Variation of the sternocleidomastoid muscle: a case report of three heads and an accessory head.

ABSTRACT

PURPOSE: Anatomical variations of the sternocleidomastoid muscle (SCM) have been observed to occupy multiple origins and insertion points and have supernumerary heads, sometimes varying in thickness. METHODS: An 85-year-old Korean male cadaver was dissected and a SMC with unilateral supernumerary heads was identified. RESULTS: Three heads and an accessory belly of the sternal head were recorded on the left side. There were two clavicular heads, one lateral and one medial. The sternal head originated inferior to the manubrium of the sternum. A small belly arose from the sternal head and merged to the lateral clavicular head on the C5 vertebrae level. These bellies inserted into the mastoid process of the temporal bone. CONCLUSIONS: These variants may be problematic during surgical approaches to the upper neck and occipital, and should thus be appreciated by the clinician.