What is Thoracic Outlet Syndrome?
Thoracic outlet syndrome (TOS) is a clinical condition caused by compression of the brachial plexus, subclavian artery, and/or subclavian vein at the superior thoracic outlet 1.
Anatomical Framework
TOS occurs when neurovascular structures are compressed as they pass through three distinct anatomical spaces at the thoracic outlet:
1. Costoclavicular Triangle
- Bounded by: clavicle (superior), anterior scalene muscle (posterior), first rib (inferior)
- All three neurovascular structures pass through this space
- Narrowing here predominantly causes venous symptoms (venous TOS or vTOS), with varying degrees of arterial or brachial plexus compression 1
2. Interscalene Triangle
- Bounded by: anterior scalene muscle, middle scalene muscle, and first rib
- Contains the brachial plexus trunks and subclavian artery
- Narrowing here causes neurological dominant TOS (nTOS), arterial dominant TOS (aTOS), or combinations 1
3. Pectoralis Minor Space
- Bounded by: pectoralis minor muscle (anterior) and chest wall (posterior)
- Functions as an extension of the thoracic outlet
- Can produce compression patterns similar to the costoclavicular space 1
Clinical Subtypes
TOS is classified into three distinct forms based on which structure is compressed:
- Neurogenic TOS (nTOS): Most common form, characterized by chronic arm and hand paresthesia, numbness, or weakness 1
- Venous TOS (vTOS): Presents with arm swelling and venous symptoms
- Arterial TOS (aTOS): Involves arterial compression with risks of thrombosis, distal embolization, and aneurysm formation 1
Etiological Factors
Anatomical Variants
- Cervical ribs (most common congenital cause)
- Anomalous first rib
- Elongated C7 transverse process
- Post-traumatic changes from prior clavicular or rib fractures 1
Acquired Causes
- Repetitive upper-extremity movement in athletes (swimmers, throwers) or workers with overhead activities 1
- Subclavius muscle hypertrophy, which further narrows the costoclavicular space
- Repetitive stress leading to thickening and fibrosis of the subclavian vein wall, with restrictive fibrotic tissue surrounding the vein
- Eventual intimal damage resulting in luminal narrowing and thrombogenic surface formation 1
Pathophysiological Progression
In vTOS, the repetitive stress causes progressive vascular changes: thickening and fibrosis of the subclavian vein wall → restrictive fibrotic tissue formation → intimal damage → luminal narrowing → scarred, thrombogenic surface 1. In aTOS, similar intimal damage occurs with additional concerns for thrombosis, distal embolization, and aneurysm formation 1.
Epidemiology
The incidence of symptomatic TOS is estimated at 10 per 100,000 population 1, though more recent prospective data suggests 2-3 cases per 100,000 people per year for nTOS and 0.5-1 per 100,000 per year for vTOS 2. The ratio of patients with nTOS to vTOS is approximately 80:20 among those referred for evaluation, though the surgical ratio shifts to approximately 75:25 2.
Key Clinical Pearls
- TOS is not a diagnosis of exclusion—there should be evidence of a physical anomaly that can be corrected 3
- Extreme shoulder abduction naturally narrows the costoclavicular space, which is why provocative maneuvers are diagnostically useful 1
- Clinical presentation is often nonspecific and may overlap with peripheral neuropathies, cervical spine disorders, and musculoskeletal conditions 4
- Diagnosis requires identifying the affected neurovascular structures, localizing the compression site, and elucidating the underlying anatomical or pathological cause 4