The Difference Between Dementia and Alzheimer's Disease

David Roeltgen, MD Health Guide April 16, 2007
  • One question I am frequently asked is "What is the difference between dementia and Alzheimer's disease?" On one level, the answer to this question is relatively easy and straightforward. Compare the definitions of dementia and Alzheimer's.

     

    Dementia:

     

    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:

     

    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. 

     

    Distinguishing Alzheimer's disease from other causes of dementia is not always as easy and straightforward as defining these terms.  In practice, people and their disorders of behavior are far more complex than simple definitions sometimes commonly imply.


    Alzheimer's: Microscopic Brain Abnormalities
     

    Alzheimer's disease is a specific form of dementia having very specific microscopic brain abnormalities.  However, in typical medical practice, we do not have the ability to see microscopic brain abnormalities.

     

    History and Examination for Alzheimer's


    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. 

     

    CT Scan or MRI for Alzheimer's

     

    After the history and examination are completed, the doctor will commonly obtain a scan of the brain, using either a CT scan or MRI scan.  The scans look at brain large-scale (macroscopic rather than microscopic) structure.  (Newer scan types are being developed to begin to examine brain function and microscopic structure.  In the future, these are likely to help better diagnose Alzheimer's disease.)  Also, the doctor, after the history and physical examination are completed, will commonly obtain certain blood tests.  Which blood tests are obtained will depend on the background history and physical examination.

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    Alternative Causes of Dementia

    In attempting to make a diagnosis of Alzheimer's disease, the doctor will be considering alternative causes of dementia.  For example, if a child, teenager or young adult has features of dementia, as described above, Alzheimer's disease is very unlikely.  Therefore, the causes of dementia in a young person would be considered.  Another setting of dementia that is not Alzheimer's disease would be someone who has dementia after a head injury.  In an older person, the presence of a head injury does not exclude the possibility of Alzheimer's disease as well.  However, any young person who was normal before a head injury and who had dementia after a head injury would be diagnosed with "post-traumatic (after injury) dementia."

     

    Distinguishing Alzheimer's disease from other late-life causes of dementia is not as easy or as obvious as the examples given above.  In fact, even the doctors most knowledgeable about Alzheimer's disease are not always accurate in making the diagnosis. 

     

    Parkinson's Disease with Dementia vs. Alzheimer's Disease with Dementia

     

    There is a list of other forms of dementia that can occur in later life.  Distinguishing some of these from Alzheimer's disease may be important.  For example, the treatment of Parkinson's disease with dementia is different from the treatment of Alzheimer's disease, because there are specific treatments for the problems of movement seen in patients with Parkinson's disease. 

     

    Therefore, one of the goals of the doctor is to distinguish disorders that have different treatments and attempt to use that treatment most appropriate for that individual person.  Another goal that the doctor has is to have enough information about the dementia to appropriately counsel the patient and family members about the diagnosis and treatment plan.

     

    In summary, Alzheimer's disease is, by definition, a type of dementia.  However, a diagnosis of dementia does not always mean that the person with that diagnosis has Alzheimer's disease.  Sometimes these distinctions are easy, and sometimes they are not.

     

    To learn more about Alzheimer's and dementia diagnosis, treatment, and prevention, check out Alzheimer's Basics.