Electrophysiological features of late-onset transthyretin Met30 familial amyloid polyneuropathy unrelated to endemic foci.

BACKGROUND: Through the development of gene diagnostic techniques, late-onset transthyretin Met30-associated familial amyloid polyneuropathy (FAP TTR Met30) has been shown to be more prevalent than is generally believed.
OBJECTIVE: To examine the electrophysiological features of late-onset FAP TTR Met30 unrelated to endemic foci.
METHODS: Nerve conduction findings in 44 cases with an onset of more than 50 years of age in a non-endemic area were assessed and compared with findings from 21 earlier-onset cases related to endemic foci.
RESULTS: The extent of the reduction of the compound muscle action potential and, especially, the sensory nerve action potential was more profound in the late-onset group even when the decline of these indices with aging in normal control subjects was taken into account. The feature of predominant lower-limb involvement seemed to be more conspicuous in the late-onset group. Electrophysiological indices tended to be aggravated as the duration of neuropathic symptoms increased in the early-onset group, while most of these indices in the lateonset group did not show this correlation. A slowing of conduction velocity and a prolongation of distal latency, which suggests demyelination, were conspicuous in some patients. Pathologically, a predominant loss of small-fibers was not conspicuous in sural nerve biopsy specimens from late-onset patients. Large myelinated fiber density showed a negative correlation with the disease duration in early-onset cases, but not in late-onset cases.
CONCLUSIONS: Electrophysiological differences between late- and early-onset cases were present, probably reflecting the different underlying pathogenic mechanisms of neuropathy. The demyelinating feature does not exclude the possibility of this disease.