46. CN XI Testing

What is Being Tested?

Weakness in the sternocleidomastoid or trapezius muscles can be caused by lesions in the muscles, neuromuscular junction, or lower motor neurons of the accessory spinal nerve (CN XI) (see KCC 12.7 in Neuroanatomy through Clinical Cases 3e). Unilateral upper motor neuron lesions in the cortex or descending pathways cause contralateral weakness of the trapezius, with relative sparing of sternocleidomastoid strength. This may be remembered by analogy to upper motor neuron facial lesions sparing the upper portion of the face. When sternocleidomastoid weakness is present with upper motor neuron lesions, there is weakness of head turning away from the side of the lesion (see also KCC 13.10 in Neuroanatomy through Clinical Cases 3e).