Lewis-Sumner Syndrome Mimicking Idiopathic Brachial Plexopathy: A Case Report with Electrophysiological, Ultrasonographic, and Magnetic Resonance Imaging Findings

Multifocal acquired demyelinating sensory and motor neuropathy (MADSAM), also known as Lewis-Sumner syndrome, is a rare subtype of chronic inflammatory demyelinating polyradiculoneuropathy.We report a patient who initially presented with features suggestive of idiopathic brachial plexopathy and was ultimately diagnosed with MADSAM.A 43-year-old man presented with a 3-year history of progressive weakness in the right distal upper extremity and paresthesia in the fingers and forearm.Initial nerve conduction studies revealed characteristic demyelinating features, including definite conduction block in the right upper extremity.Ultrasonography showed swelling of the cervical roots of the brachial plexus, and magnetic resonance imaging (MRI) demonstrated increased signal intensity and multifocal swelling of the right C5-C8 roots and brachial plexus trunks.Additional nerve conduction studies of the lower extremities revealed conduction block and temporal dispersion of the right tibial nerve around the popliteal fossa.Right sciatic and tibial nerve swelling was also demonstrated on knee MRI.The patient was ultimately diagnosed with MADSAM and received oral corticosteroids and intravenous immunoglobulin treatment.This report emphasizes the need to integrate electrodiagnostic, ultrasonographic, and MRI findings when evaluating a patient with symptoms suggestive of brachial plexopathy, particularly because this rare inflammatory neuropathy is treatable.