A degenerative brain disease characterized primarily by progressive dementia. The familial form is very rare and is completely inherited and has an early onset (usually in the 4th decade). It occurs when there is excessive production of a toxic protein (amyloid beta peptide) in the brain which collects into clumps (amyloid plaques) in the brain. These plaques cause damage to nerve cells in the brain. See also Alzheimer disease, familial