Polyneuropathy associated with IgM monoclonal gammopathy

Polyneuropathy associated with IgM monoclonal gammopathy is a chronic progressive neurological disorder in which an abnormal clone of B lymphocytes or plasma cells produces a monoclonal immunoglobulin M (IgM) protein that damages peripheral nerves. This condition is also known as anti-MAG (myelin-associated glycoprotein) neuropathy when the IgM antibodies target MAG, which is the most common subtype. The disease primarily affects the peripheral nervous system, leading to slowly progressive, predominantly sensory polyneuropathy. Patients typically experience numbness, tingling, and impaired sensation in the hands and feet (distal symmetric distribution), along with sensory ataxia (unsteadiness due to impaired proprioception), tremor, and progressive difficulty with balance and walking. Motor involvement, including distal weakness, may develop later in the disease course. Nerve conduction studies characteristically show a demyelinating pattern with disproportionately prolonged distal motor latencies. The underlying IgM monoclonal gammopathy is most often classified as a monoclonal gammopathy of undetermined significance (MGUS), though in some cases it may be associated with Waldenström macroglobulinemia (lymphoplasmacytic lymphoma) or other B-cell lymphoproliferative disorders. The condition predominantly affects older adults, with a male predominance. Diagnosis is established through a combination of clinical evaluation, nerve conduction studies, detection of IgM monoclonal protein in serum, and often testing for anti-MAG antibodies. Treatment remains challenging, as the neuropathy often responds poorly to standard immunotherapies. Therapeutic approaches include rituximab (an anti-CD20 monoclonal antibody), which has shown modest benefit in some patients, as well as plasma exchange, intravenous immunoglobulin (IVIg), and corticosteroids, though responses are variable. In cases associated with Waldenström macroglobulinemia, treatment directed at the underlying lymphoproliferative disorder may be warranted. The disease tends to follow a slowly progressive course, and while it is rarely life-threatening, it can cause significant disability over time due to progressive sensory loss and gait impairment.

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