What are most likely causes of spinal accessory nerve xi, bilateral denervation with fibrillations on emg?

Cranial nerve: Focal problems with accessory nerve have been seen with trauma, biopsy of shoulder masses or enlarged lymph nodes, and rarely accompanying peripheral neuropathies. Bilateral involvement is indeed rather unusual, unless there is a mass affecting the lower brainstem, or upper neck. If your EMG was done correctly, it would behoove you to investigate all of this.