A study of sternocleidomastoid muscular atrophy after modified neck dissection.

To elucidate the conditions of atrophy in the sternocleidomastoid muscle (SCM) after modified neck dissection (MND), we tried to scrutinize the atrophic regions in the SCM and determine the cause of atrophy, by electromyography in 40 patients with SCM atrophy following MND. We also examined the detailed anatomy of the SCM in 40 cadavers. Atrophy was observed in the caudal portion in the SCM in 90% of the patients. Electromyographic examination revealed neurogenic atrophy in 24 patients, ischemic atrophy in 11, and a mixing of both types in 5. The SCM was found to be innervated by the spinal accessory nerve (SAN) and the SCM branch of the cervical nerve. The main artery feeding the cranial half of the SCM was a branch of the occipital artery (Oc) or the external carotid artery (Ex), and the auxiliary artery was a branch of the posterior auricular artery (Ap). The main artery feeding the caudal half of the SCM was a branch of the superior thyroidal artery (St), and the auxiliary artery was a branch of the subclavian artery (Sc). Postoperative SCM atrophy is attributed to damage of the feeding artery in the SCM caudal portion and local damage in the nerve fibers running through the SCM. To prevent this type of atrophy, it is important to carefully handle this muscle itself and protect the nerve fibers running through it, as well as to conserve the SCM branches of the St and Sc.