The aim of this study was to elucidate the distribution of the accessory nerve within the sternocleidomastoid muscle (SCM) to aid identifying the optimum sites for botulinum neurotoxin (BoNT) injections and applying chemical neurolysis. Thirty SCM specimens from 15 Korean cadavers were used in this study. Sihler's staining was applied to 10 of the SCM specimens. Transverse lines were drawn in 20 sections to divide the SCM into 10 divisions vertically, and a vertical line was drawn into the medial and lateral halves from the mastoid process to the sternoclavicular joint. The most densely innervated areas were 5/10–6/10 and 6/10–7/10 along the lateral and medial parts of the muscle, respectively. We suggest injecting BoNT in the medial region 6/10–7/10 along the SCM prior to injecting in the lateral region 5/10–6/10 along the muscle to ensure safe and effective treatment. Clin. Anat. 33:192–198, 2020.
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