Diseases of the cervical (and first thoracic) roots, nerve trunks, cords, and peripheral nerve components of the brachial plexus. clinical manifestations include regional pain, paresthesia; muscle weakness, and decreased sensation (hypesthesia) in the upper extremity. these disorders may be associated with trauma (including birth injuries); thoracic outlet syndrome; neoplasms; neuritis; radiotherapy; and other conditions. (from adams et al., principles of neurology, 6th ed, pp1351-2)