Dementia due to Down syndrome

**Definition:** Dementia due to Down syndrome is a neurodegenerative disorder related to the impact of abnormal increased production and accumulation of amyloid precursor protein (APP) leading to formation of beta-amyloid plaques and tau tangles. APP gene expression is increased due to its location on chromosome 21, which is abnormally triplicated in Down syndrome. Cognitive deficits and neuropathological features are similar to those observed in Alzheimer disease. Onset is typically after the fourth decade of life with a gradual decline in functioning, and may impact 50% or more of individuals with Down syndrome.