Paralysis of the Upper Extremity Due to Acute Thoracic Outlet Syndrome Caused by the Subclavius Posticus Muscle: A Case Report With Literature Review.

The subclavius posticus muscle is a rare aberrant muscle that traverses from the costal cartilage of the first rib posterolaterally to the superior border of the scapula. We report a patient having persistent paralysis of shoulder abduction with wrist and finger extension after a humeral neck fracture. Electromyography (EMG) examination revealed injuries to several upper extremity peripheral nerves, including the radial, axillary, and musculocutaneous nerves. Magnetic resonance imaging (MRI) performed at 10 months post-injury showed severe entrapment of the left brachial plexus by the subclavius posticus muscle at the thoracic outlet. The diagnosis of brachial plexus injury due to a rare abnormal subclavius posticus muscle was typically delayed until the MRI was performed for unexplained multiple peripheral nerve palsy. Resection of the aberrant muscle and brachial plexus decompression did not yield significant improvement in the patient's radial nerve palsy until 6 months after surgery. Entrapment of the brachial plexus caused by the subclavius posticus muscle can cause symptoms of acute thoracic outlet syndrome following trauma to the upper extremity. In a case of inexplicable multiple peripheral nerve injuries in the upper extremity that are not proportional to the degree of trauma, MRI imaging along with EMG is required.