The spinal accessory nerve (XI cranial nerve) injury is an unusual clinical entity and, sometimes, of complex diagnosis. The objective of this study is to describe the syndromic picture attending to its main etiologic factors, the different forms of presentation and the value of the neurophysiological studies, especially of electromyography in its diagnosis. The information that neurophysiology brings is of great value at the moment of establishing a precocious diagnosis as well as in the evolution and prognosis of the lesion. There are few available data in the literature that describe neurophysiological techniques for its correct management.