A 43 year old woman came with a large abscess in the middle of the right posterior triangle of the neck. The physician incised and drained the abscess. Five days later the patient noticed that she could not extend her right hand above her head to brush her hair. Which of the following are the signs and symptoms of additional harm?

Overhead abduction is produced by serratus anterior-supplied by long thoracic nerve (branch of brachial plexus arising from C5, C6, C7) and trapezius-supplied by spinal part of the accessory nerve. Spinal part of accessory nerve emerges from the middle of the posterior border of sternocleidomastoid and runs through the roof of the posterior triangle. It can be damaged in the posterior triangle of the neck.