What is the exact difference between dementia and delirium in terms of clinical manifestation?
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You are asking an excellent and important question. I will provide some of the main differences between delirium and dementia and I then suggest that you go the the Merck Manual where you will find a fuller explanation. You can find the Merck Manual in the pubic library.
Delirium and dementia are easily confused and that can make diagnosis difficult. Basically, delirium is a confusional state caused by something like a high fever, toxic reaction to a drug, etc. Consequently, when the drug reaction, fever or other cause clears, the delirium clears. Generally, delirium can last from a day to a couple of weeks and, in some more serious cases, it can last much longer. What it shares in common with dementia is that it is a cognitive impairment (impairment in the ability to think and reason clearly). Delirium, because of the nature of the problems that cause it, comes on suddenly, such as becoming delirius due to a reaction to a medication.
Dementia is also a cognitive impairment but it is not reversible. It does not clear up and develops very slowly over time. Dementia is found in people with Alzherimes disease and in those with Multi Infarctions (mini brain strokes).
To complicate things further is the fact that a person with dementia can become delirius. For instance, if there is a patient with Alzheirmers disease who is given a new medication that their body has a toxic reaction to, and it can happen, they suddenly become delirius on top of their demented state. Here, the delirium will clear but the dementia will not.
I have a friend who had an very elderly father who went into surgery to repair his knee. Post surgery, he became delirius. It took a long time for the delirium to clear but it finally did. He never had dementia.
I hope this helps.