Abnormal motor unit firing rates in chronic inflammatory demyelinating polyneuropathy.

OBJECTIVE:Patients with CIDP have impairments, including muscle weakness, that could be consequences of demyelination, conduction block, and eventually axonal loss and denervation, leading to muscle atrophy. Consequently, motor unit (MU) activation of the muscle may be impaired contributing to weakness; but this has not been explored in CIDP. METHODS:MU firing rates were recorded at four levels of voluntary isometric dorsiflexion contractions (25%, 50%, 75% and 100% of maximal voluntary contraction [MVC]) in 8 (6 male, 2 female) patients with CIDP and 7 (4 male, 3 female) controls. RESULTS:Patients with CIDP were 33% weaker. The mean MU firing rates of the CIDP group were ~ 19 Hz at 25%, ~16 Hz at 50% MVC, ~18 Hz at 75% MVC and ~ 17 Hz at 100% MVC. The controls had rates of ~13 Hz at 25%, ~18 Hz at 50% MVC, ~32 Hz at 75% MVC and ~ 40 Hz at 100% MVC. Surface root mean squared electromyography normalized to the MVC was less in patients with CIDP at 50 and 75% MVC. CONCLUSIONS:As a consequence secondary to MU loss, patients with CIDP demonstrate significantly lower mean firing rates at high contraction intensities, and higher mean firing rates at low contraction intensities.