Alzheimer's disease :This is the most common among the progressive dementias and usually occurs in in people aged 65 and older. However early-onset forms of the disease can occur, usually as the result of a defective gene. It is caused by destruction of brain cells through formation of plaques (clumps of a normally harmless protein called beta-amyloid) and tangles (fibrous tangles made up of an abnormal protein called tau protein). Alzheimer's disease usually progresses slowly, over seven to 10 years, causing a gradual decline in cognitive abilities including memory, and language, judgment, behavior and abstract thinking and ultimately leading to sever disability including movement disorders.
Lewy body dementia :In this disorder the brain cells are destroyed by abnormal clumps of protein called Lewy bodies. The symptoms are predominantly fluctuations between confusion and clear thinking (lucidity), visual hallucinations and tremor and rigidity. Other symptoms may be similar to Alzheimer's disease. People with this disorder will often have acting out dreams, including thrashing or kicking during sleep.
Vascular dementia :In general, vascular dementia is more common with age. It is a result of damage to the brain caused by "mini stokes" (lacunar infarcts) that destroy small parts of the brain. It occurs due to interference with blood supply to the brain. The symptoms may be similar to Alzheimer or Lewy body dementia but occur suddenly and progress in a "step ladder" pattern, with sudden onset of a group of symptoms, a period (sometimes months) with no worsening, and then sudden onset of another group of symptoms with worsening of the condition. People with high blood pressure, diabetes, high cholesterol, or previous strokes or heart attacks are at risk of developing this disorder.
Frontotemporal dementia : This disorder is caused by damage to the brain cells in the frontal and temporal lobes of the brain associated with personality, behavior and language. Symptoms of frontotemporal dementia — include socially inappropriate behaviors, loss of mental flexibility, language problems and difficulty with thinking and concentration It usually occurs between the ages of 40 and 65.