A dementia that is characterized by the development of abnormal proteinaceous (alpha-synuclein) cytoplasmic inclusions, called Lewy bodies, throughout the brain that results in progressive decline in mental abilities that has_material_basis_in accumulation of alpha-synuclein, SCNA, (protein). The causal mutation might be in SCNA or SCNB or some other gene(s) but the end result is always accumulation of SCNA in Lewy bodies.