Widening of myelin lamellae in polyneuropathy with immunoglobulin-M monoclonal gammopathy, without activity against myelin-associated glycoprotein, responsive to treatment

•In anti-MAG neuropathy; widenings of myelin lamellae (WML) are classically observed in the nerve biopsy (electron microscopy).•WML may be observed in polyneuropathy associated to IgM monoclonal gammopathy without anti-MAG activity.•IgM may target an unknown myelin antigen responsible for the nerve lesions similar to those observed in anti-MAG polyneuropathy.•The good response to plasma exchange and/or rituximab suggests an autoimmune origin.•Nerve biopsy may help to confirm the dysimmune process and to select an efficient treatment.