Sternocleiodomastoid muscle with five fleshy bellies and thirteen heads of origin

Sternocleidomastoid (SCM) is the main landmark muscle of the neck. Knowledge of variations of this muscle is very important for radiologist, surgeons, chiropractitioners, plastic surgeons and clinicians in general. We report a unique unilateral variation of SCM observed in an adult male cadaver. The clavicular head of right SCM had four fleshy bellies (B1, B2, B3 and B4). The most medial belly (B1) had three tendons of origin; the next belly just lateral to it (B2) had three tendons of origin; third belly (B4) had four tendons of origin and the lateral most belly (B4) had two tendons of origin. Thus, including the tendon of sternal head, in total, the right SCM had thirteen heads of origin. To the best of our knowledge, this is the first report on a thirteen headed sternocleidomastoid muscle. We review the literature and discuss the clinical importance of the variation in this report.