Distinguishing Delirium from Dementia

Decline in cognitive function in older people is most commonly caused by delirium, depression or dementia and although these conditions are distinctly different, it can be difficult to distinguish between them in the initial stages of assessment. Evaluation is usually based on history, observation, and physical examination and it is important to know about the person's baseline level of functioning in order to determine the extent and rate of change. Determining whether a patient has delirium and the cause of the delirium is often a matter of urgency as the underlying problem may be life-threatening (e.g., drug toxicity or electrolyte imbalance).

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When you assess older people it’s sometimes difficult to distinguish between delirium and dementia and sometimes in older people even depression can look like dementia.

The first thing to remember is that confusion can be caused by something and you really need to determine, firstly what causes the confusion and as in most solutions it is best to go from the simple logical explanation to the more complex, so first of all look for things like fever, or dehydration, or something simple and physical that can cause confusion in an older person.

Rule that out first and then when the confusion is still there, look for secondary causes like metabolic disturbances, rule that out and by process of elimination you would find out whether it was an acute temporary state, which would be delirium or whether it was dementia or depression and usually the difference between depression and dementia can be picked up by what the person remembers and what they can tell you.

Quite often with depression the person is not interested in telling you but they can remember what you asked them, where as the person with cognitive impairment and dementia will forget what the conversation was about. Those kinds of things can give you an initial clue for where you’re headed with the particular patient.

Diagnosing dementia is by elimination, you go through this whole process of ruling out the delirium, ruling out the depression, ruling out everything and in that process you would be doing scans and you’ll pick up shrinkage in the brain and no metabolic disturbance, no fever, no dehydration, no physical things, then you’re starting to think maybe you’re looking at dementia. Particularly from the nurses perspective you do the simple things first.

The onset of delirium tends to be relatively sudden, but may be superimposed upon existing dementia. Severity often fluctuates over the course of the day and there is often disturbance in the sleep-wake cycle and psychomotor behaviour. Dementia is caused by several different brain disorders, including Alzheimer's Disease, and is characterised by multiple cognitive deficits that include memory impairment. The onset of dementia is usually quite gradual.

Thinking Challenge

The links below detail some of the ways that delirium can be distinguished from dementia.

  • In what ways is the clinical history of an older patient with delirium likely to differ from that of a patient suffering from dementia?
  • What are some of the common clinical causes of delirium?
  • What types of cognitive changes might an older patient need to have to make you suspect that they are suffering from cognitive impairment?
  • How can you distinguish whether a patient is suffering from depression, rather than dementia or delirium?
  • Formal testing of mental status is often left until the latter part of the assessment interview. Why is this?