Autism and Neurodegeneration: Distinct Disorders or a Shared Biological Continuum?

BACKGROUND/OBJECTIVES: Autism spectrum disorder (ASD) is traditionally classified as a neurodevelopmental condition, whereas neurodegenerative diseases are defined by progressive neuronal decline in later life. This separation has shaped research and clinical practice, yet emerging evidence suggests potential biological overlap. This review aims to evaluate whether ASD and neurodegenerative disorders represent distinct entities or are linked through shared mechanisms operating across the lifespan. METHODS: This narrative review synthesizes findings from genetic, molecular, cellular, circuit-level, and epidemiological studies examining ASD and major neurodegenerative conditions, including Alzheimer's disease, Parkinson's disease, and Amyotrophic lateral sclerosis. Emphasis is placed on identifying convergent pathways and evaluating evidence within a lifespan-oriented framework. RESULTS: Across multiple levels of analysis, ASD and neurodegenerative diseases share partially overlapping biological mechanisms, including mitochondrial dysfunction, impaired proteostasis, neuroimmune alterations, and network-level instability. Genetic and molecular data reveal pleiotropic pathways influencing both early neurodevelopment and later neuronal resilience. Circuit-level studies highlight shared principles of network vulnerability, including cerebellar involvement and excitation-inhibition imbalance. Epidemiological data further indicate increased risk of dementia and parkinsonian features in autistic adults. These convergences suggest that early neurodevelopmental alterations may establish latent vulnerabilities that, under specific conditions, intersect with neurodegenerative processes later in life. CONCLUSIONS: ASD and neurodegenerative diseases are best understood as distinct clinical conditions that share partially overlapping biological substrates. Rather than implying a deterministic progression, the evidence supports a model of lifespan convergence in which timing, context, and individual susceptibility shape outcomes. This framework highlights the need for integrated research and clinical approaches that consider brain health as a continuous process from development through aging.