Absence of the sternocleidomastoid muscle in a patient that underwent neck dissection for squamous cell carcinoma of the tongue.

The congenital absence of a skeletal muscle is a rare cause of congenital muscular torticollis, and the condition is associated with various unusual anatomical structures. We describe a rare case of congenital absence of the sternocleidomastoid muscle associated with squamous cell carcinoma of the tongue in a patient who underwent neck dissection. In this case, both the external jugular vein and the spinal accessory nerve were absent. However, we found that branches of the C3 nerve extended from the cervical plexus to the trapezius muscle and seemed to be acting as trapezius muscle motor nerves in place of the accessory nerve. After the operation, the patient was able to lift and abduct his right arm, and his shoulder did not droop.