There are several types of dementia and Alzheimer's is just one of them. With Alzheimer's patients tend to lose short term memory but can remember their earlier years quite clearly. My father-in-law has frontal lobe dementia and exhibits bipolar tendencies as well as some paranoia. Dementia is a general term for brain dysfunction.
A lot of confusion arises in making the distinction between Dementia and Alzheimer since the two terms are often used interchangeably. Dementia and Alzheimer are different but still closely related to each other. Dementia describes the symptoms that occur when the brain is affected by specific diseases and Alzheimer is the most common type of Dementia accounting for more than 60 per cent of all Dementias. Alzheimer's is a gradually progressive form of dementia. Source: http://www.differencebetween.info/difference-between-dementia-and-alzheimer