Motor unit number index and compound muscle action potential amplitude

Area of Science:

• Neurology
• Electromyography
• Motor Unit Analysis

Background:

• Motor unit number index (MUNIX) is increasingly used as an alternative to motor unit number estimation (MUNE).
• Previous studies noted MUNIX values correlating more with compound muscle action potential (CMAP) amplitudes than MUNE values in amyotrophic lateral sclerosis (ALS) patients and healthy controls.

Purpose of the Study:

• To investigate the reasons behind MUNIX's closer resemblance to CMAP amplitudes than MUNE values.
• To determine the relationship between MUNIX, CMAP, and surface EMG interference pattern (SIP) amplitudes.

Main Methods:

• Theoretical analysis of MUNIX, CMAP, and SIP amplitude relationships.
• Reanalysis of existing data from ALS patients and healthy controls.

Main Results:

• Theoretical analysis indicates MUNIX is dependent on CMAP area and power when motor unit potentials extensively overlap, losing information on motor unit size and number.
• MUNIX values were sensitive to CMAP amplitude changes but not SIP amplitude changes.
• Reproducibility of MUNIX in healthy controls was primarily linked to CMAP properties.

Conclusions:

• MUNIX provides misleading information regarding motor unit numbers in healthy individuals.
• MUNIX offers limited insight into motor unit loss in ALS patients beyond what CMAP amplitude measurements provide.
• MUNIX should not be considered a valid MUNE method.