Accessory nerve palsy.

Accessory nerve palsy. Clinically this refers to paralysis of the spinal-originating fibres of the accessory nerve (eleventh cranial nerve) which innervates the sternocleidomastoid and trapezius muscles. The spinal portion is more commonly damaged in the mid-portion of the posterior triangle of the neck (a) where it courses superficially just underneath the fascial roof where it can be damaged from stab wounds or iatrogenically from lymph node biopsy (b). Other pathologies such as posterior triangle abscess (c) or malignant nodal involvement can also cause paralysis of the trapezius muscle, with weakness in shoulder raising and a permanent droop on that side. Where the nerve is damaged proximal to the sternocleidomastoid muscle (which the nerve pierces) difficulty in turning the head towards the opposite side is experienced.