1. Alzheimer's disease (AD) ─ the commonest form of dementia. The precise causes of this degenerative disease are unclear but early aging process and genetic factors have been identified to be possible causes. People with AD would experience a progressive deterioration.
2. Multi-infarct dementia (MID) is caused by multiple strokes and vascular diseases, originating from the damage of multiple small vessels which results in accumulative damage to the brain. People with MID often experience a stepwise deterioration in brain function.
3. Other diseases such as alcoholism, thyroid, drugpoisoning, Creutzfeldt-Jakob disease may impair the intellectual function.
