Monoclonal gammopathy of undetermined significance (MGUS) occurs in some patients with chronic inflammatory demyelinating polyneuropathy (CIDP), but its impact on clinical phenotype and treatment response remains unclear. We assessed the prevalence of paraproteinemia in CIDP and compared disease features between CIDP patients with and without MGUS. We used data from the International CIDP Outcome Study (ICOS), a prospective cohort study. We compared the prevalence and causes of paraproteinemia in CIDP to matched disease controls (axonal polyneuropathy or motor neuron disease) and compared disease features and treatment responses between CIDP patients with and without MGUS. Treatment response, defined as a ≥ 1-point improvement on the modified Rankin scale, was retrospectively assessed. IgG paraproteinemia was more common in CIDP than in controls (9%, 17/193 vs. 3%, 6/192; p = 0.03). IgM and IgA paraprotein prevalences did not differ. One CIDP patient had Waldenström macroglobulinemia; others had MGUS. Patients with IgG MGUS less often had an acute clinical presentation (6% vs. 33%; p = 0.02), more often had sensory deficits (94% vs. 67%; p = 0.02), and prolonged distal CMAP duration (64% vs. 31%; p = 0.02), compared to patients without MGUS. First-line treatment response rates were comparable (80% [IgG MGUS] vs. 67% [no MGUS]; p = 0.39). IgG MGUS is more prevalent in CIDP than in controls. Presence of IgG MGUS is weakly associated with some CIDP disease features, but not treatment response. These findings indicate that, although IgG MGUS is associated with CIDP, the presence of IgG MGUS does not constitute a distinct subgroup with unique clinical features or treatment implications.

This article reviews recent clinical updates and research on the evaluation and management of patients with peripheral neuropathy in association with monoclonal gammopathies. Recent studies have elucidated pathogenic mechanisms of IgM paraprotein associated neuropathies, including nodal and paranodal targets and complement mediated processes, suggesting novel therapeutic targets. New chemotherapeutic regimens have improved outcomes and neurotoxic side effect profiles in the treatment of patients with light chain (AL) amyloidosis and POEMS syndrome. Establishing when a monoclonal gammopathy is causative of a peripheral neuropathy remains a clinical challenge. New therapeutic drugs for treatment of IgM paraprotein associated neuropathies show promise. Identifying AL amyloidosis and POEMS early in patients presenting with neuropathy are important. Clinical phenotyping and antibody testing are critical to evaluating patients with paraproteins and peripheral neuropathy.

Antimyelin-associated glycoprotein (MAG) neuropathy is a rare autoimmune demyelinating peripheral neuropathy caused by IgM autoantibodies targeting MAG. The typical presentation is that of a slowly progressive, distal, length-dependent, predominantly sensory, sometimes ataxic neuropathy, frequently accompanied by upper limb tremor. Distal motor weakness may subsequently occur. The clinical presentation may vary and rarely be consistent with that of typical chronic inflammatory demyelinating polyradiculoneuropathy (CIDP), as well as have an aggressive and rapidly disabling course. The diagnosis of anti-MAG neuropathy is based on the detection of anti-MAG antibodies through ELISA or western blot analysis, primarily in presence of an IgM monoclonal gammopathy. Anti-MAG neuropathy may occur without or with haematological malignancy. Electrophysiology is characteristic of a predominantly distal demyelinating neuropathy. Intravenous immunoglobulins and plasma exchange have unproven benefits, but may provide short-term effects. Cytotoxic therapies are commonly used, although without an evidence base. Rituximab, an anti-B-cell monoclonal antibody was studied in two randomised controlled trials, neither of which achieved their primary outcome. However, a meta-analysis of these two studies demonstrated improvement of disability at 8-12 months. A recent trial with lenalidomide was interrupted prematurely due to a high rate of venous thromboembolism. There are currently two ongoing trials with Bruton's tyrosine kinase inhibitors. Symptom control is otherwise frequently needed. Outcome measures used for other inflammatory neuropathies present limitations in anti-MAG neuropathy. International registries such as the planned IMAGiNe study may, in future, provide answers to the many remaining questions.

Polyneuropathy associated with an immunoglobulin M (IgM) monoclonal gammopathy is characterized by slowly progressive, predominantly distal sensorimotor deficits, sensory ataxia, and electrophysiologic features of demyelination. IgM antibodies against myelin-associated glycoprotein (MAG) are present in serum from most patients. Nerve damage most likely results from the concerted action of binding of anti-MAG antibodies to nerves, followed by complement activation. The interaction of anti-MAG antibodies with complement activation and their relation to clinical characteristics have not been studied in detail. We studied the correlation among anti-MAG antibody titers, complement activation, and IgM-associated polyneuropathy disease severity. We used serum samples from 101 patients with IgM-associated polyneuropathy to assess IgM anti-MAG titers by ELISA and antibody-mediated complement deposition using both an ELISA-based system and a cell-based system of primate peripheral nerve slides. We studied correlations of complement activation with anti-MAG ELISA titers and clinical characteristics. IgM anti-MAG titers varied from negative to strongly positive. Complement deposition in the ELISA-based system correlated significantly with anti-MAG antibody titer (Spearman rho 0.80; p < 0.0001) despite large variability between serum samples with comparable anti-MAG titers. This variability was even larger in the cell-based assay, which also showed complement deposition in IgM anti-MAG negative patients, indicating the presence of autoantibodies directed against epitopes other than MAG in a subset of patients with IgM-associated polyneuropathy. Clinical characteristics did not correlate with anti-MAG titers or complement activation. Anti-MAG antibody titers correlate with the level of complement activation in both ELISA and cell-based systems. However, clinical characteristics of IgM-associated polyneuropathy do not or only weakly correlate with titers or the level of complement deposition. The lack of clear correlations between complement activation and clinical characteristics does, at this stage, not support the use of complement inhibitors in the treatment of IgM-associated polyneuropathy.

Monoclonal gammopathy is observed in approximately 10% of patients with peripheral neuropathy, which is particularly common in those with the immunoglobulin M monoclonal component. The diagnostic challenge lies in determining whether the neuropathy is causally related to the paraprotein or merely coincidental. Physicians must be familiar with the spectrum of monoclonal gammopathy-associated neuropathies, including anti-myelin-associated glycoprotein neuropathy, amyloid light chain amyloidosis, POEMS (polyneuropathy, organomegaly, endocrinopathy, monoclonal gammopathy, and skin changes) syndrome, CANOMAD (chronic ataxic neuropathy, ophthalmoplegia, IgM paraprotein, cold agglutinins, and disialosyl antibodies), and cryoglobulinemia, as well as their clinical, electrophysiologic, and immunologic characteristics. Initial workup includes serum protein electrophoresis, immunofixation, free light chain analysis, and targeted antibody testing based on the paraprotein and neuropathy phenotype. Close collaboration between neurologists and hematologist-oncologists is essential for accurate diagnosis and appropriate treatment, which may involve both immunotherapy and targeted therapies for the underlying hematologic disorder.