• infant;
  • normal;
  • sternocleidomastoid muscle;
  • torticollis;
  • ultrasonography

Introduction: This study was performed to obtain normative ultrasonographic data of the sternocleidomastoid (SCM) muscle in infants and to describe an ultrasound method for evaluating muscular torticollis. Methods: The thickness and cross-sectional area (CSA) of the sternal and clavicular heads of the SCM in 84 subjects were analyzed retrospectively. The diagnostic performance of ultrasonography (US) was estimated using receiver-operating characteristic analysis. Results: Muscle thickness increased with age until 10 months. There were no significant differences in thickness or CSA between the right and left sides (P > 0.05) in normal subjects. A ratio of the thickness of the affected side to the unaffected side for the sternal head of the SCM muscle of >1.19 showed a diagnostic sensitivity of 97.9% and specificity of 96.4%. Conclusions: Distinguishing the heads of the SCM muscle and comparing the thickness of the same head on both sides using US may be helpful for evaluating torticollis in infants. Muscle Nerve 000: 000–000, 2013