Amyloid neuropathy: a retrospective study of 35 peripheral nerve biopsies

We performed a retrospective study of 35 peripheral nerve biopsies (PNBs) with amyloid deposits in the endoneurium. In every case, nerve lesions were studied on paraffin-embedded fragments (PEFs) and by ultrastructural examination (USE). In addition, muscle fragments were taken and embedded in paraffin. Immunohistochemistry was performed with anti-transthyretin (TTR) serum on 19 nerve and 15 muscle PEFs. Direct immunofluorescence with anti-light-chain sera was performed on frozen nerve fragments in 19 cases. Endoneurial amyloid deposits were easily identified on routine PEF in 26 cases, after Congo red or thioflavine staining in three, and by USE in six. A dramatic myelinated fiber loss was evidenced in 34 cases (77-2970 per mm2), and features of axonal degeneration were present in every case. Segmental demyelination was observed in 10 cases. A mutation in the TTR gene was present in 14 cases, with Met30 mutation in 10 and Ala49 in four members of the same family. Amyloid deposits were strongly marked by the anti-TTR serum in 11 other cases, twice in the endoneurium, five around muscle fibers, and four in both locations. In eight patients, light-chain positivity was evidenced in endoneurial deposits, lambda in six and kappa in two. Two other patients with monoclonal gammopathy did not present any light-chain fixation. In 17 cases, amyloidosis was disclosed by PNB and 13 had a TTR pathology; eight of them, over 65 years old, correspond to a late-onset form of familial amyloid polyneuropathy which is an underdiagnosed condition.