SUPERNUMERARY HEADS OF THE STERNOCLEIDOMASTOID MUSCLE AND THEIR IMPLICATIONS FOR INTERNAL JUGULAR VEIN VENOUS ACCESS: AN ANATOMICAL CASE REPORT

Abstract

The sternocleidomastoid muscle (SCM) is an important anatomical landmark for cervical procedures, particularly internal jugular vein cannulation. Variations involving supernumerary heads may alter cervical neurovascular relationships and increase procedural complexity. This study reports an anatomical variation identified during routine cadaveric dissection at the Human Anatomy Laboratory of the Universidade do Estado do Amazonas. In a formalin-fixed adult male cadaver, the right SCM presented four heads of origin, consisting of one sternal head and three independent clavicular heads. The variation resulted in narrowing of the lesser supraclavicular triangle and greater muscular coverage over the presumed course of the internal jugular vein. No associated vascular alterations were identified. These findings demonstrate the potential impact of SCM variants on landmark-guided internal jugular vein access, as additional muscular heads may distort conventional anatomical references and increase the risk of technical difficulty and iatrogenic complications. Furthermore, such variations are relevant in surgical planning and imaging interpretation. This report reinforces the importance of anatomical knowledge and cadaveric studies for safer cervical procedures and improved diagnostic accuracy.