Atypical Multifocal Motor Neuropathy Presenting With Isolated Spinal Accessory and Pectoral Nerve Involvement

Multifocal motor neuropathy (MMN) is a rare, immune-mediated disorder typically characterized by slowly progressive, asymmetric distal limb weakness. Cranial nerve involvement is distinctly uncommon, and isolated involvement of the spinal accessory nerve (CN XI) is exceedingly rare. We present the case of a 68-year-old male patient who presented with severe neck and shoulder wasting and a profound head drop. Despite the fact that standard conduction studies failed to demonstrate classic motor conduction blocks due to the highly proximal nature of the nerve involvement, a clinical diagnosis of MMN was suspected based on the asymmetric motor-only presentation, elevated Anti-GM1 antibodies, and steroid nonresponsiveness. The patient showed a robust and sustained functional recovery following treatment with intravenous immunoglobulin (IVIG), highlighting the importance of clinical judgment in atypical presentations of treatable neuropathies.