Counting motor units in chronic motor neuropathies

Area of Science:

• Neurology
• Electrophysiology
• Neuroscience

Background:

• Chronic motor neuropathies like spinal muscular atrophy (SMA) and Charcot-Marie-Tooth (CMT) present challenges in quantifying motor unit loss.
• Routine electrodiagnostic testing and pathological examination have limitations in accurately assessing the degree of axonal loss.

Purpose of the Study:

• To utilize motor unit number estimation (MUNE), an electrophysiologic technique, to quantify axonal loss in SMA and CMT.
• To investigate the pattern and degree of motor unit loss in different types and muscle groups affected by SMA and CMT.

Main Methods:

• Employed MUNE, calculating the ratio of maximal compound muscle action potential (CMAP) to average surface-recorded motor unit potential (S-MUP).
• Studied hypothenar muscles in infant and older SMA subjects using multiple point stimulation MUNE.
• Examined hypothenar and biceps-brachialis muscles in adult CMT1A and CMT2 subjects using spike-triggered averaging MUNE.

Main Results:

• Profound motor unit loss was observed in SMA types 1 and 2, with moderate loss in type 3.
• Significant distal motor unit loss was noted in both CMT1A and CMT2, with proximal involvement in CMT2.
• SMA demonstrated marked distal muscle denervation, contrary to its classification as a proximal disorder.

Conclusions:

• MUNE is an effective method for assessing motor unit loss in chronic motor neuropathies.
• Axonal loss, rather than slow nerve conduction velocity, is a critical pathological feature in CMT1A.
• Distinct patterns of proximal motor unit loss in CMT1A and CMT2 suggest underlying differences in axonal pathology.