Causes and Implications of Type I Myofiber Grouping in Parkinson's Disease: A Brief Scoping Review

Abstract

Purpose: The purpose of this study was to review the alterations in muscle fibers in Parkinson's disease(PD). The following were our review questions. (1)How has research on type I myofiber grouping in PD developed over time? (2)What kinds of muscles are affected in which patients? (3)What are some possible pathophysiology and mechanisms? Does type I myofiber grouping in PD differ from primary sarcopenia that occurs with aging? (4)What are the clinical implications and possible therapeutic approaches for type I myofiber grouping in PD?.
 Method: To investigate the questions, we used combinations of keywords such as “Parkinson”, “skeletal muscle”, “myofiber type”, “fast twitch”, “slow twitch”, “myofiber grouping”, and “motor unit” in PubMed and Google Scholar. Articles on PD patients and normal elderlies that dealt with type I myofiber grouping and motor unit alterations were included. References in the included articles were also considered.
 Results: Research over the past five decades has identified various motor abnormalities and myofiber alterations in PD patients, including the hypertrophy of slow-twitch type I myofibers and atrophy of fast-twitch type II myofibers across different muscles. One important finding is that Type I myofiber grouping, which is common in aging, is more severe in PD, which could be due to the selective activation of low-threshold motor units and could be also linked to abnormal alpha-synuclein aggregation, a factor associated with PD.
 Conclusion: Research suggests that type I myofiber grouping in muscles, not just dopaminergic cell damage in the substantia nigra, could influence motor symptoms of PD, indicating that alternative treatments beyond dopaminergic drugs, such as high-intensity exercise, might be beneficial. However, given the limitations in these studies, such as small participant numbers and the complexity of PD pathophysiology, future research is needed to fully understand the phenomena in different PD subtypes and to develop more effective treatments.