IgM deposits on skin nerves in anti-myelin-associated glycoprotein neuropathy.

Anti-myelin-associated glycoprotein (anti-MAG) neuropathy is a chronic demyelinating neuropathy with predominant involvement of large sensory fibers and deposits of IgM and complement on sural nerve myelinated fibers. We assessed the presence of IgM deposits on skin myelinated nerve fibers and the involvement of unmyelinated axons in anti-MAG neuropathy. Skin biopsies were performed in 14 patients with anti-MAG neuropathy, in 8 patients with chronic inflammatory demyelinating polyradiculoneuropathy (CIDP), and in 2 patients with IgM paraproteinemic neuropathy. Biopsies were taken at the proximal thigh in 20 patients, at the distal leg in 21 patients, at the proximal arm in 13 patients, and at the hand or fingertip in 10 patients. We found IgM deposits on dermal myelinated fibers in all anti-MAG neuropathy patients, with a greater prevalence at the distal site of the extremities. Deposits were located throughout the length of the fibers and at the paranodal loops. CIDP and IgM paraproteinemic neuropathies did not show any deposit of IgM. Anti-MAG neuropathy and CIPD patients showed a decrease in epidermal nerve fiber density reflecting an associated axonal loss. In anti-MAG neuropathy, both large- and small-diameter nerve fibers are affected, and specific deposits of IgM are found on skin myelinated nerve fibers.