Motor unit integrity in multifocal motor neuropathy: A systematic evaluation with CMAP scans

Area of Science:

• Neurology
• Neurophysiology
• Motor Neuron Diseases

Background:

• Multifocal motor neuropathy (MMN) involves progressive axonal loss, typically assessed by nerve conduction studies (NCS) measuring maximum compound muscle action potentials (CMAPs).
• Reinnervation can mask significant motor unit (MU) loss until advanced stages, limiting the diagnostic utility of standard NCS.

Purpose of the Study:

• To investigate motor unit (MU) characteristics in a large cohort of MMN patients using advanced CMAP scans.
• To compare MU parameters derived from CMAP scans with clinical, laboratory, and standard NCS findings.

Main Methods:

• CMAP scans were used to derive maximum CMAP amplitude (CMAPmax), motor unit number estimate (MUNE), and stimulus current thresholds (S5, S50, S95).
• Relative ranges of MU thresholds and largest MU amplitudes were calculated.
• These metrics were analyzed in 40 MMN patients and 24 healthy controls.

Main Results:

• MMN patients exhibited reduced CMAPmax and MUNE (P < .001) and increased largest MU amplitude as a percentage of CMAPmax (P < .001).
• A larger relative MU threshold range was observed in patients with anti-GM1 antibodies compared to those without and controls (P = .016, P < .001).
• Patients without anti-GM1 antibodies had larger maximal MU amplitudes than those with anti-GM1 antibodies and controls (P = .037, P = .044).

Conclusions:

• Motor unit loss is prevalent in MMN and accompanied by compensatory enlargement of remaining MUs.
• Anti-GM1 antibody status correlates with specific alterations in MU threshold ranges and amplitudes.
• CMAP scans provide valuable complementary information to routine NCS for monitoring MMN progression and treatment response.