A typically distal, symmetrical, slowly progressive, sensorimotor, demyelinating neuropathy is caused by monoclonal IgM against myelin-associated glycoprotein (MAG) or, in benign IgM paraproteinaemia, antibodies against MAG, SGPG and SGLPG glycolipids.
This paper reports a patient with a neuropathy that fulfilled the diagnostic criteria for chronic inflammatory demyelinating polyradiculoneuropathy (CIDP) in whom IgM kappa anti-MAG/SGPG /SGPLG antibodies were present.
A 57-year-old man developed a slowly progressive, distal, sensorimotor neuropathy in the lower and then the upper limbs, associated with cranial nerves palsies in oro-pharyngo-laryngeal territory. EMG and nerve conduction studies confirmed a demyelinating neuropathy but with additional features, namely a disproportionate slowing of distal conduction and evidence for axonal degeneration in the lower limbs. Initial routine biochemical and haematological tests were normal. Serum protein electrophoresis was normal on a number of occasions. The third CSF sample taken was acellular with a moderately raised protein level. Monoclonal IgM–kappa against MAG/SGPG/SGLPG was detected and a serum anti-MAG antibody titre of 20059 BTU (n=≪1.000) was measured. A small IgM-kappa paraprotein was identified by immunofixation. Electron microscopy failed to show any nerve fibres with the expected widening of the outer myelin lamellae. There was no improvement after IV immunoglobulins, corticosteroids, plasma exchange or rituximab.
The conclusion is that it remains unclear whether this neuropathy is an atypical form of MAG associated polyneuropathy or a CIDP associated with anti-MAG. When EMG shows disproportionate distal motor slowing serum should be screened by immunofixation to look for small monoclonal components. Where IgM-MGUS are present, a search for anti-MAG/SGPG/ SGLPG should be undertaken to help in the choice of optimal treatment before exhibition of more aggressive treatments with agents such as cyclophosphamide.
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