A Rare Bilateral Tripartite Configuration of the Sternocleidomastoid Muscle: A South Indian Cadaveric Case Report with Clinical Pertinence

The sternocleidomastoid (SCM) muscle is an important landmark muscle of the neck that normally arises by two heads, namely the sternal and clavicular heads. Variations in its origin, insertion, and number of muscular slips are uncommon but clinically significant. During routine dissection sessions for undergraduate students, a rare tripartite variation of the SCM muscle was observed in a male cadaver. In this variation, the muscle presented with three distinct heads instead of the typical bipartite arrangement described in the standard anatomical literature. Knowledge of such anatomical variations is important for anatomists, radiologists, surgeons, and clinicians to prevent diagnostic and surgical errors. During cadaveric dissection of the neck region in a male cadaver, the SCM muscle was found to possess an additional muscular slip, resulting in a tripartite arrangement. The accessory slip appeared to arise from the clavicular or sternal attachment along with the normal heads of origin. The muscle fibers merged superiorly and followed the normal course toward their insertion. This anatomical variation altered the typical morphology of the muscle and its spatial relationship with adjacent neurovascular structures of the neck. Embryologically, this anomaly may have resulted from atypical splitting of the mesoderm of the postsixth branchial arch during organogenesis. The presence of a tripartite SCM muscle has important clinical implications. Additional muscle slips may modify the biomechanics of neck movements, particularly rotation and lateral flexion of the head. Such variations may also alter the anatomical relations of the carotid sheath, internal jugular vein, spinal accessory nerve, and cervical plexus. Awareness of this variation is essential during surgical procedures involving the neck, central venous catheterization, reconstructive surgeries, and radiological interpretation to avoid misdiagnosis or accidental injury to adjacent structures. Furthermore, these muscular anomalies may be mistaken for soft-tissue masses during imaging studies if not properly recognized. The tripartite SCM muscle represents a rare anatomical variation with significant educational and clinical relevance. Recognition and documentation of such variations contribute to a better understanding of cervical anatomy, embryological development, and surgical anatomy. Awareness of these anomalies is crucial for clinicians, surgeons, and radiologists to minimize procedural complications and ensure accurate anatomical interpretation.