Activity-dependent conduction block in multifocal motor neuropathy: magnetic fatigue test

Area of Science:

• Neurology
• Clinical Electrophysiology

Background:

• Multifocal motor neuropathy (MMN) is frequently misdiagnosed as motor neuron disease (MND), particularly when conventional electrodiagnostic criteria for conduction block (CB) are not met.
• Activity-dependent conduction block (ADCB), a transient CB induced by exercise, has been identified in MMN but not in amyotrophic lateral sclerosis (ALS).

Purpose of the Study:

• To evaluate the diagnostic value of ADCB in differentiating MMN from ALS.
• To identify sensitive electrophysiological markers for MMN in patients lacking overt CB.

Main Methods:

• Nine MMN patients lacking conventional CB criteria underwent a magnetic fatigue test involving nerve root stimulation before and after a 1-minute maximal voluntary contraction.
• Compound muscle action potentials (CMAPs) were recorded from small hand muscles to assess for postexercise CB and temporal dispersion.

Main Results:

• Six out of nine MMN patients exhibited postexercise CB or temporal dispersion, primarily in the immediate postexercise period.
• Activity-dependent conduction block (ADCB) improved in an MMN patient following intravenous immunoglobulin (IVIg) treatment.
• Prolongation of CMAP duration (onset to positive peak) emerged as the most sensitive indicator for MMN, potentially due to phase cancellation obscuring other abnormalities.

Conclusions:

• The magnetic fatigue test is effective in detecting mild, potentially proximal, conduction block in MMN patients who do not meet standard electrodiagnostic criteria.
• This test aids in differentiating MMN from ALS, especially in challenging diagnostic cases.