Lewy body dementias are two similar and common subtypes of dementia—dementia with Lewy bodies (DLB) and Parkinson's disease dementia (PDD). Both are characterized by changes in thinking, movement, behavior, and mood. The two conditions have similar features and may have similar causes, and are believed to belong on a spectrum of Lewy body disease that includes Parkinson's disease. As of 2014, they were more often misdiagnosed than any other common dementia.
The exact cause is unknown, but involves widespread deposits of abnormal clumps of protein that form in neurons of the diseased brain. Known as Lewy bodies (discovered in 1912 by Frederic Lewy) and Lewy neurites, these clumps affect both the central nervous system and the autonomic nervous system. The fifth revision of the Diagnostic and Statistical Manual of Mental Disorders (DSM-5) gives Lewy body disease as the causative subtype of dementia with Lewy bodies, and Parkinson's disease as the causative subtype of Parkinson's disease dementia. Dementia with Lewy bodies is marked by the presence of Lewy bodies primarily in the cortical regions, and Parkinson's disease dementia with Lewy bodies primarily in the subcortical basal ganglia.