Most cases are due to direct injury to the
long thoracic nerve or its origin in the brachial plexus, eg. stabbing. However, the comments below seem to indicate that it may arise indirectly due to injury.
a doctor at the Cleveland Clinic*.
As you are already apparently aware, a “winged scapula” results from paralysis or weakness of the serratus anterior muscle, which normally helps stabilize the scapula, particularly during foreward pushing motion with the arms. It is supplied by the long thoraccic nerve, which arises very soon after cervical spinal nerve roots C5,C6, and C7. It is rare for this nerve to be injured occupationally or for it to become entrapped. Entrapment of another two nerves that supply two other shoulder muscles in the rotator cuff (supra- and infraspinatus muscles) is much more common. The most common neurologic disease that afflicts the long thoraccic nerve is called neuralgic amyotrophy, or Parsonage-Turner syndrome. *Usually the disease follows a triggering event (virus, surgery, injury etc.) *and then begins as intense pain in one or both shoulders or arms, follwed 3 or so weeks later by weakness and wasting of muscles supplied by one or more of the peripheral nerves that supply the arms. Involvement of shoulder girdle
muscles is very common and fairly distinctive for this syndrome. A particular branch of the median nerve called the anterior interosseous nerve, but can affect other parts of this and the other arm nerves as well. Recovery from the syndrome is usually spontaneous, but on occasion permanent residual weakness may occur. The condition is caused by an intense inflammation of the affected nerves.
[sub][sup]* emphasis added[/sup][/sub]