Activity-dependent hyperpolarization and conduction block in chronic inflammatory demyelinating polyneuropathy

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.