Having had delirium raises the risk of being diagnosed with dementia later. Most people who have delirium do not go on to develop dementia soon afterwards.
There are two reasons for the link. The first is that delirium can be the earliest visible sign of a brain that was already becoming vulnerable. The illness did not cause the problem. It brought it to light.
The second is that delirium may itself do some harm to the brain, particularly when it is severe or goes on for a long time. Researchers are still working out how much of the link is explained by each of these.
How big is the risk?
The risk is roughly two to four times higher than in people who have not had delirium. That figure already takes account of age, and of how unwell people were to begin with.
You will find bigger numbers than this, sometimes eight or more. Those are raw comparisons. People who develop delirium are older to begin with, frailer, and more unwell. Some already have early changes in the brain that nobody has noticed. Once you allow for all that, the extra risk linked to delirium is smaller.
Two to four times is an average across large groups. It is not a prediction about one person.
What to do
Make sure the word delirium appears in the hospital letter, and that the GP knows about it. Mention it before any future operation, because a past episode is the strongest warning sign that it may happen again.
Watch memory and concentration over the next few months. Ask the GP for an assessment if they are getting worse rather than better.
And look after the brain in the ordinary ways: hearing aids if they are needed, blood pressure managed, staying active, and company.
If confusion comes on suddenly again, do not assume dementia has arrived. Ask: could it be delirium?
There is more about the difference between delirium and dementia.
When to get help
New confusion that has come on over hours or days needs medical advice the same day. Phone the GP practice, or 111 when it is closed. Phone 999 if the person cannot be woken, has collapsed, or has new weakness or new trouble speaking. When you speak to someone, say what has changed. Then ask: could it be delirium?