Multifocal Motor Neuropathy masquerading as a primary peripheral nerve tumor

Objective: To describe a case multifocal motor neuropathy without conduction block (CB) misdiagnosed as a primary peripheral nerve tumor Background: Multifocal motor neuropathy (MMN) is a rare immune-mediated motor neuropathy with slowly progressive, predominantly distal, asymmetrical primarily upper limb weakness in individual motor nerve distribution with or without persistent conduction block preferentially affecting young man. Clinical response to intravenous immunoglobulin (IVIG) has been reported (PRuppers et al, JPNS: 20: 306–318) but there is a subgroup with irreversible axonopathy. While diagnosis is based on electrodiagnosis and clinical presentation, MRI maybe performed to rule out other etiologies of nerve injury. Design/Methods: We report the case of a 16 year-old man with acute onset right-sided finger drop with progression to the median nerve and ulnar over time. Inflammatory work up, spinal fluid indices (glucose, protein, cell count), HNPP, Anti-GM1 and cervical spine and upper limb MRI were unrevealing. Results: MMN was diagnosed and treatment with high dose IVIG (1g/kg) was given for 6 months every 3 weeks without improvement in finger extension but in grip. The patient has undergone 3 years later further studies due to isolated PIN weakness. MRI showed a mass lesion and enhancement of the PIN and nerve sheath tumor was suggested. Nerve biopsy showed axonopathy and onion bulb formation confirming the prior diagnosis of MMN. After the biopsy patient has experienced new wrist extension weakness but has refused treatment with IVIG. Conclusions: This is a patient with permanent PIN axonal injury due to MMN and MRI mistakenly diagnosed as primary nerve sheath tumor. The case highlights the importance of recognition of MRI findings of MMN. Disclosure: Dr. Meira Benchaya has nothing to disclose. Dr. Bauman has nothing to disclose. Dr. Wu has nothing to disclose. Dr. Jani-Acsadi has nothing to disclose.