Which anatomical region forms the most common etiology of thoracic outlet syndrome?

• A. Interscalene triangle
• B. Costoclavicular space
• C. Subcoracoid/sub-pec minor space
• D. Supraclavicular space

Answer: (A)

The interscalene triangle is the most common site where thoracic outlet syndrome arises. This space lies between the anterior and middle scalene muscles and the first rib, through which the brachial plexus trunks and the subclavian artery pass. Hypertrophy or tightness of the scalene muscles, postural strain, or an extra rib can narrow this passage, making compression there more likely. Because the brachial plexus is directly affected in this region, neurogenic symptoms predominate, which is why this site is the typical primary source of TOS.

Other spaces, like the costoclavicular area (between clavicle and first rib) or the subcoracoid/pectoralis minor space, can contribute to compression, and there can be tandem involvement, but they are less commonly the initial or most frequent etiology. Supraclavicular space compression is also possible but not the most common starting point.