Polyneuropathy associated with monoclonal gammopathy of undetermined significance: further evidence that IgM-MGUS neuropathies are different than IgG-MGUS.

We evaluated the clinical characteristics and electromyographic features of 39 patients with monoclonal gammopathy of undetermined significance (MGUS) and neuropathy. Twenty-three patients had a monoclonal IgM protein, 13 had an IgG, and three had an IgA. In 15 patients of the IgM group, the M protein reacted with myelin-associated glycoprotein (MAG). Comparing IgM-MGUS and IgG-MGUS neuropathies, we found the following differences: (1) There was a statistically significant higher frequency of sensory loss in the IgM group. (2) Nine attributes of nerve conduction abnormality were statistically worse in the IgM group, with slowing of conduction velocities and prolonged distal latencies. (3) The frequency of monoclonal IgM was overrepresented in the MGUS neuropathy group. In general, the clinical and electrophysiologic features of the IgM-MGUS MAG-reactive group were not significantly different than the MAG-nonreactive group. Our cases are similar to those previously reported and suggest that monoclonal IgM-MGUS should be separated conceptually from monoclonal IgG neuropathies.