Handgrip maximal voluntary isometric contraction does not correlate with thenar motor unit number estimation

Area of Science:

• Neurology
• Electrophysiology
• Genetics

Background:

• Motor neurone disease (MND) involves significant motor unit loss before clinical weakness appears.
• Maximal voluntary isometric contraction (MVIC) is currently used to monitor MND progression, but its sensitivity is questioned.
• Early detection of motor unit loss is crucial for understanding disease progression.

Purpose of the Study:

• To compare the sensitivity of motor unit number estimation (MUNE) versus MVIC in detecting early signs of motor unit loss.
• To investigate the correlation between isometric grip strength and thenar MUNE in asymptomatic subjects.
• To evaluate MUNE as a more sensitive biomarker for monitoring disease progression in conditions like MND.

Main Methods:

• Motor unit number estimation (MUNE) was performed on the thenar muscles of 69 subjects.
• Subjects included asymptomatic Cu, Zn superoxide dismutase 1 (SOD 1) mutation carriers, family controls, and population controls.
• Isometric grip strength was measured using a Jamar hand dynamometer.

Main Results:

• A lack of correlation was found between isometric grip strength and thenar MUNE in asymptomatic individuals.
• MUNE demonstrated a stronger correlation with the number of functional motor neurons compared to MVIC.
• This suggests MUNE is a more sensitive indicator of early motor unit loss.

Conclusions:

• Motor unit number estimation (MUNE) is a more sensitive electrophysiological technique than MVIC for monitoring disease progression in MND.
• MUNE can detect motor unit loss earlier than MVIC, which may remain normal despite significant denervation.
• This highlights the potential of MUNE for earlier and more accurate disease monitoring in slowly progressive neurological conditions.