In TOS, which structure is most commonly compressed?

• A. Subclavian artery
• B. Subclavian vein
• C. Brachial plexus
• D. Axillary artery

Answer: (C)

The structure most commonly compressed in thoracic outlet syndrome is the brachial plexus. Why this is best: the brachial plexus traverses the thoracic outlet and is susceptible to compression in the narrow spaces formed by the first rib, clavicle, and surrounding scalene muscles. When the plexus is compressed, patients typically present with neurogenic symptoms in the arm—numbness, tingling, and weakness—often in the distribution of the ulnar nerves and intrinsic hand muscles. This neurogenic presentation accounts for the vast majority of TOS cases, making the brachial plexus the most frequent structure affected.

Compression of the subclavian artery or subclavian vein can occur, but they are less common and lead to vascular signs such as claudication or edema, respectively. The axillary artery is not the usual site of primary compression in TOS.