A Unilateral Sternocleidohyoid Muscle Replacing the Omohyoid Muscle: an Anatomical Case Report with Surgical and Radiologic Implications

Abstract The omohyoid muscle is the most variable component of the infrahyoid musculature and frequently displays atypical origins, insertions, and fusions with neighboring muscles. During routine dissection of the neck in a middle-aged female cadaver fixed in 10% formalin, we identified an unrecorded unilateral variant in which the omohyoid was absent and replaced by a sternocleidohyoid muscle. This muscle arose from distinct sternal and clavicular heads that joined to form a common belly, continued into a true intermediate tendon firmly attached to the wall of the internal jugular vein, and ascended to insert into the hyoid bone while remaining completely separate from a normally developed sternohyoid muscle. Within the recently proposed classification of the omohyoid, this configuration is consistent with a Type IIIA variant, combining an additional sternal belly with an anomalous clavicular origin. Awareness of such omohyoid-replacing variants is essential to avoid misinterpretation of imaging, to prevent inadvertent injury during neck surgery and reconstructive procedures, and to maintain accurate identification of muscular and venous landmarks during ultrasound-guided or open cervical interventions.