To report an unusual case IgM lambda anti MAG neuropathy, with CIDP like presentation and rapid progression.

Anti-MAG (myelin-associated glycoprotein) neuropathy is a rare immune-mediated polyneuropathy characterized by IgM monoclonal gammopathy, usually lambda predominant. The hallmark presentation is slowly progressive, sensory-predominant distal symmetric polyneuropathy with characteristic distal acquired demyelinating symmetric (DADS) pattern on electrodiagnostic studies. Patients typically experience gradual onset over years, with immunotherapy generally reserved for significant functional impairment.

A 72-year-old woman presented with six months of progressive numbness and tingling ascending from toes to ankles. Neurological examination demonstrated length-dependent sensory loss affecting pinprick and vibration modalities, absent ankle reflexes, gait ataxia, and positive Romberg sign. Electrodiagnostic studies revealed DADS pattern with markedly prolonged distal motor latencies and F-wave latencies exceeding 130% of normal, resembling a CIDP-like presentation. Serologic testing confirmed the diagnosis with a highly elevated anti-MAG antibody titer exceeding 62,000 units and IgM lambda monoclonal gammopathy. Bone marrow evaluation excluded lymphoproliferative malignancy. Despite initiation of symptomatic treatment with gabapentin, the patient experienced rapid clinical deterioration over three months, with sensory symptoms extending to the knees, proximal and distal lower extremity weakness, and development of upper extremity proprioceptive deficits. Rituximab therapy was initiated with a planned regimen of 1g every six months for one year. The patient tolerated initial infusion well.

This case demonstrates an atypically rapid progression of anti-MAG neuropathy with the markedly elevated IgM lamba antibody titer, in contrast to the typical IgM kappa, potentially correlating with rapid disease progression. It underscores the importance of recognizing the DADS electrodiagnostic pattern, pursuing anti-MAG antibody testing in appropriate clinical contexts. Early immunotherapeutic intervention in rapidly progressive anti-MAG neuropathy may prevent irreversible disability, though treatment response remains variable and requires long-term monitoring.