Accessory Clavicular Sternocleidomastoid Causing Torticollis in an Adult

BACKGROUND: Sternocleidomastoid anomalies are mostly discovered in cadavers during routine dissection. Such anomalies causing torticollis are rare in adults. We report a case of accessory mid-clavicular head of the sternocleidomastoid causing torticollis in an adult. CASE: A previously healthy 27-year-old man presented with neck and shoulder pain following heavy manual work. On examination, he had mild restriction of left tilt of the head and right rotation of the neck. Palpation revealed a bipartite right sternocleidomastoid. There was no abnormal posturing of other body parts. The range of motion of the cervical spine was limited to rotation of 70° and tilt of 38° on the left side and rotation of 65°and tilt of 46° on the right side. Neck extension was 40°. The accessory sternocleidomastoid muscle belly was visible and inserted at the middle of the clavicle. Musculoskeletal ultrasound imaging confirmed the diagnosis. Radiological images of the cervical spine and electromyography were normal. Myectomy of the lateral accessory clavicular belly of the sternocleidomastoid was planned, but the patient declined this treatment. Currently, he uses oral analgesics and participates in occasional sessions of physical therapy. DISCUSSION: An accessory sternocleidomastoid can result in torticollis because it causes a physical restriction preventing the neck from tilting and rotating to the opposite side. In mild cases, the anomaly may remain undiagnosed until adulthood and can be confused with cervical dystonia and fibromatosis colli. Knowledge of the anatomy and possible variants of the sternocleidomastoid muscle is of the utmost importance to medical practitioners involved in the diagnosis and management of problems in the neck area. Variants of the sternocleidomastoid can be a concern for surgeons, physicians, and anesthetists performing interventional procedures on the neck because of possible confusion of anatomical landmarks.