a neurologic disorder, of unknown cause, characterized by the sudden onset of severe pain, usually about the shoulder and often beginning at night, soon followed by weakness and wasting of various forequarter muscles, particularly shoulder girdle muscles; both sporadic and familial in occurrence with the former much more common; often preceded by some antecedent event, such as an upper respiratory infection, hospitalization, vaccination, or nonspecific trauma; usually attributed to a brachial plexus lesion, because the nerve fibers involved are most often derived from the upper trunk. SYN: acute brachial radiculitis, brachial neuritis, brachial plexitis, brachial plexus neuropathy, Parsonage-Turner syndrome, shoulder-girdle syndrome.