ANATOMICAL VARIATION OF THE STERNOCLEIDOMASTOID MUSCLE; A CASE REPORT

Purpose: The sternocleidomastoid (SCM) muscle is a prominent neck muscle. It is the main flexor muscle of the neck and an accessory muscle during deep inspiration. Variations of the SCM have been widely recognized and described in medical literature. Supernumerary clavicular origins separating into individual bellies are a defined variant. However, the existence of a thyrohyoid insertion has not previously been described. Methods: Medical students performed a routine cadaver dissection in the University of North Texas Health Science Center's Gross Anatomy Lab. Results: A sternocleidomastoid muscle with four heads was identified on the left side of a 59 year old male cadaver whose cause of death is liver cancer. A typical SCM has origins on the sternal head and clavicular head, and an insertion on the mastoid process. This male cadaver however has two distinct abnormal findings. The SCM shows three clavicular origins separating into individual bellies. Additionally a unilateral thyrohyoid insertion is present, originating from the distal end of the sterno-mastoid belly. While the supernumerary SCM bellies can be explained by abnormal mesodermal splitting, the thyrohyoid insertion presents as a genetic abnormality not previously reported. Conclusions: Surgical awareness of abnormalities is significant, especially in regards to the existence of supernumerary posterior triangles and the additional thyrohyoid head. Furthermore, the clinical implications suggest possible tension headaches, torticollis and even infringement on the thyroid cartilage during swallowing and speech.