3 Answers

Dementia is an impairment of thinking and memory that interferes with a person's ability to do things which he or she previously was able to do.  

Alzheimer's disease is the common cause of dementia, and is particularly common in older people.  Because it is the most common cause of dementia, Alzheimer's disease is commonly equated with the general term dementia. However, there are many other causes of dementia. 

Therefore, distinguishing Alzheimer's disease from other forms of dementia requires a doctor to establish certain background information (the history) and define a patient's abilities (the examination), and then perhaps obtain results from certain tests. 

The last step is guided by the results from the first two. Alzheimer's disease is typically a slowly progressive disorder that involves memory for recent information (short-term memory) and one or more other abilities, such as speech and language, personality, decision-making and judgment or awareness and ability to interact with the environment.  

A doctor attempting to distinguish Alzheimer's disease from another form of dementia asks questions about these abilities and examines them as well.  Additionally, the doctor also asks questions about and examines abilities that are typically not impaired in a patient with Alzheimer's disease.  These abilities include, among others, memory for information of long ago (long-term memory), vision, ability to feel things and muscle strength.  In doing these things, the doctor is attempting to determine if the pattern of impairments that the patient has are typical or not typical for Alzheimer's disease. 

Dementia is a prelude to alzheimers. Here is some information:

http://www.alzheimersreadingroom.com/2010/06/whats-difference-between-alzheimers-and.html