Neurodegenerative Diseases: Epidemiology, Risk Factors, Classification, Pathophysiology, Clinical Features,...

Neurodegenerative diseases are a growing cause of disability worldwide, particularly as populations age. Although these disorders differ in etiology and clinical presentation, advances in genetics, neuropathology, and biomarker research have identified shared disturbances in protein homeostasis, mitochondrial function, and immune regulation. These processes may contribute to selective neuronal vulnerability and neural circuit dysfunction. These findings are refining disease classification and diagnosis, yet effective disease-modifying therapies remain limited. Disease-specific abnormalities remain essential to clinical interpretation and treatment selection. Existing reviews often focus on individual disorders or isolated mechanisms, leaving the links between shared biology and disease-specific phenotypes insufficiently clear. We synthesize evidence on the epidemiology, risk factors, classification, and pathophysiology of Alzheimer's disease, Parkinson's disease, amyotrophic lateral sclerosis, Huntington's disease, frontotemporal dementia, Friedreich's ataxia, and multiple sclerosis. We compare their clinical manifestations and diagnostic approaches, including the evolving role of biomarkers, and critically assess established and emerging therapies. By distinguishing shared mechanisms from disease-specific abnormalities, we identify key translational gaps and explore how common biological disturbances may lead to distinct clinical trajectories. This cross-disease perspective may inform diagnostic reasoning, treatment development, and future comparative research.