What causes loss of muscle mass in primary sarcopenia: A MUNIX analysis and correlation with plasma biomarkers.

Introduction: To determine if the loss of muscle mass in primary sarcopenia is secondary to motor neuron loss or directly related to muscle fiber loss using electrophysiological methods.

Methods: We recruited individuals with primary sarcopenia and non-sarcopenic individuals of similar age and gender for the study. After a detailed evaluation of geriatric syndromes and neurological disorders through systemic, neurological, and electrophysiological examinations, we recorded the motor unit number index (MUNIX) and motor unit size index (MUSIX) of the first dorsal interosseus (FDI) and tibialis anterior (TA) muscles in all participants.

Results: Individuals with sarcopenia exhibited reduced FDI MUNIX and lower TA CMAP amplitude and MUNIX values. The FDI MUNIX showed an inverse correlation with ferritin levels.

Discussion: Primary sarcopenia is characterized by motor unit loss without significant reinnervation. Motor unit loss in individuals with sarcopenia can be demonstrated in both the upper and lower extremities using the MUNIX method, and functional iron deficiency contributes to the etiology of primary sarcopenia.

Conclusion: Demonstration of motor unit loss in primary sarcopenia supports the hypothesis that the disease has a neuromuscular origin.