The nosological status of multifocal motor neuropathy remains controve
rsial. The clinical and electrodiagnostic hallmarks suggest selective
motor fiber involvement. In this study, we asked to what extent sensor
y nerves might be involved pathologically in multifocal motor neuropat
hy. Examination of sensory nerve biopsy specimens from 11 patients did
reveal pathological findings in all, but they were very mild. An incr
eased number of thinly myelinated, large-caliber fibers was the unifyi
ng feature common to each specimen. By electron microscopy, each biops
y specimen had thinly myelinated fibers surrounded by minor onion bulb
s. Active demyelination, though scant, was seen in 3 nerves. Myelinate
d fiber density was normal. Subperineurial edema and inflammation were
not present We conclude that multifocal motor neuropathy is not an ex
clusively motor abnormality, although it appears to be so clinically a
nd electrophysiologically. The frequent, albeit mild, pathological abn
ormalities in sensory fibers suggest that the demyelinating pathophysi
ology also affects sensory fibers, but to a lesser degree than motor f
ibers. Some investigators maintain that multifocal motor neuropathy is
within the spectrum of chronic inflammatory demyelinating polyneuropa
thy. The very mild degree of sensory fiber involvement, the absence of
inflammation or edema, and the distinctive clinical features support
the concept of multifocal motor neuropathy as distinct from chronic in
flammatory demyelinating polyneuropathy.