Ultrasound-Guided Injection of the Sternocleidomastoid Muscle: A Cadaveric Study with Implications for Chemodenervation

Background: Ultrasound guidance may improve the accuracy of botulinum toxin injection, but studies of its potential for cervical dystonia treatment are lacking. Objective: To determine the accuracy of ultrasound-guided injection in the sternocleidomastoid muscle (SCM). Design: Observational study. Setting: Tertiary care university hospital. Participants: Eighteen embalmed cadavers. Interventions: In total, 36 SCMs from 18 embalmed cadavers were examined. One physician performed ultrasound scans to divide each SCM into quarters and evaluated its cross-sectional area (CSA) and thickness at each of three meeting points between adjacent quarters. Under ultrasound guidance, another experienced physician injected methylene blue solution at one of the three points, using the in-plane technique (12 specimens per point; right SCM 3 mL, left SCM 5 mL). One anatomist dissected all cadavers and measured the distance of dye dispersion along the longitudinal axis of each muscle. Dispersion ratio was calculated as longitudinal dye dispersion divided by SCM length. Main Outcome Measures: SCM thickness and CSA; dye dispersion patterns (dispersion distance and dispersion ratio). Results: SCM thickness and CSA were greatest at the middle injection point (mean ± SD of 6.6 ± 2.0 mm and 1.4 ± 0.6 cm², respectively). All injections were successful, except in one case where the SCM was thin and the dye reached the omohyoid muscle. Mean longitudinal dye dispersion and dispersion ratio were significantly greater when the volume was 5 mL. There were no statistically significant differences in dispersion patterns among the three injection points. Conclusions: Ultrasound-guided intramuscular injection can be performed with good accuracy in the SCM, as ultrasound can be used to evaluate SCM thickness and CSA. Higher volumes of injection solution appear to diffuse better, but further clinical studies are required to determine optimal injection volume.