Long thoracic nerve palsy results from injury and damage to the long thoracic nerve. This nerve runs from the neck vertebrae along the side of the chest to the muscle that holds the scapula bone to the chest wall. When this nerve is damaged, the scapula — or shoulder blade — becomes abnormally positioned, resembling a protruding wing. This odd positioning has led to the name "winged scapula" being given to this condition. Shoulder pain and loss of movement occur, as the imbalance in muscle functioning causes stress to the muscles of the shoulder.

Injuries to the long thoracic nerve may also involve the brachial plexus nerve. At one point above the collarbone, the two nerves run parallel to each other. If the injury occurs at this position, the brachial plexus — which runs the entire length of the arm, innervating the muscles of the arm and hand — could also be affected. Injuries near the vertebrae of the neck where these nerves originate may also affect both nerves, causing long thoracic nerve palsy and loss of function in the arm and hand.

There are numerous possible causes of long thoracic nerve palsy, but most cases are due to direct injury or strenuous repetitive movements. The nerve runs fairly close to the surface and can be easily affected by impact to the upper body. Sports injuries are a common cause. Activities like weight lifting, where load bearing repetitive movement stresses the nerve, may also cause damage to the long thoracic nerve. Surgical procedures, such as radical mastectomies, and deep tissue massage might also damage the nerve.

The long thoracic nerve passes through the middle scalene muscle that attaches the neck vertebrae to the first rib. Medical professionals believe that injurious compression of this muscle may contribute to long thoracic nerve palsy. Contraction of the muscle and resulting nerve damage can occur during exercise. This view is supported by the existence of numerous patients suffering from long thoracic nerve palsy who have a history of strenuous upper body activities. These include weight lifting or lifting and carrying heavy loads.

Diagnosis of long thoracic nerve palsy involves a test of the electrical conductivity of the nerve using the nerve conduction velocity test or similar test. Once nerve damage is detected, appropriate treatment is given. Nerve injuries can take a relatively long time to resolve and may only partially heal. Both physical therapy and surgery are used to treat long thoracic nerve palsy. Decompression of the nerve or surgical nerve transfer are possible treatment options.