An unusual sternalis with variation of the contralateral sternocleidomastoid muscle: a case report

Purpose To report a previously undocumented variant of sternalis. Methods An unusual muscle was observed during routine dissection. Results The sternalis muscle located in the right thoracic region originated from the superior portion of the rectus abdominis sheath and 5–6th costal cartilages, crossed the midline and attached at the sternum. The muscle fibers then ascended with the left sternocleidomastoid muscle as an additional fasciculus, of which the superior ends were finally terminated at the left mastoid process. The sternalis muscle of the thoracic region was innervated by the anterior cutaneous branches of right intercostal nerve, while the additional fasciculus ascended with the left sternocleidomastoid muscle was innervated by the branches of left accessory nerve. Conclusions This study presents a unilateral sternalis muscle with the contralateral sternocleidomastoid variation. It will enhance the exhaustive classification of sternalis, and provide significant information to radiologists, angiologists and surgeons for better interpretation of images and safer interventions.