Motor Unit Firing Rate Alterations Found in CIDP

Patients with chronic inflammatory demyelinating polyneuropathy (CIDP) show significant abnormalities in motor unit (MU) firing rates, according to study results published in the Journal of the Neurological Sciences.

Generation and control of muscle force is affected by the functioning numbers of MU and their firing rate. Previous studies revealed that CIDP usually results in multifocal and segmental demyelination, along with conduction block, axonal loss and denervation, and eventually muscle atrophy and MU loss.

In the current study the researchers aimed to investigate the MU firing characteristics in patients with CIDP, and as these patients often have problems with gait and balance, they chose to focus on a leg muscle, the tibialis anterior.

The study cohort included 7 healthy participants serving as controls (4 male, 3 female) and 8 patients with CIDP (6 male, 2 female). MU firing rates were recorded at 4 levels of voluntary isometric dorsiflexion contractions (25%, 50%, 75% and 100% of maximal voluntary contraction [MVC]).  

Patients with CIDP were 33% weaker with regards to dorsiflexor MVC strength compared to controls. The mean MU firing rates of the CIDP group were 18.8±1.1, 16±6.3, 18.1±2.0, and 17.1±1.7 at 25%, 50%, 75% and 100% of MVC, respectively. The respective MU firing rates of the control group were 13±2.3, 18.2±4.4, 31.6±3.8, and 40.1±2.3.

These findings indicate that while in the control group there was a strong positive correlation between MU firing rate and MVC%, in the CIDP group there was a weak negative correlation, as the MU firing rates were higher at lower contraction intensity and not significantly higher at intensities of 75% and 100% MVC.

Compared to controls, patients with CIDP demonstrated higher MU firing rate range at 25% of MVC, but the rates in the CIDP group were significantly lower at 75% and 100% of MVC. At 50% of MVC there was no difference in MU firing rate between the groups.

Surface root mean squared electromyography (EMG) normalized to MVC indicated that both groups had a positive linear relationship between contraction intensity bins and surface EMG. However, surface EMG was lower in patients with CIDP compared to controls at 50 and 75% MVC.

“These findings of altered MU firing rates and possible MU block in patients with CIDP may be fundamental in explaining some of the functional consequences related to weakness and atrophy”, conclude the researchers.

Reference

Gilmore KJ, Kirk EA, Doherty TJ, Kimpinski K, Rice CL. Abnormal motor unit firing rates in chronic inflammatory demyelinating polyneuropathy[published online April 29, 2020]. J Neurol Sci. doi:10.1016/j.jns.2020.116859