Subclavian, Innominate, Axillary Artery Disease and Thoracic Outlet Syndrome
Debabrata Mukherjee
Upper extremity arterial disease is significantly less common than disease of the lower extremities. Large-vessel occlusive disease of the arm typically involves the subclavian arteries. The Joint Study of Extracranial Arterial Occlusion reported a 17% incidence of subclavian or innominate artery stenosis, but clinical symptoms or angiographic steal occurred in only 2.5% of cases. The most common site of disease is the left subclavian artery, and this vessel is involved three to four times more frequently than the right side. Significant proximal subclavian artery stenosis may result in subclavian steal syndrome (SSS) leading to vertebrobasilar insufficiency, upper extremity claudication, and myocardial ischemia due to hyperfusion of the internal mammary artery (IMA) in patients with an IMA bypass. The axillary artery is only rarely involved by atherosclerosis, and more commonly, axillary artery stenosis or aneurysm is related to long-term use of axillary crutches. Table 16.1 lists etiologies of large-artery involvement. For the most part, large-vessel disease has limited etiologies [embolic, aneurysm, vasculopathy (vasculitis and atherosclerosis), and entrapment]. Long-term use of crutches may cause axillary artery stenosis or aneurysms. Giant cell arteritis (typically Takayasu’s) should be considered in young individuals presenting with upper extremity involvement, whereas atherosclerosis is a common etiology in older individuals. Thoracic outlet syndrome (TOS) should be considered in any individual with acute or chronic upper extremity symptoms.
CLINICAL FEATURES
Innominate Arterial Disease
Involvement of the innominate artery is uncommon, and the etiologies are limited to atherosclerosis and inflammatory vasculitis. Involvement of the innominate artery is seen in 5 to 17% of patients undergoing arteriography of the arch/cerebral vessels. While most cases are asymptomatic, other presentations include upper extremity-related symptoms (right-sided claudication or microembolization) or cerebrovascular ischemic symptoms related to the vertebrobasilar distribution.
Subclavian Artery Disease
Most individuals with subclavian artery stenosis are asymptomatic, and involvement is detected as an incidental finding during Doppler ultrasound examination of the carotid and vertebral arteries. Arm claudication may occur but is generally rare, owing to exuberant collateral formation
in most individuals but may be seen with very high-grade stenosis or subclavian occlusions. Radiologic vertebral-subclavian steal occurs in the setting of subclavian artery stenosis proximal to the origin of the vertebral artery, associated with flow reversal in the vertebral artery. This may be demonstrated by ultrasound, phase-contrast magnetic resonance angiography (MRA), or retrograde opacification of the vertebral artery on diagnostic angiography. Symptoms suggestive of vertebrobasilar or posterior cerebral circulation ischemia such as dizziness, unsteadiness, vertigo, or visual changes may occur in response to upper extremity arm exertion. The combination of retrograde vertebral flow and neurologic symptoms in response to upper extremity exertion has been labeled SSS. In this disorder, blood is shunted through the ipsilateral vertebral artery from the contralateral posterior circulation to ameliorate blood flow to the subclavian artery beyond a tight proximal stenosis. This is mostly encountered in the context of contralateral carotid or posterior circulation disease or hypoplastic posterior communicating arteries (common anomaly), resulting in inadequate blood flow to the ipsilateral vertebral artery and resultant posterior circulation ischemia. The Joint Study of Extracranial Arterial Occlusion found that 80% of patients with SSS had concomitant lesions in the contralateral carotid or vertebral circulation. The existence of this clinical entity has, however, been questioned, as there can be imprecise correlation between the presence of these symptoms and the existence of stenosis. Moreover, individuals with atherosclerosis of the subclavian artery also harbor concomitant cerebrovascular disease, making precise attribution of symptoms difficult. Individuals with left internal mammary artery (LIMA) coronary bypass grafts may develop angina with arm exercise related to coronarysubclavian steal where there is flow reversal in the LIMA and blood is diverted from the coronary circulation resulting in myocardial ischemia.
in most individuals but may be seen with very high-grade stenosis or subclavian occlusions. Radiologic vertebral-subclavian steal occurs in the setting of subclavian artery stenosis proximal to the origin of the vertebral artery, associated with flow reversal in the vertebral artery. This may be demonstrated by ultrasound, phase-contrast magnetic resonance angiography (MRA), or retrograde opacification of the vertebral artery on diagnostic angiography. Symptoms suggestive of vertebrobasilar or posterior cerebral circulation ischemia such as dizziness, unsteadiness, vertigo, or visual changes may occur in response to upper extremity arm exertion. The combination of retrograde vertebral flow and neurologic symptoms in response to upper extremity exertion has been labeled SSS. In this disorder, blood is shunted through the ipsilateral vertebral artery from the contralateral posterior circulation to ameliorate blood flow to the subclavian artery beyond a tight proximal stenosis. This is mostly encountered in the context of contralateral carotid or posterior circulation disease or hypoplastic posterior communicating arteries (common anomaly), resulting in inadequate blood flow to the ipsilateral vertebral artery and resultant posterior circulation ischemia. The Joint Study of Extracranial Arterial Occlusion found that 80% of patients with SSS had concomitant lesions in the contralateral carotid or vertebral circulation. The existence of this clinical entity has, however, been questioned, as there can be imprecise correlation between the presence of these symptoms and the existence of stenosis. Moreover, individuals with atherosclerosis of the subclavian artery also harbor concomitant cerebrovascular disease, making precise attribution of symptoms difficult. Individuals with left internal mammary artery (LIMA) coronary bypass grafts may develop angina with arm exercise related to coronarysubclavian steal where there is flow reversal in the LIMA and blood is diverted from the coronary circulation resulting in myocardial ischemia.
Giant Cell Arteritis (Takayasu’s)
Takayasu’s arteritis commonly involves the subclavian arteries. Typically, type I (arch vessels) and type II (arch and abdominal vessels) variants of the disease display subclavian artery involvement. In contrast to the involvement with atherosclerosis, the involvement with vasculitis tends to be diffuse (proximal and distal to the origin of the vertebral artery), andconsequently, patients do not present with SSS. The involvement is typically asymptomatic, presenting with evidence of a bruit over the subclavian artery, absent radial artery pulses, or a blood pressure differential between upper and lower extremities. A few patients may complain of upper extremity claudication. All patients with Takayasu’s should have blood pressures measured in all four extremities.
Aneurysms of the Subclavian and Axillary Artery
Subclavian Artery Aneurysms
These result from atherosclerosis, trauma, TOS, and rarely collagen-vascular diseases. This is an unusual site, and the presence of an aneurysm at this location should prompt an investigation for arterial aneurysms in other sites.
Clinical features include embolization and pain in the neck/shoulder region due to impingement on the brachial plexus with or without Raynaud’s symptoms. Uncommon manifestations include hoarseness due to compression of the recurrent laryngeal nerve and dyspnea (compression of the trachea) and transient ischemic attacks due to retrograde embolization in the vertebral and carotid arteries. Pulsatile masses in the supraclavicular region may represent tortuous rather than aneurysmal subclavian arteries. Aneurysms in the distal subclavian artery are usually secondary to TOS (almost always a cervical rib).
Clinical features include embolization and pain in the neck/shoulder region due to impingement on the brachial plexus with or without Raynaud’s symptoms. Uncommon manifestations include hoarseness due to compression of the recurrent laryngeal nerve and dyspnea (compression of the trachea) and transient ischemic attacks due to retrograde embolization in the vertebral and carotid arteries. Pulsatile masses in the supraclavicular region may represent tortuous rather than aneurysmal subclavian arteries. Aneurysms in the distal subclavian artery are usually secondary to TOS (almost always a cervical rib).
“Kommerell’s Diverticulum” and Aneurysm of an Aberrant Right Subclavian Artery
An aneurysm of an aberrant right subclavian artery (ARSA) arising from a left aortic arch is a common abnormality of the aortic arch, occurring in 0.4 to 2% of the population. ARSA originates in the thoracic aorta, distal to the left subclavian artery, and crosses the midline between the spinal column and the esophagus to the right axilla. In 60% of cases, its origin in the thoracic aorta is saccular, as a diverticulum first described by Kommerell (hence referred to as Kommerell’s diverticulum). This abnormality tends to be asymptomatic and discovered by chance when a chest radiograph is taken or endoscopic examination is performed. When symptomatic, it commonly causes intermittent dysphagia for solids because of its retroesophageal course, a condition known as dysphagia lusoria. These aneurysms display a marked propensity to rupture.
Axillary Artery Aneurysm
This is usually a result of repetitive trauma as is the case with “crutch aneurysm” or may be seen with blunt or penetrating trauma to the area (as occurs with motor vehicle accidents, humeral fractures, and anterior dislocation of the shoulder). In a crutch aneurysm, the presentation is often one of upper extremity ischemia and should always be suspected in anybody using a crutch who has an abnormal pulse exam. With trauma associated with brachial plexus injury or new neurologic findings (in the context of previously normal findings), the presence of an expanding axillary artery aneurysm should be considered as an etiology and an arteriogram or a carefully performed duplex exam ordered as part of the workup.
DIAGNOSTIC APPROACH FOR SUSPECTED ARTERIAL DISEASE
Physical examination may suggest significant subclavian or innominate stenosis through detection of weak or absent radial and ulnar pulse on the ipsilateral side and reduced blood pressure (difference greater than 15 mm Hg) when compared to the contralateral arm. Detection of a bruit over the subclavian artery (subclavian artery stenosis) or proximal (base of carotid) carotid and subclavian artery is a suggestive clue. Upper extremity embolization (unilateral) should prompt a workup for proximal disease in the innominate and subclavian arteries. In some individuals, bilateral
involvement of subclavian or innominate artery as well as the subclavian vessels may result in blood pressures being low in both upper extremities. Thus, lower-than-expected blood pressures in the upper extremity should prompt measurement of lower extremity blood pressure. The evaluation of large-vessel disease of the upper extremity would not be complete without performing diagnostic maneuvers for TOS as described in detail in Chapter 1 and later in this chapter, including palpation for tenderness over the scalenus anticus. Every patient with suspected TOS should have a radiograph to rule out a thoracic rib. Figure 16.1 provides a general outline for testing in patients presenting with upper extremity symptoms. Also refer to the Diagnostic Testing section in that chapter for specific tests. Aspects specific to large-arterial involvement are outlined here.
involvement of subclavian or innominate artery as well as the subclavian vessels may result in blood pressures being low in both upper extremities. Thus, lower-than-expected blood pressures in the upper extremity should prompt measurement of lower extremity blood pressure. The evaluation of large-vessel disease of the upper extremity would not be complete without performing diagnostic maneuvers for TOS as described in detail in Chapter 1 and later in this chapter, including palpation for tenderness over the scalenus anticus. Every patient with suspected TOS should have a radiograph to rule out a thoracic rib. Figure 16.1 provides a general outline for testing in patients presenting with upper extremity symptoms. Also refer to the Diagnostic Testing section in that chapter for specific tests. Aspects specific to large-arterial involvement are outlined here.
Segmental Limb Pressures, Finger Pressures, and Doppler Pulse Contours
The normal pressure differential between adjacent levels of the same limb or compared to the same level of the contralateral limb is less than 7 mm Hg. A significant pressure drop (more than 10 mm Hg) or an index below 0.80 indicates disease at that level [ratio of brachial/brachial, forearm/forearm, wrist/wrist, wrist/brachial (same arm) or forearm/brachial indices (same arm) is normally above 0.90, with ratios below 0.85 being considered abnormal]. The presence of monophasic or damped signals is adjunctive qualitative evidence of disease. Normally, the pulse contours in the upper extremity arteries (evaluated by Doppler or pulse volume recordings) are triphasic or biphasic (see Chapter 16). Monophasic or damped signals indicate disease at that level. Concomitant finger pressures should also be obtained in cases with evidence of Raynaud’s or small-vessel disease (see Chapter 16).
Duplex Ultrasonography
Duplex evaluation of the arteries of the upper extremity is extremely helpful in suspected large-vessel involvement. The proximal left subclavian artery and segments of the subclavian system under the clavicle are one area that may be difficult to image. Duplex scanning facilitates an image-directed Doppler exam of specific upper extremity arteries and their branches. Normal systolic velocity in the subclavian and innominate arteries is 80 to 120 cm/s. A greater than twofold elevation in velocities in the stenotic zone versus the prestenotic zone indicates a more than 50% narrowing. Spectral characteristics that suggest turbulence are very helpful adjunctive findings. Arterial occlusion is diagnosed by diagnosing the absence of flow within the artery, taking care not to confuse various solid nonvascular structures with an occluded artery. In individuals with arm claudication and normal patterns at baseline, Doppler flow should be remeasured after 3 to 5 minutes of arm exercise.
Magnetic Resonance Angiography
3D contrast-enhanced magnetic resonance angiography may be used to evaluate the aortic arch vessels as well as the upper extremity arteries. The arch evaluation provides information on the proximal and
middle portions of the subclavian artery that may be sufficient for the diagnosis of atherosclerotic involvement. Additional phase-contrast MRA (PC-MRA) measurements may be required to demonstrate retrograde flow in a vertebral artery in cases of subclavian artery steal (most commonly seen in the left side). Improvements in coil technology are enabling simultaneous demonstration of the arch vessels and the carotid bifurcations. Two artifacts that are particularly relevant to imaging arch and subclavian disease that may simulate a stenosis in an artery are (a) Coil drop-out artifact: In patients with low aortic arch or elevated shoulders, the coil sensitivity may drop out in the periphery resulting in loss of signal and simulation of a stenosis. (b) Venous susceptibility artifact:
middle portions of the subclavian artery that may be sufficient for the diagnosis of atherosclerotic involvement. Additional phase-contrast MRA (PC-MRA) measurements may be required to demonstrate retrograde flow in a vertebral artery in cases of subclavian artery steal (most commonly seen in the left side). Improvements in coil technology are enabling simultaneous demonstration of the arch vessels and the carotid bifurcations. Two artifacts that are particularly relevant to imaging arch and subclavian disease that may simulate a stenosis in an artery are (a) Coil drop-out artifact: In patients with low aortic arch or elevated shoulders, the coil sensitivity may drop out in the periphery resulting in loss of signal and simulation of a stenosis. (b) Venous susceptibility artifact:
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