C. Cappelen-smith et al., Activity-dependent hyperpolarization and conduction block in chronic inflammatory demyelinating polyneuropathy, ANN NEUROL, 48(6), 2000, pp. 826-832
Voluntary activity produces activity-dependent hyperpolarization of the act
ive motor axons. The present study investigated whether this hyperpolarizat
ion produces conduction block in chronic inflammatory demyelinating polyneu
ropathy (CIDP). Studies were performed in 10 healthy control subjects, 7 pa
tients with CIDP, and 3 patients with multifocal motor neuropathy. The comp
ound muscle action potential (CMAP) of the abductor pollicis brevis was rec
orded in response to supramaximal stimuli to the median nerve at the wrist,
alternating with measurements of axonal excitability. After a maximal volu
ntary contraction for 60 seconds, the amplitude of the maximal CMAP was sig
nificantly reduced in symptomatic CIDP patients by 40%, but there were only
slight changes in the CMAPs of healthy controls, asymptomatic CIDP patient
s, and multifocal motor neuropathy patients. In symptomatic CIDP patients,
the activity-dependent conduction block paralleled the activity-dependent h
yperpolarization and was presumably precipitated by it. In these patients,
the safety margin for impulse conduction was estimated to be about 12%, Act
ivity-dependent conduction block may be clinically important in chronic dem
yelinating diseases and can be demonstrated electrophysiologically if testi
ng occurs across pathological sites.