Delirium vs dementia: What’s the difference?

Delirium and dementia both affect memory, thinking and behaviour, but they’re not the same thing. It’s common to mix them up, particularly in older people. Knowing the difference matters because delirium comes on suddenly and is often reversible, while dementia develops gradually and is usually progressive.

This guide explains the key differences between delirium and dementia, how to recognise the signs and when to get medical help.

Delirium vs dementia at a glance

The quickest way to tell delirium and dementia apart is to look at how fast the changes appeared and whether they come and go.

Source: Alzheimer's Society.

There's one important exception. Lewy body dementia can cause symptoms that change from day to day, including hallucinations, which can look a lot like delirium. Dementia UK notes that it can be hard to tell them apart without knowing how long the symptoms have been there.

What is delirium?

Delirium is a sudden change in someone's mental state. It usually develops over one to two days, and the Alzheimer's Society explains that delirium is often the first sign someone is becoming unwell.

It’s common, especially in hospitals. A 2022 study across UK hospitals found that around one in six people over 65 had delirium, and that staff were least likely to spot it in the frailest people. It can happen at home too, often when an infection or another illness is starting.

The NHS advises getting medical help for sudden confusion in adults, so it's always worth acting on, even if the change seems small.

Common symptoms of delirium

If your loved one has delirium, they might experience:

• Sudden confusion or disorientation
• Difficulty concentrating or following conversations
• Rapid changes in alertness, from drowsy to agitated
• Seeing or hearing things that aren’t there
• Feeling suspicious or paranoid
• Sleeping in the day and staying awake at night
• Sudden changes in mood or behaviour
• Losing interest in food and drink

Symptoms often come and go throughout the day. Your loved one might seem almost like themselves in the morning and far more confused by the evening. This changing pattern is one of the clearest signs of delirium.

Types of delirium

There are three types of delirium. Knowing about all three helps you spot the quieter signs, which are the easiest to miss.

• Hyperactive delirium: Your loved one becomes restless, agitated or distressed. They might pace, try to get up again and again, or refuse help. This is the type people tend to notice first.
• Hypoactive delirium: Your loved one becomes quiet, withdrawn and sleepy. They might move and speak less, or seem to be in a daze.
• Mixed delirium: Your loved one switches between the two, sometimes within the same day.

If someone you know well suddenly seems flat, drowsy or not themselves, trust your instincts. You know their usual self better than anyone, and that knowledge helps doctors.

What is dementia?

Dementia is the name for a group of symptoms caused by ongoing changes in the brain. The NHS explains that dementia affects memory, thinking and the ability to carry out everyday activities, and it gets worse over time.

Unlike delirium, dementia develops slowly, and symptoms tend to stay similar from day to day. The early signs of dementia are often small changes that build up gradually, such as forgetting recent conversations or finding familiar tasks harder.

There's no cure for most types of dementia, but the right support makes a real difference. Many people go on living well with dementia for years, at home and doing the things they enjoy.

Can someone have delirium and dementia at the same time?

Yes, and it's common. Dementia is one of the biggest risk factors for delirium. Dementia UK estimates that delirium affects around 25% to 30% of older people on hospital wards and up to 50% of people living with dementia.

When delirium develops in someone with dementia, it can look like their dementia has suddenly got worse. Dementia usually changes slowly, so a noticeable change over a day or two points to something else, like an infection or pain.

Your loved one might also find it harder to tell you something's wrong, especially if dementia affects how they communicate. A change in behaviour, appetite or sleep can be their way of showing they're unwell.

What causes delirium in older people?

Delirium usually has a physical trigger, and there's often more than one cause at the same time. Common causes include:

• Infections, especially urinary tract infections (UTIs) in older people and chest infections
• Dehydration or not eating enough
• New medicines, changes to doses or stopping a medicine suddenly
• Pain that isn't well managed
• Constipation or difficulty passing urine
• Surgery or a stay in hospital
• A change of surroundings, such as moving home or moving between hospital wards
• Poor sleep
• Low oxygen levels in the blood, for example because of a problem with the heart or lungs
• Low blood sugar in people with diabetes
• Kidney or liver problems
• A stroke, a mini-stroke (TIA), a head injury or a seizure
• Less commonly, an infection or inflammation of the brain, or a brain tumour

Some people are more likely to get delirium. Guy’s and St Thomas’ NHS Foundation Trust says people over 80 who are living with dementia are particularly at risk, as are people who are seriously ill or have had surgery.

Why delirium is often missed

Delirium is easy to overlook, especially the quieter, hypoactive type. People sometimes mistake it for:

• Dementia getting worse
• Depression or low mood
• Tiredness, or “just getting older”
• Sundowning

Sundowning in dementia is when someone becomes more confused, restless or anxious in the late afternoon or evening. It tends to follow a regular pattern. Delirium is different because it's new, it starts suddenly and it has a physical cause. If evening confusion appears out of nowhere, or it's much worse than usual, it's worth getting checked.

When to get urgent medical help

Contact your loved one's GP or call NHS 111 the same day if they:

• Become suddenly confused or disorientated
• Are much sleepier or more agitated than usual
• Start seeing or hearing things that aren't there
• Seem “not themselves” in a way you can't explain

Call 999 or go to A&E if your loved one seems seriously or suddenly unwell. Dementia UK has more advice on what to do if you spot delirium.

How you can help the doctor

You know your loved one's usual self, so the information you share makes a real difference. It helps to tell the doctor:

• When you first noticed the change and how quickly it happened
• What they're usually like, including their memory and how they manage day to day
• Any new medicines or recent changes to their medicines
• Any recent falls, illness or pain, and any changes in eating, drinking or going to the toilet

Doctors and nurses often use a short test of memory and attention, called the 4AT, to check for delirium. The Alzheimer's Society guide to delirium diagnosis explains how it works.

Can delirium be treated?

Yes, in most cases. Doctors treat delirium by finding and treating what's causing it, for example an infection, dehydration or a medicine that doesn't suit your loved one. At the same time, a calm, familiar environment helps the brain recover.

Most people recover from delirium. Guy’s and St Thomas’ NHS Foundation Trust says symptoms clear within 6 days for 6 in 10 people, but some people have symptoms for longer. People living with dementia are less likely to get back fully to how they were before, which is why acting quickly matters so much.

If your loved one is still more confused than usual a few weeks after an illness or hospital stay, let their GP know. The GP can look into what's behind it and arrange a memory assessment if needed.

Can you prevent delirium?

You can’t always prevent delirium, but Dementia UK suggests several ways to reduce the risk.

Simple, everyday steps help. At home, you can:

• Encourage regular drinks and meals throughout the day
• Make sure glasses and hearing aids are clean, working and worn
• Keep a regular routine, with daylight and gentle activity during the day and good sleep at night
• Use a clock or calendar to help with orientation, as part of a dementia-friendly home environment
• Ask the GP or pharmacist for a regular medicines review
• Watch for early signs of infection, pain or constipation
• Keep familiar faces around

If your loved one goes into hospital, take in familiar items from home and tell the ward team what helps them feel settled. Visits from people they know can help them feel safe and less disorientated.

Supporting your loved one at home

The risk of delirium stays higher after a hospital stay, while your loved one is still recovering and getting used to being home. Our guide to arranging care after a hospital discharge explains the process and the support available.

Having someone around who knows your loved one's usual routine helps you spot changes early and keeps things calm and familiar. With live-in care, a self-employed carer lives with your loved one and offers one-to-one support at home. A carer can help with drinks, meals, routines and reassurance, and let you know quickly if something seems different.

If your loved one is living with dementia, find out how dementia care at home can help them stay in familiar surroundings. And if you're caring for someone yourself and need a break, respite care for dementia gives you time to rest.

Explore live-in care options

Getting the right support

Telling delirium and dementia apart helps you act quickly when it counts. Sudden confusion is a reason to get medical help the same day, while slow, gradual changes are a reason to talk to a GP about memory.

Whichever you're facing, you don't have to work it out alone. Our Care Advisors can talk you through your options and help you find a self-employed carer who suits your loved one's needs, so they can stay safe and comfortable at home.

Sources

• Alzheimer’s Society: Delirium, symptoms, diagnosis and treatment
• Dementia UK: Delirium and dementia, symptoms, causes and treatment
• Geriatric Medicine Research Collaborative: Increasing frailty is associated with higher prevalence and reduced recognition of delirium in older hospitalised inpatients
• Guy’s and St Thomas’ NHS Foundation Trust: Delirium (sudden confusion)
• University of Edinburgh: Hospital delirium linked to later dementia risk
• NHS: Sudden confusion (delirium)
• NHS: Dementia
• nidirect: Sudden confusion (delirium)