the obstruction of one of the main vessels supplying the retina, generally due to thromboembolism. It is associated with severe, often permanent unilateral visual impairment.
The most common symptom of CRAO is a sudden, severe, unilateral visual impairment. The majority of patients presents a visual acuity of 20/400 or less . Some patients may merely have problems to count fingers, while light perception in general may be severely reduced in others. CRAO is painless.
It is not uncommon for CRAO patients to report recent episodes of amaurosis fugax, a condition that does not necessarily prompt patients to seek medical attention since symptoms may remit spontaneously within seconds or minutes. However, amaurosis fugax may indicate serious problems with retinal blood flow. Differential diagnoses for amaurosis fugax are thromboembolism, hypercoagulability, arteritis, migraine and vasospasm . The majority of these pathologies predisposes for CRAO.
Additionally, symptoms triggered by the underlying disease may be present.
Clinical presentation and severe reduction of visual acuity is highly suspicious for CRAO and should prompt a thorough ophthalmologic examination. CRAO patients often show a relative afferent pupillary defect, i.e., light perception is diminished in the affected eye but unaltered on the contralateral side and consequently, pupil constriction can only be induced by illumination of the healthy eye. With regards to the ocular fundus, a cherry-red spot can usually be observed in an otherwise pale, ground-glass retina. Of note, retinal alterations may only become visible hours after symptom onset and may resolve within a few days. In some cases, the causative embolus may be visible as a Hollenhorst plaque .
It is important to identify the underlying disease. In order to do so, laboratory analysis of blood samples should be carried out as well as coagulation tests, measurement of blood pressure and evaluation of cardiac function. With regards to the former, hyperglycemia, hypercholesterolemia, hemolytic anemia, polycythemia and other hematological disorders may be detected. Inflammatory parameters are generally elevated in patients suffering from arteritis. Blood cultures may be required to identify the causative agent in cases of endocarditis. Echocardiography and angiography are the methods of choice to detect cardiovascular pathologies.
There are no guidelines regarding treatment of CRAO and most recommendations given are based on empiric evidence collected in ophthalmology as well as stroke research.
In this context, administration of tissue plasminogen activator within the first four hours of symptom onset may prove beneficial  . If thrombolysis can be achieved, retinal blood flow may be reestablished before permanent damage to retinal cells occurs. Both intravenous and intra-arterial thrombolysis should be considered.
Pentoxifylline, isosorbide dinitrate as well as carbogen and hyperbaric oxygen may contribute to restoration of retinal blood flow by means of improved rheological properties of the blood, vasodilation and increased oxygen provision.
Prognosis worsens significantly as time passes after symptom onset. The above mentioned results regarding retinal survival time as well as data collected in stroke research suggest an unfavorable prognosis for recovery of eye sight if more than four hours have passed. However, even shorter periods of retinal ischemia often cause extensive and severe visual impairment due to irreversible retinal damage.
As has been indicated above, virtually all cases of CRAO result from thromboembolism. Thus, any condition predisposing for thrombus formation and subsequent embolism may be considered a risk factor for CRAO. According to current knowledge, neither the time nor the localization of vessel occlusion due to displacement of thrombi can be predicted. Consequently, patients at risk of CRAO are also at risk of ischemic heart disease, cerebral infarction or ischemic stroke, and vice versa.
In this context, a large number of predisposing factors has been identified:
Local or systemic vasculitis does increase the risk for thrombus formation within the central retinal artery. Pathologies that may be considered here are polyarteritis nodosa and temporal arteritis / giant cell arteritis.
Finally, iatrogen CRAO has been reported. Occlusion of retinal arteries has been related to drug administration in order to treat comorbidities and to cosmetic facial injections .
The overall incidence of CRAO has been estimated to be about 1 per 100,000 inhabitants.
CRAO is mostly diagnosed in the elderly and this age distribution is presumably due to high prevalence rates of obesity, diabetes mellitus and hypertension among people older than 50 years. Additionally, it is not uncommon to observe CRAO in patients with a medical history of ischemic heart disease or ischemic stroke. However, CRAO may also be diagnosed in pediatric patients presenting neither of the aforementioned risk factors .
The retina is a very complex organ that consists of many different cell types arranged in multiple distinct layers. Put in highly simplified terms, retinal ganglion cells, amacrine cells, bipolar cells, photoreceptors and pigment epithelium can be encountered in this order when passing through the retina from its inner layers to the outer strata. Single layers are partially separated by limiting membranes.
All retinal layers are supplied by branches of the ophthalmic artery that, in turn, originates from the internal carotid artery. After entering the orbita through the optic canal, it releases the central retinal artery and various, very small ciliary arteries. The former supplies the outer layers of the retina, mainly its photoreceptors, while retinal ganglion cells and underlying strata receive oxygen and nutrients from ciliary arteries. Thus, CRAO does not interfere with metabolism of inner retinal layers, but does interrupt blood supply to photoreceptors and adjacent tissues.
The central retinal artery eventually divides into four end branches, the superior and inferior nasal and temporal branches, respectively. Therefore, proximal CRAO will cause ischemia in all photoreceptors of the affected eye. In contrast, distal occlusion of end branches of the central retinal artery or obstruction of any one of the ciliary arteries disrupts blood supply to a restricted part of photoreceptors or retinal ganglion cells, respectively, and causes less detrimental effects than proximal CRAO. In no case, occlusion of end branches can be compensated by collateral blood flow before permanent damage to retinal cells occurs.
The fact that most CRAO patients claim complete or almost entire loss of vision argues for rather proximal occlusion of the central retinal artery. This vessel's lumen is constricted at certain anatomical sites, e.g., at the penetration point into the optic nerve. It seems plausible that thrombi are frequently captured here.
Although CRAO is usually associated with permanent visual impairment due to ischemic retinal cell death, a thrombus lodged in the central retinal artery may crumble, be partially or totally removed with blood flow. In these cases, patients may claim transient visual impairment, amaurosis fugax. Because it cannot be ruled out that the artery is still partially blocked - a condition that greatly facilitates renewed thromboembolism - this symptom should be taken very seriously.
There is no precise duration of CRAO that guarantees full recovery or that surely leads to permanent retinal damage. According to studies conducted in atherosclerotic and hypertensive rhesus monkeys, ischemia for less than two hours is generally well tolerated while arterial occlusion for more than four hours results detrimental . These results imply a very short time window for diagnosis and initiation of treatment.
In general, any measure aiming at losing overweight, reducing hypertension, hypercholesterolemia and hyperglycemia may also contribute to lower the risk of CRAO. Unfortunately, certain risk factors are beyond the control of patient or physician and cannot be prevented by means of specific measures.
However, if a patient presents with CRAO, timely identification and adequate treatment of the underlying disease may largely contribute to reducing the risk of contralateral CRAO, ischemic heart disease and cerebral infarction.
Central retinal artery occlusion (CRAO) refers to the obstruction of the vessel supplying the inner layers of the retina, the central retinal artery. CRAO may be considered a type of ischemic stroke and indeed, etiology and pathogenesis of this disease largely resemble those of cerebrovascular accidents like middle cerebral artery occlusion. Both ocular and cerebral ischemic strokes are painless, reduce or interrupt blood flow to dependent tissues, and functional failure of these cells occurs within very short periods of time, usually within minutes or a few hours maximum. Therefore, CRAO is an ophthalmic emergency.
CRAO is caused by embolic artery occlusion and while distinct types of embolism may account for this disease, thromboembolism is the direct cause of CRAO in virtually all cases. Only few case reports exist regarding other sources of emboli, e.g., fat embolism . CRAO is associated with sudden, severe, unilateral visual impairment.
Unfortunately, CRAO and cerebral infarction also share lack of treatment options. Recombinant tissue plasminogen activator has long since been the treatment of choice for cerebral infarction if patients receive medical attention within a few hours after symptom onset. This thrombolytic agent may also be administered to CRAO patients and similar to ischemic stroke, improvement of prognosis seems to depend on early administration of the drug . Of note, controlled clinical trials regarding the use of tissue plasminogen activator to treat CRAO have not yet been conducted and are urgently required .
Additionally, pentoxifylline, vasodilators, diuretics, corticosteroids or hyperbaric oxygen are frequently used to improve blood supply to the retina, to reduce intraocular pressure and to avoid concurrent edema, but scientific evidence for their efficacy is scarce.
The retina coats the back of the eye and contains photoreceptors, neurons and supportive cells that allow perception of light and are therefore indispensable for vision. The retina is a very complex organ consisting of various layers that are differentially supplied with oxygen and nutrients: Only photoreceptors and cell in close proximity depend on central retinal artery blood flow.
If this vessel is obstructed, central retinal artery occlusion (CRAO) occurs. Etiology, pathophysiology and treatment largely resemble those of cerebral infarction - a very similar condition that does affect other vessels, though.
Virtually all cases of CRAO are caused by thromboembolism, i.e., a thrombus forms in any part of the cardiovascular system and is carried to the small vessels supplying the retina, lodge here and interrupt blood flow. If this condition persists for more than a few hours, irreversible damage to the retina occurs and symptoms become permanent.
Obese patients as well as those diagnosed with hypertension, diabetes mellitus, atherosclerosis, endocarditis, coagulopathies or vasculitis have increased risks of thromboembolism and thus of CRAO, ischemic heart disease and ischemic stroke.
Some patients report having experienced periods of transient loss of vision, a condition referred to as amaurosis fugax. Here, vision returns within a few minutes. While amaurosis fugax does not leave any sequelae, it indicates problems with retinal blood flow and should be taken seriously. Amaurosis fugax results from embolic occlusion of the retinal artery and subsequent displacement of the embolus. It is likely, though, that the source of thromboembolism persists and another thrombus may easily be carried to the retinal artery and may cause CRAO.
Patients presenting with symptoms characteristic for CRAO are usually submitted to a thorough ophthalmologic examination. In some cases, the physician may be able to visualize the embolus. Typical alterations of the ocular fundus will support their tentative diagnosis.
Additional diagnostic measures will be undertaken to identify the underlying disease. Such measures may consist in laboratory analysis of blood samples, coagulation tests and diagnostic imaging to assess cardiac function.
CRAO is an ophthalmologic emergency and immediate treatment with thrombolytic agents is required to improve the chance for recovery. Additional drug therapy is often recommended, but scientific evidence regarding its efficacy is scarce: Vasodilators may facilitate blood flow to the retina and reduction of intraocular pressure should reduce physical resistance against retinal blood supply.