collective title for a number of conditions attributed to compromise of blood vessels or nerve fibers (brachial plexus) at any point between the base of the neck and the axilla; formerly classified on the basis of presumed injurious structure or mechanism, i.e., scalenus anticus syndrome, hyperabduction syndrome, costoclavicular syndrome; currently classified on the basis of the structure known or presumed to be compromised, and divided into two main groups: vascular and neurologic (simultaneous compromise of both neural and vascular structures is rare); vascular subdivisions include arterial and venous; some neurologic subdivisions remain in dispute.