Spinal Accessory Neuropathy (SAN)! The spinal accessory nerve, one of the two roots of CN XI (the other root is the cranial), innervates the Trapezius and Sternocleidomastoid muscles. Spinal accessory neuropathy is commonly a result of injury during neck dissection/surgery. Often a brachial plexus MRI is ordered to confirm the clinical symptoms of trapezius muscle weakness. Similar to other neuropathies, positive MRI findings are usually related to edema and/or atrophy of the muscle innervated by the damaged/affected nerve. Although an indirect finding, muscle edema is sensitive for neuropathy but obviously not specific. With MRI, we may be able to see the culprit causing the neuropathy, such as a mass or transaction. But in the case of neuritis, for example, the nerve usually looks normal on MRI. In the case of SAN, the overwhelming most common MRI finding is edema and/or atrophy of the trapezius muscle. The sternocleidomastoid muscle is usually uninvolved. When you see homogeneous muscle edema or atrophy, think about denervation or neuropathy.
