Classification of Sternocleidomastoid Muscle Abnormalities and Comparison of Their Incidence among Populations

We classified sternocleidomastoid muscle abnormality and calculated its incidence in a Thai population. Such abnormality should be considered to prevent complications from central venous catheterization.This study aimed to classify and investigate abnormalities of the sternocleidomastoid (SCM) muscle, which is commonly used as an anatomical landmark to indicate the correct position for central venous catheterization, in a Thai population. Thirty-five embalmed cadavers from northeast Thailand (19 females and 16 males) were systemically dissected to reveal the SCM muscle anatomy. Variations in SCM origin and insertion were recorded. The prevalence of abnormality was approximately 11.4% (4 of 35 cadavers) and did not differ by sex. Abnormalities were classified into 5 types based on origin, insertion, and presence of additional heads, as follows: type I (n=31; 88.6%), type II (n=1; 2.85%), type III (n=1; 2.85%), type IV (n=1; 2.85%), and type V (n=1; 2.85%). As the incidence of abnormality was more than 10%, surgeons must seriously consider this before insertion of a central venous catheter to avoid complications.