Delirium or dementia? How to tell the difference

Delirium and dementia are the two conditions most often confused with each other, by families and by professionals. Getting it right changes what happens next, because a sudden change usually has a treatable cause behind it.

The speed of the change

Dementia develops slowly. The diseases behind it, such as Alzheimer’s disease, damage the brain over years, and the changes in memory and thinking build up so gradually that many families only see the pattern when they look back. No one can point to the day it began, because there was no such day.

Delirium arrives over hours or days, so there is a before and an after. If you can put a date on the change, or even a time of day, think delirium.

Consider two women in their eighties. May’s family noticed, over about two years, that she was repeating questions and losing track of her bills. There was no single moment when anything changed; she simply needed more help this year than last. A memory clinic later diagnosed Alzheimer’s disease. Farida was her usual self on Friday and phoned her son as normal on Saturday morning. By Sunday evening she was accusing her carers of stealing, could not say what year it was, and had been awake all night. Her doctor found a chest infection and dehydration; this was delirium, and within two weeks she was back to herself.

Good hours and bad hours

Delirium fluctuates. A person can be settled and fairly clear at eleven in the morning, then disorientated and suspicious by teatime. Families describe good hours and bad hours, sometimes good minutes and bad minutes. The symptoms are often at their worst in the evening and overnight.

Many dementias are steadier, with their own slow rhythms. Some differ: dementia with Lewy bodies can cause marked fluctuations and hallucinations, and vascular cognitive impairment may change in steps. Even so, any new or clearly worse change needs urgent medical assessment for delirium or another acute medical problem.

Can they follow the conversation?

The central problem in delirium is attention: the person cannot hold on to the thread. They lose track mid-sentence, answer the first question and drift during the second, or are too drowsy to attend at all.

In typical early Alzheimer’s disease, memory often fails first while attention still holds. A person may ask the same question three times in an hour, yet follow the conversation while it is happening. Other dementias can present differently. In delirium, attention usually changes suddenly and the conversation itself keeps falling apart.

The differences at a glance

When the person already has dementia

The person most likely to develop delirium is precisely the person who already has dementia. In hospital, up to half of older people with dementia will develop delirium at some point during the admission. Doctors call this “delirium superimposed on dementia”, and wards see it every week.

It is also the situation in which delirium is most often missed. When someone with dementia deteriorates, everyone reaches for the explanation already written in the notes. Families are told the dementia is progressing. Sometimes it is. A sudden drop below the person’s normal level should be treated as possible delirium or another acute medical problem until assessed.

The delirium may also be easy to overlook because there is nothing dramatic to see. In people with dementia the hypoactive form (quiet, sleepy, withdrawn) is common. Nobody shouts or pulls out a drip; the person simply goes quiet and starts sleeping through visits. On a busy ward that can look like a patient who is no trouble at all.

Missing it matters. Delirium on top of dementia carries higher risks than either condition alone: longer hospital stays, more complications, a higher chance of not getting home again, and a higher risk of dying. The earlier it is found, the earlier the causes can be treated.

Say it plainly

Staff meeting your relative for the first time see a confused older person. They cannot know that on Tuesday she was feeding herself and taking an interest, because they were not there on Tuesday. You were. The tests staff use to detect delirium, such as the 4AT, specifically ask whether there has been a recent change from the person’s normal state. Often only a family member can answer that question.

Even experienced clinicians can find this distinction difficult. Once you notice a sudden change, report when it began and ask whether it could be delirium. If staff put the change down to dementia, the Raising concerns with staff page gives examples of what to say.

If the sudden change happens at home

Many episodes of delirium begin at home or in a care home. New sudden confusion needs medical assessment now. Contact the urgent or emergency medical service where the person is. In England, call 999 or go to A&E. In Scotland, contact the GP urgently if open; otherwise phone NHS 24 on 111. Outside the UK, follow local health-service guidance. In a care home, tell the senior carer or nurse immediately and ask them to arrange medical assessment now. Use the words “sudden change” and “possible delirium”.

Why the difference matters

The two conditions lead to different actions. A sudden change needs medical assessment now, because there may be a treatable cause behind it. A gradual change over months also needs proper assessment, but on a different timescale: a doctor’s appointment, then usually a memory clinic, over weeks.

There is one more reason to keep the two apart in your mind: thinking and memory cannot be fairly judged during delirium. A new diagnosis of dementia should not normally be made while a person is delirious, and major decisions, such as whether someone can manage at home again, should not rest on how the person is in the depths of a delirium. If you find such decisions being made about someone who is still clearly delirious, it is reasonable to ask for them to be delayed.