Peripheral nervous system involvement in Parkinson's disease: Peripheral neuropathy, neuromuscular junction dysfunction, and clinical implications.

Parkinson's disease (PD) has long been recognized as a central nervous system disorder, yet growing evidence indicates that the peripheral nervous system (PNS) plays a clinically relevant role in disease initiation, progression and heterogeneity. Peripheral sensory, autonomic, and motor pathways, including the neuromuscular junction (NMJ) and enteric circuits, show PD-associated structural and functional abnormalities that contribute to pain and symptoms, orthostatic and visceral dysfunction, gait instability, weakness, and reduced neuromuscular restoration. This review provides a conceptually integrated synthesis of PNS involvement in PD. To clarify how peripheral pathology relates to central neurodegeneration, we use a three-concept framework that distinguishes causal, parallel, and secondary pathophysiological processes. In this framework, peripheral abnormalities may precede central pathology, occur in parallel through shared mechanisms, or arise secondarily from disease progression, treatment exposure, reduced mobility, or comorbid factors. We summarize clinical and pathological evidence supporting peripheral neuropathy and PNS involvement in PD, including motor, autonomic, and sensory phenotypes. We outline key physiological mechanisms that maintain peripheral nerve function, including neurotrophic factors, NMJ integrity, calcium signaling, and mitochondrial homeostasis. We integrate converging mechanisms, including α-synuclein (α-syn) pathology, immune activation, mitochondrial injury, oxidative stress, and PD-related genetic and environmental factors to explain how these processes disrupt peripheral nerve homeostasis. Advances in peripheral diagnostic evaluation, including nerve conduction studies, electromyography, and peripheral α-syn detection, are also discussed. Finally, we summarize therapeutic approaches and rehabilitation strategies targeting peripheral manifestations and highlight the importance of incorporating peripheral mechanisms into PD research to improve early detection and guide future therapeutic strategies.