A peripheral neuropathy that affects movement and sensation in the shoulder. Idiopathic brachial plexopathy usually results from brachial nerve entrapment by vigorous exercise. It may cause atrophy of the muscles surrounding the shoulder (the “shoulder girdle”). Nonidiopathic (i.e., readily identifiable) causes of the neuropathy sometimes are found in persons with brachial nerve injury caused by tumors or radiation.