A term covering a number of conditions responsible for dementia having its onset at a younger age than ALZHEIMER'S DISEASE. It results in ATROPHY of brain cells in the frontal and termporal lobes of the brain. The illnesses usually start with progressive loss of speech and a marked change in personality, sometimes misdiagnosed as a psychiatric disorder. It occurs in some patients with PARKINSONISM or MOTOR NEURONE DISEASE (MND) (see also DEMENTIA).