Delirium vs dementia: sudden confusion compared with gradual decline

The simplest distinction between delirium and dementia is time. Delirium appears suddenly, over hours or a day or two, and tends to improve when the cause is found and treated. Dementia comes on slowly, over months or years, and gets worse over time.

The Alzheimer's Society describes delirium as a change in a person's mental state that happens suddenly over 1 to 2 days, and as often the first sign that someone is becoming unwell. It stresses that delirium is not the same as dementia, but that the two can look similar to someone who does not know the person well. The NHS describes sudden confusion, or delirium, as having many different causes and as something for which you should get medical help immediately.

The two can also occur together. People with dementia are more likely to develop delirium, and in that case the delirium may be mistaken for the dementia getting worse.

The table is drawn from the Alzheimer's Society page on delirium and the NHS pages on sudden confusion and on dementia symptoms.

The Alzheimer's Society says the most important part of identifying delirium is noticing a sudden change in a person's mental state, that they are "not themselves". Family, friends and carers are often best placed to notice this because they know the person well. Signs include being less alert or responsive, being easily distracted, being less aware of where they are or what time it is, suddenly being less able to do something such as walk or eat, unclear speech, sudden swings in mood or behaviour, and hallucinations or delusions.

It describes three types. Hyperactive delirium involves restlessness, agitation, pacing, resisting care, wariness, delusions or hallucinations. Hypoactive delirium involves being withdrawn, sluggish, tired, moving less, unusually sleepy or eating and drinking less, and can easily be missed because the person may just look tired. Mixed delirium switches between the two. It says that among older people, including those with dementia, hypoactive and mixed delirium are most common.

The NHS lists signs of confusion as not being able to think or speak clearly or quickly, not knowing where you are, struggling to pay attention or remember things, and seeing or hearing things that are not there. It says that if someone seems unsure of their name, age and today's date they probably need medical help.

The Alzheimer's Society says delirium is usually the brain's reaction to a separate problem. The causes it lists include pain, infection, a major operation with general anaesthetic, a physical injury such as a fracture or head injury, poor nutrition or dehydration, constipation or urinary retention, low blood oxygen, effects of medication, abnormal metabolism such as low salts or sugar, and organ failure. There is often more than one cause, which is why a thorough examination matters.

The NHS list of common causes of sudden confusion includes infections such as urinary tract infections (UTIs), which are a common cause in older people or people with dementia, a stroke or TIA, low blood sugar in people with diabetes, a head injury, some prescription medicines, alcohol poisoning or withdrawal, drug use, carbon monoxide poisoning, a severe asthma attack or other lung or heart problems, and certain seizures. It cautions not to try to self-diagnose.

Those at higher risk include people with dementia, people over 65, frail people or those with several medical conditions, people with poor hearing or vision, people in unfamiliar surroundings, sleep-deprived people, those who cannot sit up or move around, people taking certain drowsy-making medicines, and anyone who has had delirium before.

The NHS says dementia is not a disease itself but a collection of symptoms that result from damage to the brain caused by different diseases such as Alzheimer's. The common early symptoms are memory loss, difficulty concentrating, finding it hard to carry out familiar daily tasks, struggling to follow a conversation or find the right word, being confused about time and place, and mood changes. It says they are often mild and may get worse only very gradually.

Different types of dementia can differ. The NHS describes vascular dementia, in which symptoms can sometimes develop suddenly and quickly get worse, and dementia with Lewy bodies, in which people typically have periods of being alert or drowsy, fluctuating levels of confusion, visual hallucinations, slowed movement and repeated falls. This is why the Alzheimer's Society lists dementia with Lewy bodies as an important exception: it has many of the same symptoms as delirium, so it can be hard to tell the difference without knowing the person well.

The NHS stresses that dementia is not a natural part of ageing and that it is important to talk to a GP sooner rather than later if you are worried about memory problems or other symptoms. It says that if you are worried that you or a relative have gradually become more forgetful or confused, to see a GP, noting that in older people forgetfulness and confusion are sometimes signs of dementia.

The NHS says to go to A&E or call 999 if someone suddenly becomes confused, and not to drive them yourself. While waiting for an ambulance, it suggests staying with the person, telling them who you are and where they are, reassuring them, using simple words and short sentences, making a note of any medicines they are taking, not asking lots of questions, and not stopping them moving around unless they are in danger.

The Alzheimer's Society advice for a person with dementia who may have delirium is to seek medical help straight away by calling the GP or NHS 111, not to leave it and hope it gets better by itself, and to call 999 if the person appears seriously unwell. It suggests noting when the new symptoms began, how the person is different from usual, whether they are sleepier, more agitated or confused, whether they are seeing things that are not there, any recent illness, injury or medicine changes, and the type and length of time they have had dementia.

It also suggests that doctors may use a short screening test called the 4AT, which checks alertness, awareness, attention and acute change and takes about two minutes. In England, it notes that Martha's Rule allows families to ask for an urgent review by another clinical team if a condition is getting worse and concerns are not being responded to.

The Alzheimer's Society says delirium is treated by resolving the health problems that caused it, and that most people get better in a few days once the causes are found and treated, though some causes, such as a serious infection, take longer. A calm, supportive environment helps: talking calmly in short sentences, reminding the person where they are, familiar objects, regular eating and drinking, working glasses and hearing aids, a clock and calendar, a healthy sleep routine and getting up and about as soon as it is safe.

It says some people do not make a quick or full recovery and may still have problems with memory and thinking weeks or months later, particularly older people after a major operation. It adds that prolonged and severe delirium over several weeks can increase the risk of developing dementia, and that this does not mean delirium causes dementia, but it may speed up decline in someone already having problems.