Thoracic outlet syndrome (TOS) was first described by Coot in 1861,1,2 and the term was coined by Peet and colleagues3 in 1956 to cover a spectrum of conditions caused by dynamic compression of the brachial plexus (neurogenic), subclavian artery (arterial), or subclavian vein (venous). The estimated incidence of TOS is 10 in 100,000.4 However, cadaveric studies have suggested that up to 90% of the population may have what is considered abnormal anatomy of the thoracic outlet,5 which in turn suggests a multifactorial etiology for symptomatic disease. TOS is most commonly diagnosed in patients 20 to 40 years of age, with females affected in a 4:1 ratio.6 Although historically TOS is a clinical diagnosis, advanced imaging is often helpful in determining the nature and location of the structure undergoing compression and the structure producing compression, which help guide management. Computed tomography angiography (CTA) and magnetic resonance imaging (MRI) performed in association with postural maneuvers aid in the diagnosis in patients with dynamically acquired compression.7
Pathophysiology
The pathophysiology of TOS is attributable to the unique anatomy of the thoracic outlet. Compromise of the neurovascular structures can occur through congenital or acquired narrowing in 3 distinct compartments: the interscalene triangle, the costoclavicular space, and the retropectoralis minor space. The interscalene triangle is the most medial of the compartments. Containing the subclavian artery and the 3 trunks of the brachial plexus, it is bordered anteriorly by the anterior scalene muscle, posteriorly by the middle and posterior scalene muscles, and inferiorly by the first rib. The interscalene triangle is the most frequent site of neurologic compression.8 The middle compartment is the costoclavicular space, which is bordered superiorly by the clavicle, anteriorly by the subclavius muscle, and posteriorly by the first rib and the middle scalene muscle. The costoclavicular space is the most frequent site of arterial compression,8 where the artery lies directly anterior to the subclavian vein and is surrounded by the 3 cords of the brachial plexus. The most lateral compartment is the retropectoralis minor space, which is bordered anteriorly by the pectoralis minor muscle, superiorly by the subscapularis muscle, and inferiorly by the anterior chest wall. Sources of neurovascular compression within any of the spaces include cervical ribs9; elongated C7 transverse processes; hypertrophy of the anterior or middle scalene, subclavius, or pectoralis minor muscles10; anomalous scalenus minimus muscle; repetitive overhead arm movements (pitching, swimming)11; anomalous fascial bands; degenerative spine disease; bone destruction from primary or secondary neoplasms (Pancoast tumor); hyperextension/flexion injury of the neck12; and malunion of clavicle fractures, among others.13
Classification
Three distinct TOSs have been described, individually or combined, depending on the injured component: neurogenic from brachial plexus compression, arterial from subclavian artery compression, and venous from subclavian or axillary vein compression.14,15
Neurogenic TOS has 2 reported types: true (classic) and disputed. True neurogenic TOS is rare, with an estimated incidence of 1 in 1 million.16 First described in 1970 as a lower trunk plexopathy involving slowly progressive unilateral weakness of the intrinsic hand muscles and sensory abnormalities in the ulnar and medial antebrachial cutaneous nerve distributions, true neurogenic TOS was originally called Gilliatt-Sumner hand syndrome.17 A congenital band extending between the first rib and an elongated C7 transverse process was thought to be the location of brachial plexus injury in true neurogenic TOS. Conversely, disputed neurogenic TOS is the most common form of TOS, occurring in 3 to 80 per 100018 and accounting for 90% to 95% of all TOS cases.13,19 In contrast to true neurogenic TOS, in which anatomical and electrodiagnostic evidence supports the diagnosis, objective clinical findings are often lacking in the disputed form.18 Patients with disputed neurogenic TOS present with a diverse array of symptoms, including pain, numbness, and weakness affecting the neck, shoulder, and arm, exacerbated by activities requiring elevation or sustained use of the extremity.20
Arterial TOS accounts for 1% to 5% of all TOS cases.21 Arterial TOS typically affects patients who perform repetitive movements of the upper extremities with their arms above their shoulders, resulting in compression of the subclavian artery. Symptoms of arterial TOS include pain, weakness, coolness, pallor, and paresthesia.18,22 In severe cases of compression, subclavian artery damage can result in thrombosis with distal embolization, poststenotic aneurysm, or even retrograde extension causing stroke.22,23
Last, representing 2% to 3% of all TOS cases, venous TOS results from compression of the subclavian or axillary vein.18,24 Two mechanisms for vascular compromise have been described. The first involves compression of the vein between the clavicle and the first rib with overhead activities.18 Patients often experience intermittent “heaviness” of the extremity with repeated overhead use. The second mechanism involves repeated stress between the clavicle and vein, causing an intravascular thrombosis.18 Patients may experience pain, edema, cyanosis, venous distention, and even spontaneous venous thrombosis, referred to as Paget-Schroetter syndrome, which can lead to pulmonary embolism.6,25,26