Multifocal Motor Neuropathy: Complementary Role of Ultrasound

Abstract

Background: Multifocal motor neuropathy (MMN) is an uncommon, asymmetric motor neuropathy. As MMN is a treatable disorder, its differentiation from lower motor neuron disease is important. Evidence of conduction block (CB) or positive IgM anti-GM1 is considered one of important markers for the diagnosis. However, some patients with atypical MMN have no detectable CB or anti-GM1 antibodies. Methods: A 40-year-old man presented with twoyear history of progressive left-sided weakness without sensory symptoms. We experienced a case of MMN with focal nerve enlargement on ultrasound. Results: Nerve conduction study revealed partial conduction block in two nerves. Anti-GM1 antibody is negative and other laboratory studies were unremarkable. Ultrasound showed focal nerve enlargement of right median nerve at the forearm and left ulnar nerve at the elbow (Fig. 1). Weakness improved completely after intravenous immunoglobulin. Ultrasound image shows focal enlargement of median and ulnar nerve (arrow). (A and B) The cross-sectional area (CSA) of right (A) and left (B) median nerve at the wrist is normal. (C and D) The CSA of right (C) median nerve at the forearm is increased at 15.2 mm2 and left (D) median nerve is normal (5.9 mm2). (E and F) The CSA of right (E) and left (F) median nerve at the upper arm is normal. (G and H) The CSA of right (G) and left (H) ulnar nerve at the wrist is normal. (I and J) The CSA of right (I) ulnar nerve at the elbow is normal (3.5 mm2), but left (J) ulnar nerve is increased at 11.1 mm2. t; tendon, c; carpal bone, fds; fl exor digitorum superficialis, fdp; fl exor digitorum profundus, b; brachial artery, br; brachialis, u; ulnar artery, me; medial epicondyle. Conclusion: Ultrasound can be a valuable tool in supporting the diagnosis of MMN.