Dissociated weakness of sternocleidomastoid and trapezius muscles with lesions in the CNS.

We describe three patients with lesions in the CNS that produced dissociated weakness of trapezius (TM) and sternocleidomastoid muscles (SCM). One patient with a right cerebral hemisphere lesion had left TM weakness and right SCM weakness. A second, with a brainstem lesion, and a third, with a high cervical cord lesion, had TM weakness with preserved SCM strength. We discuss the findings according to the known neuroanatomy.