Which feature is a typical problematic area of Thoracic Outlet Syndrome, but less often part Anterior Scalene Syndrome?

The Subclavian Vein. The subclavian vein is not restricted by the Anterior Scalene Triangle, as it exists medially to the triangle. For the vein to be affected, the costoclavicular ligament would likely have to be inflamed as well, or the clavicle itself could be restricted/displaced.