Ocular Manifestations of Subclavian Steal Syndrome
Subclavian steal syndrome (SSS) occurs when proximal subclavian artery stenosis or occlusion leads to reversal of flow in the ipsilateral vertebral artery. Intracranially, the bilateral vertebral arteries join to form the basilar artery and terminates in the right and left posterior cerebral arteries (PCA). The PCA form the posterior part of the circle of Willis and are joined to the anterior circulation via the posterior communicating arteries. Blood flow in the ipsilateral vertebral artery can be reversed if demand increases in the subclavian artery distal to the occlusion. For example, during periods of upper extremity use or exertion, patients can experience symptoms of arm and/or cerebral ischemia including reduced blood flow to the occipital cortex or the eye. In addition, some cases after a coronary artery bypass graft can experience anginal chest pain upon exertion, termed coronary-subclavian steal. As many patients with SSS can be asymptomatic it is likely that SSS is more common than previously thought due to screening of asymptomatic patients with ultrasound and magnetic resonance angiography.
SSS may be caused by stenosis or occlusion of one or both subclavian arteries. Atherosclerosis is the most common cause.5 Other causes however include Takayasu arteritis, inflammation secondary to radiation, compression syndromes, fibromuscular dysplasia, neurofibromatosis, and a congenital steal involving a right-sided aortic arch.
Either the right or left subclavian artery may be affected, but stenosis of the left subclavian artery is four times more common than stenosis of the right, as it was found in one study to be left sided in 82.3% of subjects. Bilateral subclavian stenosis was seen in 13% of patients in one study. Of patients with SSS, 81% had atherosclerotic disease in more than one vessel feeding the brain.
The risk factors for SSS are the same as the risk factors for atherosclerosis, which include vasculopathic risk factors (e.g., hyperlipidemia, hypertension, diabetes mellitus, increasing age, smoking, and family history of cardiovascular disease).
Stenosis or occlusion of the proximal subclavian artery impedes blood flow distal to the occlusion. The takeoff for the ipsilateral vertebral artery arises from the subclavian artery and is distal to the occlusion in SSS. A drop in blood pressure or blood flow distal to the occlusion draws blood flow from the vertebral artery in a retrograde manner. The blood flow from the contralateral vertebral artery also may reverse across the union of the vertebral arteries at the basilar artery. Many cases of SSS are asymptomatic. Symptoms tend to occur in the setting of atherosclerosis of other intracranial vessels, particularly upon exertion. Exertion of the arm causes increased steal from the brain and may result in arm claudication or symptoms of cerebral ischemia.
There is also a coronary-subclavian steal syndrome, which occurs in the setting of a patient with severe atherosclerotic disease who has undergone a coronary artery bypass graft that uses the internal mammary artery. In this case, stenosis/occlusion of the subclavian artery causes retrograde flow of blood in the internal mammary artery to the subclavian artery, resulting in angina pectoris.
Prevention of SSS includes control of vasculopathic risk factors (e.g., hypertension, diabetes, and hyperlipidemia, and smoking cessation). Antiplatelet therapy (e.g., aspirin or clopidogrel), lipid-lowering therapy (e.g., statin) blood pressure control and blood glucose control are recommended in SSS.
A blood pressure difference between the two arms > 20 mmHg may be present in patients who have SSS.4 In one study, a blood pressure difference > 50 mm Hg was associated with symptoms of SSS in 38.5% of patients. Screening can be done with color Doppler or transcranial Doppler ultrasound as it is non-invasive and may be performed at bedside. Ultrasound may show stenosis of many vessels at once and transcranial Doppler can indicate flow changes in posterior circulation. CT or MR angiography may be performed as well.
The first documented case of SSS was published in 1960 in a patient with an absent radial pulse in whom retrograde flow was observed via angiography. In 1961, a case report described two patients who experienced symptoms of cerebral ischemia secondary to stenosis of the subclavian artery who had subsequent retrograde flow in the ipsilateral vertebral artery. In 1961, Fisher then coined the term “subclavian steal.”
Many patients may be asymptomatic due to collateral circulation via the ipsilateral vertebral artery. Patients most likely to experience symptoms will have concomitant artery disease involving intracranial or extracranial vessels.
A thorough physical examination may include auscultation of the subclavian, carotid, and vertebral (suboccipital region) arteries as well as palpation of the carotid, radial, and ulnar pulses. Blood pressure may be measured in each arm. Examination of the skin for ulcers, gangrene, and nail bed splinter hemorrhages may be considered.
Findings on physical exam include cervical or supraclavicular systolic bruits, including bruits over the subclavian artery. Blood pressure measurements in both arms may show a lower pressure reading in the arm ipsilateral to the stenosis. Radial and ulnar pulses in the arm ipsilateral to the stenosis may be diminished. Skin changes may be present secondary to atheroemboli, such as cyanotic digits, livedo reticularis, ulcerations, and nail bed splinter hemorrhages.
Various ophthalmic signs have been reported in the literature. One patient with left sided SSS had blood pressure of 140/80 mmHg in the right arm and 95/75 mmHg in the left. This patient had a retinal arterial pressure of 120/20 gm in the right and 30/20 gm in the left eye, and had experienced episodes of blurred vision for 5 minutes at a time, 2-3 times per day for 3 weeks.
Ophthalmic findings in another patient with a left sided SSS included a Hollenhorst plaque in the inferior temporal arcade in the eye ipsilateral to the subclavian stenosis initially. With time, this patient developed a Hollenhorst plaque in the contralateral eye as well. This patient had a systolic blood pressure differential of 44 mmHg between the two arms, and was found to have retrograde flow in the ipsilateral (left) vertebral artery with an occlusion of the left subclavian artery. The patient also had plaques of the carotid arteries at the bifurcation and the proximal internal carotids bilaterally. Upon initial visit when the Hollenhorst plaque in the left eye was identified, his vision was 20/25-3 OD and 20/25-2 OS, and he had age related macular degeneration that he had been monitoring with a home Amsler grid. Patients with SSS may have other markers of more diffuse atherosclerotic large vessel disease including Hollenhorst plaques or platelet-fibrin plaques. Takayasu arteritis can produce SSS. Interestingly, patients with SSS may have retinal findings (e.g., retinal hemorrhage, multiple petechiae, and microaneurysms) and may report unilateral or bilateral visual loss.
In some cases, patients fail to appreciate the homonymous nature of their visual loss and report the visual loss as monocular when really bilateral. In one patient, an innominate steal syndrome was identified due to an inter-eye difference of the ocular pulse amplitude (OPA). Before the stenotic innominate artery was revascularized, the OPA of the right eye was 2.00 (+/- .49) mm Hg and of the left eye was 3.46 (+/- .53) mm Hg. After revascularization, the OPA of the right eye was 3.26 (± 0.51) mm Hg the left eye had an OPA of 3.25 (± 0.99) mm Hg. This patient had symmetrical intraocular pressure measurements within normal limits and slit lamp exam showed no abnormalities before treatment. Blood pressure was 70/40 mm Hg in the right arm and 120/80 mm Hg in the left. Stenosis was found at the proximal brachiocephalic trunk. Innominate steal syndrome is related to subclavian steal syndrome as a stenosis causes stealing of blood from the contralateral circulatory system, and can manifest with ocular symptoms and with vertebrobasilar insufficiency. The decreased OPA in the right eye indicated less blood flow to the right eye.
SSS was found in another patient who experienced transient blindness diagnosed as incomplete central retinal artery occlusion. The transient blindness was attributed to ophthalmic artery spasms resulting in occlusion of the short posterior ciliary artery. During the episode of transient blindness, the patient had reduced visual acuity and visual field defects. On exam, there was a mild cherry red spot on the macula, and mild retinal edema. One day after the initial exam and after administration of vasodilator and antiplatelet agent, the visual acuity improved to 20/20.
A patient with a congenital right sided aortic arch with isolation of the left subclavian artery had subclavian steal that resulted in ocular nerve palsies. She had a history of progressive horizontal diplopia with rightward gaze, a palsy of the right cranial nerve VI. She also had a palsy of right cranial nerve V. Right arm blood pressure was 110/74 mm Hg and left arm blood pressure was 86/60 mm Hg, and the patient was diagnosed with left subclavian steal on angiography.
Vertebrobasilar hypoperfusion can result in visual disturbances, dizziness, syncope, limb paresis, intermittent claudication, paresthesias, ataxia, vertigo, dysphagia, dysarthria, and facial sensory deficits.Visual disturbances may include an oculomotor nerve palsy, which may result in diplopia, visual field deficits, as well as monocular or binocular visual changes. Patients who have undergone internal mammary artery (IMA) grafts experience angina pectoris primarily, secondary to coronary-subclavian steal syndrome.
A correlation between blood pressure differential in the arms and symptom occurrence has been shown. Patients with blood pressure difference greater than or equal to 50 mmHg were reported to be more likely to experience symptoms than patients with a smaller blood pressure differential. In patients with > 50 mmHg, 38.5% of patients experienced symptoms, compared to 1.38% of patients with a blood pressure differential of 20-30 mmHg. Symptoms of ischemia are likely due to inadequate collateral circulation due to disease affecting multiple arteries, resulting in decreased perfusion of brain, arm, or heart.
The severity of SSS is classified into three grades based on the severity of hemodynamic changes in the vertebral artery.
Grade I, pre-subclavian steal: reduced anterograde vertebral flow
Grade II: alternating flow, anterograde flow in the diastolic phase and retrograde flow in the systolic phase
Grade III: permanent retrograde vertebral flow
The grade of steal correlates with the blood pressure differentials. Blood pressure differential > 40 mmHg is more commonly seen with Grade II and III steal. Blood pressure differential < 40 mmHg was associated with no steal observed on ultrasound.
A lipid panel may be ordered to monitor LDL. Diabetes Mellitus may be monitored via hemoglobin A1C.
In a patient with signs of cerebral ischemia, the differential diagnosis can include orthostatic hypotension, cardiac arrhythmia, anemia, seizure, TIA, and subclavian steal syndrome. Other causes include embolism, atherosclerosis, arterial dissection, migraine, fibromuscular dysplasia, coagulopathies, and drug abuse.
Arm claudication is most commonly due to peripheral artery disease and thus atherosclerosis, but can be attributed to atheroembolism, thromboembolism, thoracic outlet syndrome, arterial injury, arterial dissection, fibromuscular dysplasia, thrombosed aneurysm, radiation induced vascular disease, Raynaud’s phenomenon, occupational/sports injury, recreational drug abuse, and medication side effects. Congenital defects of the arterial wall such as pseudoxanthoma elasticum and Ehlers-Danlos syndrome may also cause upper extremity ischemia. Large and small vessel vasculitis can cause upper extremity ischemia, such as Takayasu arteritis, giant cell arteritis, and thromboangiitis obliterans. Connective tissue diseases that can cause arm ischemia include rheumatoid arthritis, systemic lupus erythematosus, scleroderma, CREST syndrome, Sjogren syndrome, polymyositis/dermatomyositis and mixed connective tissue disease.
Medical management may be sufficient in some carefully chosen asymptomatic patients. In patients experiencing symptoms of ischemia and claudication, surgery or endovascular intervention may be considered.
Medical therapy for patients with SSS primarily focuses on controlling hypertension, diabetes, and hyperlipidemia, as well as smoking cessation and antiplatelet medications.
Medical follow up
Patients receiving medical management may be monitored with ultrasound in out-patient clinics.
Surgical options include: axillary–axillary bypass, carotid–subclavian bypass, and transposition of the subclavian artery.
Percutaneous transluminal angioplasty was originally used to treat subclavian steal. Currently, endovascular stenting is more commonly performed. Endovascular stenting of the subclavian artery has a patency rate of 83-89% at 5 years. Endovascular stenting has overall lower rates of complications than surgery, making it a popular treatment option for symptomatic patients.
Surgical follow up
After stenting, the patient may receive dual anti-platelet therapy for one month, continuing to take baby aspirin indefinitely if they also have coronary artery disease. Stent thrombosis is rare, likely benefited by the large size of the subclavian artery.
Surgical repair has a mortality rate of 0.5% and stroke rates of less than 3.8%. Carotid-subclavian bypass surgery has a mortality rate of 0.8% and complication rates of 8%. Subclavian transposition has a 30 day mortality rate of 1.2%. Stenting has a stroke rate of 0.6-1%, low mortality rate, and minor complication rate of 4.5-5.3%.24,25 Minor complications as listed in the study include dissection, transient ischemic attack, distal emboli, thrombosis, bleeding, and access site issues.
The treatment options for SSS have high patency rates, meaning that most patients will not have to undergo a second surgery. Endovascular options have lower patency rates than surgical correction, but are associated with fewer complications and thus are preferred to surgery. Endovascular stenting has a patency rate of 83-89% at 5 years. In contrast, surgical repair has been shown to have patency rates of 91% at 5 years and 82% at 10 years. Axillary-axillary bypass surgery has a patency rate of 89% at 10 years. Carotid-subclavian bypass surgery has a patency rate of 95% at 5 years and 83% at 10 years. Subclavian transposition has a patency rate of 98% at 5 years. Percutaneous transluminal angioplasty has a restenosis rate of 6% at 3 years, but was associated with more complications than endovascular stenting, and thus is not preferred.References
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