Morphological changes in IgM paraproteinaemic neuropathy.

Sural nerve biopsies were examined from two patients with neuropathy associated with IgM kappa [anti-myelin-associated glycoprotein (MAG)] paraproteinaemia. Both nerves had a moderate loss of myelinated fibres. The pathology in one was of a chronic primary demyelinating type, in the other it was associated with axonal atrophy. Widened myelin (WM) was seen in both nerves affecting over 80% and 50% of myelinated fibres, respectively. The WM was associated with deposition of material which sometimes appeared granular but could also display a highly organised pattern, an appearance not previously described in these neuropathies. Granular material was also identified at the external surface of the Schwann cells of myelinated, but not of unmyelinated, fibres. WM was seen not only at the outer lamellae (a commonly observed site) but also at terminal myelin loops at the paranode, at Schmidt Lanterman incisures and at the inner and outer mesaxon. Material was also seen on the inner (adaxonal) Schwann cell surface. These are all sites associated with the presence of MAG. Other pathological features are described, including evidence of impairment of remyelination, abnormal Schwann cell/axon specialisations and the presence of tomaculous bodies. The implications of these findings are discussed.