Arterial thoracic outlet syndrome is rare and may be associated with a bony anomaly. Patient presentation can range from mild arm discoloration and claudication to severe limb-threatening ischemia. For patients with subclavian artery dilation without secondary complications, thoracic outlet decompression and arterial surveillance is sufficient. Patients with subclavian artery aneurysms or distal embolization require decompression with reconstruction or thromboembolectomy and distal bypass respectively.
Key points
- •
Arterial thoracic outlet syndrome is associated with a cervical rib.
- •
Patient presentation can range from mild arm discoloration and claudication to severe limb-threatening ischemia.
- •
For patients with mild subclavian artery dilation without secondary complications, thoracic outlet syndrome decompression and arterial surveillance is sufficient.
- •
Patients with subclavian artery aneurysms require replacement of the aneurysm with arterial bypass graft.
Introduction/History/Definitions/Background
The least common form of thoracic outlet syndrome (TOS), arterial TOS (aTOS), typically represents approximately 1% of TOS cases. Mayo has been credited as being the first to describe arterial compression due to a cervical rib, whereas Coote was the first to report successful resection of a bony abnormality resulting in subclavian artery aneurysm in 1861. William Halsted wrote about post-stenotic dilation of a subclavian artery and the relationship to a cervical rib more than a century ago. To test his hypothesis that the abnormal dilation was due to stenosis, he performed aortic banding on 30 canine subjects of which 17 (56.6%) showed evidence of post-stenotic dilation. At the time of his work, he had discovered only 716 instances of cervical ribs; 525 were from clinical cases and the rest from autopsy findings and museum specimens. Of the clinical cases, only 19 demonstrated vascular symptoms alone, 6 of which with severe enough ischemia to produce gangrene of the fingers with an associated aneurysmal subclavian artery. Halsted postulated possible explanations for this dilation 2 years later, including (1) weakening of the wall of the subclavian artery from erosion by the rib, (2) variable or intermittent pulse pressure occasioned by the normal excursions of the arm, and (3) vasomotor and vasa vasorum disturbances leading to modified nutritional activities in the wall of the artery. Approximately a decade later, Adson and Coffey would describe the physical examination finding that would take Adson’s namesake in association of a cervical rib by compression by the anterior scalene muscle. In fact, it had yet to be called TOS and was still being referred to as scalenus anticus syndrome. It was not until Peet and colleagues coined the term “thoracic outlet syndrome” in 1956 to describe compression of the neurovascular bundle by the thoracic outlet that it was used widely among the surgical community. This term is a misnomer because the superior thoracic aperture is also known as the thoracic inlet. Although the least common form of TOS, aTOS has arguably the most serious consequences as a result of upper extremity ischemia, gangrene, and possible limb loss. To universalize diagnostic criteria for the syndromes associated with TOS, the Society of Vascular Surgery outlined reporting standards. aTOS is defined as an objective abnormality of the subclavian artery caused by extrinsic compression and subsequent damage by an anomalous first rib or analogous abnormal structure (cervical rib or band) at the base of the scalene triangle.
Epidemiology/Prevalence/Incidence
The true prevalence of aTOS is unknown given many patients may be asymptomatic. In one of the largest single-center series out of Baylor University, Urschel and Kourlis described their 50-year experience with all TOS patients. Of 5147 extremities treated, only 240 (4.7%) were for associated arterial complications. Patients are typically younger and active with a mean age in published series of 33 to 40 years old and affects men and women equally though some series have a bias toward women. Unlike in adults where it is the most rare form of TOS, aTOS seems to be more common in children and adolescents who are treated surgically for TOS. ,
Cervical ribs exist in approximately less than 2% of the population and are a known cause of subclavian artery compression and aneurysm. The approximated incidence of subclavian aneurysms caused by a cervical rib is 0.019% or 1 in 5000 people. In a study to assess the incidence of cervical ribs on more than 3400 computed tomography (CT) images of the neck, Viertel and colleagues discovered that only 67 (2%) had cervical ribs and of those, 27 (40.3%) had bilateral cervical ribs. In this study, radiologists commented on the cervical rib in just a quarter of the patients leading investigators to conclude they are often overlooked. Weber and Criado retrospectively reviewed imaging of 400 TOS cases at a single institution to describe the prevalence of bone anomalies including cervical ribs, clavicular anomalies, and first rib aberrations. The prevalence of a bone anomaly was 29% in treated TOS cases. Interestingly, when they stratified TOS cases based on subtypes, the likelihood of arterial compression was much higher in the presence of a bony anomaly (odds ratio [OR] 4.0; P <.001).
Presentation
aTOS may be symptomatic (ischemia or embolization) or asymptomatic (aneurysm, occlusion, or silent embolization). Symptoms can range from acute ischemia, pain, paresthesia, and weakness or changes in color or coldness of the hand. Because of the rarity of the syndrome it is often misdiagnosed and the time from symptom onset to accurate diagnosis can be more than 6 months, resulting in chronic and repeated embolization. Rest pain and ischemic ulceration can develop in advanced disease. Although less common, aTOS can present as an asymptomatic neck mass or as an incidental finding on cross-sectional imaging of subclavian artery dilation. The association of aTOS and retrograde embolism from a subclavian artery aneurysm thrombus as the embolic etiology of a posterior stroke has been described. , Because stroke is uncommon in young healthy patients, aTOS should be considered in the differential diagnosis. It has been noted that as many as 56% to 68% of patients presenting with venous or arterial TOS have neurogenic symptoms as well, or so called mixed-type TOS. , Likes and colleagues evaluated patients with neurogenic TOS (nTOS) with concomitant arterial pathology and found them to be younger (25 vs 40 years old) than pure nTOS patients with a shorter length of symptom onset before presentation. An important conclusion was that unlike patients with strictly nTOS, those with a mixed-type TOS did not have symptomatic relief from physical therapy and these patients should be offered surgery sooner to prevent repetitive injury to the arterial wall resulting in stenosis or thrombosis.
Evaluation
Patients suspected of aTOS should have a thoroughly history and physical examination. Symptoms of rest pain, paresthesia, loss of dexterity, or coldness in the hand or fingers as well as ischemia with exertion or overhead positioning can suggest aTOS. Any prior history of trauma to the shoulder or fractures of the clavicle and first rib should be noted. Physical examination should include pulse examination of the brachial, radial, and ulnar arteries as well measurement of blood pressure in both upper extremities. The hand and fingers should be examined for discoloration, muscle atrophy, and the presence of ischemic ulcers or tissue loss. The supra and infraclavicular fossa should be palpated for a pulsatile mass and auscultated for a bruit.
Compressive maneuvers use positioning the patient to place tension on the thoracic outlet and palpating the radial artery for abolition of a pulse. Loss of pulse is considered a positive result. These tests include (1) Adson’s test: abducting the arm 30° at extension, turning the head through 90° toward the tested arm, while maintaining deep inspiration; (2) costoclavicular test: bracing the shoulders back and down in an exaggerated military posture; (3) erect and supine hyperabduction test: abducting the arm through 180° while standing and supine. , Unfortunately, the false-positive rate in normal healthy volunteers can be as high as 57% depending on the maneuver being tested. Due to the inaccuracy of these tests, further testing with diagnostic imaging is used to establish the diagnosis.
Imaging
Diagnostic imaging includes plain chest radiography, hemodynamic testing with digital plethysmography, pulse volume or segmental pressure recordings, and duplex ultrasonography, as well as cross-sectional imaging with computed tomography arteriography (CTA) or magnetic resonance arteriography (MRA) and conventional angiography. Imaging choices are influenced by the nature and acuity of the initial patient presentation as well as the need for anatomic assessment for intervention.
Chest radiography
A plain chest radiograph is often the initial imaging modality if aTOS is suspected given the ability to evaluate for osseous abnormalities ( Fig. 1 ). These films are easy to access with low cost and safety profile that can identify cervical ribs, anomalous first ribs, elongated transverse processes, congenital osseous malformation and clavicular trauma or callus. , Rarely, plain film may identify bone destruction related to a primary or secondary neoplasm that results in TOS symptoms. Unfortunately, isolated rib anomalies can be missed on chest radiographs, specifically fibrocartilaginous bands which can better be visualized with the aid of CT or MR to better delineate the anatomy.
Hemodynamic studies
Noninvasive tests can be used to identify the presence and location of a stenosis or occlusion resulting in a physiologic deficit. Segmental pressure measurements may demonstrate a pressure gradient across 2 levels. Digital and upper extremity plethysmography can display occlusion or stenosis by compression on the subclavian artery by evidence of a delayed upslope, rounded waveform, and loss of a dicrotic notch. These studies should be performed both at rest and with provocative maneuvers. If they are normal at rest they may be repeated with exercise. As with compressive maneuvers, these hemodynamic studies are limited by the prevalence of abnormal findings in the normal population. Chen and colleagues used photoplethysmography to evaluate arterial flow in normal volunteers. They found absent tracings and dampened waveforms in 13% of limbs tested and noted the hyperextension position to produce the greatest arterial flow anomalies. Given that these changes only occurred in a small percent of the normal population, any patients with upper extremity symptomatology suggesting aTOS may represent true anatomic compression at the thoracic outlet.
Ultrasound
Ultrasound can be used to evaluate the thoracic outlet both at rest and with provocative maneuvers. B-mode evaluation detects anatomic abnormalities of the subclavian artery including atherosclerotic plaque, aneurysmal dilation, extrinsic compression, narrowing via cross-sectional area, deviation, and a fibrous band ( Fig. 2 ). Doppler imaging can assess for any significant arterial stenosis or occlusion. After being performed at rest, compressive maneuvers and upper extremity abduction aid in identifying changes in peak systolic velocity or complete cessation of arterial flow suggesting aTOS. Others have questioned the value of duplex ultrasound in diagnosis of symptomatic patients. Gergoudis and Barnes evaluated the prevalence of thoracic outlet compression on 130 healthy individuals. Significant arterial obstruction was noted in 78 individuals (60%) and was bilateral in 33 (43%) during provocative maneuvers. Given this rate of abnormalities in healthy subjects, ultrasound should be used only to confirm a suspected diagnosis of aTOS and not the sole diagnostic tool.
Computed tomography angiography
A protocol for intravenous contrast administration and evaluation of the thoracic inlet typically involves 2 helical CT angiograms in the same session. The first is with the arm along the patients side during full inspiration and the second with the arm in hyperabduction with the head turned to the contralateral side ( Fig. 3 ). In symptomatic patients, CTA can be used to analyze the anatomic relationship between the subclavian and axillary artery with the surrounding musculoskeletal structures identifying the exact point of compression and length of arterial disease that requires treatment. A detection of 30% stenosis is considered significant and CTA findings in patients with aTOS have shown correlation to both operative findings and success of decompression. Importantly, subclavian artery compression in normal subjects is rarely affected with normal arm motion. Matsumura and colleagues examined vascular compression of the thoracic outlet in 10 healthy volunteers with the arm in a neutral position and with the arm abducted using helical CT. Unlike the subclavian vein, which was universally compressed, most subclavian arteries showed less than 10% narrowing. With the ability to produce 3-dimensional reconstruction, CTA has largely supplanted catheter-based angiography as the main diagnostic and preoperative planning imaging modality ( Fig. 4 ).
