Spontaneous muscle activity in multifocal motor neuropathy – Insights from axonal excitability testing

Objective
To investigate motor axonal excitability in multifocal motor neuropathy (MMN) associated with involuntary muscle activity.
Methods
Two MMN patients with continuous involuntary finger movements (MMNifm) were compared to 11 patients without movements (MMNnfm). Clinical examination, EMG of the abductor pollicis brevis muscle, nerve conduction studies, motor unit number estimation, excitability studies, and mathematical modeling were conducted in the patients with MMN and compared to controls.
Results
Weakness, axonal loss, conduction block, or both occurred in the median nerve in the MMNifm and the MMNnfm patients. Ultrasound studies (US) in MMNifm showed enlargement of the nerves at the axilla/brachial plexus at the site of the conduction block. In MMNifm, EMG and the US showed continuous involuntary contractions, and excitability studies of the median nerve at the wrist showed increased threshold reduction during early depolarizing electrotonus and at early recovery cycle (superexcitability). Mathematical modeling was consistent with reduced fast K+ current more pronounced in MMNifm than in MMNnfm.
Conclusions
MMN may have a spectrum of changes associated with instability of the axonal membrane which may be due to paranodal myelin loosening.
Significance
In addition to motor conduction block and axonal loss, MMN has pathophysiological changes that suggest more widespread involvement of motor myelinated fibers.