Delirium in Seniors With Dementia: Why It Can Be Easy to Miss

Is it dementia or delirium?

When a senior is living with dementia, changes in memory, communication, mood, or behaviour may become part of everyday life. But when something changes suddenly, it deserves attention.

Someone who was participating in everyday activities yesterday may suddenly seem unusually confused, sleepy, withdrawn, agitated, or simply “not themselves.” Families may understandably wonder whether their dementia is progressing.

But sometimes, the change may be delirium.

Dementia and delirium can look similar, particularly when someone already has dementia. However, families and caregivers can watch for important differences.

Dementia vs. Delirium: What Can You Look For?

One of the biggest differences is how quickly the change happens.

According to the Alzheimer’s Society, dementia generally develops slowly over months or years, while delirium develops suddenly, often over one or two days. Delirium symptoms may also change considerably from hour to hour.

How it begins:

Dementia: Usually develops gradually.

Delirium: Usually develops suddenly, over hours or days.

Pattern of changes:

Dementia: Changes generally develop over time.

Delirium: Symptoms may change significantly throughout the day.

Attention:

Dementia: Attention can be affected, depending on the person and stage of dementia.

Delirium: The person may suddenly have difficulty focusing or following a conversation.

Alertness:

Dementia: Alertness may vary from person to person.

Delirium: The person may suddenly become unusually sleepy, withdrawn, restless, or agitated.

Everyday abilities:

Dementia: Abilities may gradually decline.

Delirium: The person may suddenly struggle with something they could do recently.

Behaviour:

Dementia: Changes may develop gradually over time.

Delirium: The person may seem noticeably different from their usual self.

This comparison isn’t a diagnostic tool. The Government of Canada notes that dementia symptoms can also vary from day to day and from time to time. Dementia and delirium can also occur together.

What matters is recognizing a new and sudden change from that person’s usual pattern.

The Easiest Clue: “This Isn’t Like Them”

For someone who knows the senior well, perhaps the most useful clue is surprisingly simple:

Sudden + noticeably different from their normal = don’t automatically assume it’s dementia.

The Alzheimer’s Society explains that one of the most important parts of identifying delirium is noticing a sudden change in someone’s mental state—that they are “not themselves.” Family, friends, and caregivers can be especially well placed to notice this because they know the person.

For example, a senior with dementia may normally repeat questions or need reminders. Those behaviours are familiar. But a new change might look like:

• suddenly becoming much more confused than usual
• being unusually sleepy or difficult to engage
• struggling to follow a conversation they could follow recently
• suddenly becoming restless or agitated
• eating or drinking much less than usual
• suddenly being unable to do something they could recently manage
• seeing or hearing things that aren’t there
• becoming noticeably withdrawn or less responsive

HealthLink BC similarly describes delirium as a sudden change in mental status that usually develops over several hours to a few days. Signs can include confusion, disorientation, disorganized thinking, a short attention span, sleep-wake changes, and periods of hyperactivity or extreme sleepiness.

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When “This Isn’t Like Him” Made a Difference

In January 2025, Dementia Solutions shared Christian’s story as part of its Meaning Behind the Behaviour series.

Christian was a well-loved gentleman whose personality could light up a room. But after developing a respiratory infection, something suddenly changed.

He stopped eating. He stayed in bed. He withdrew from activities he normally enjoyed and became confused and disengaged.

Some believed his dementia had progressed, and discussions began about transitioning him to palliative care.

But two caregivers who knew Christian well recognized something important:

This wasn’t Christian.

His decline had happened suddenly, and what they were seeing was very different from his usual behaviour. They raised concerns that these changes could be related to delirium rather than simply the progression of dementia.

With medical treatment and ongoing support, including encouragement, hydration, and nutrition, Christian gradually returned to his familiar self. He began eating again, interacting with others, and enjoying life.

His experience is a powerful reminder of why knowing a person’s usual patterns matters. A sudden change shouldn’t automatically be assumed to be dementia progression.

(Dementia Solutions, formerly Personalized Dementia Solutions Inc., was founded in Vancouver, BC, by dementia consultant, educator, and author Karen Tyrell. Dementia Solutions provides dementia education and training for professional caregivers, emphasizing compassionate, person-centred care.)

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Delirium Can Be Quiet—and Easy to Miss

Not everyone experiencing delirium becomes visibly agitated.

Some people experience hypoactive delirium, where they become unusually quiet, sleepy, sluggish, or withdrawn. They may interact less, move less, have difficulty staying focused, or eat and drink less.

The Alzheimer’s Society specifically warns that this form of delirium can be easy to miss because the person may simply appear tired or sleepy.

For a family member or familiar caregiver, the first signs might therefore be very ordinary observations:

“She never sleeps this late.”

“He always finishes breakfast.”

“She normally talks to me when I arrive.”

“He could do this himself yesterday.”

These observations don’t diagnose delirium. But they can tell you that something has changed.

What Can Cause or Contribute to Delirium?

Delirium can be the brain’s response to another health problem. According to the Alzheimer’s Society, possible causes or contributing factors include:

• infection
• pain
• dehydration or poor nutrition
• constipation or urinary retention
• low blood oxygen
• medication effects
• surgery or physical injury
• other medical problems

HealthLink BC also notes that infection, other health problems, and some medicines can lead to delirium, and that people with dementia are more prone to developing it.

Because there are many possible causes, families and caregivers shouldn’t try to diagnose the underlying problem themselves.

When Should You Seek Medical Attention?

A sudden change should never automatically be dismissed as “just the dementia.”

The Alzheimer’s Society advises that when someone with dementia develops a sudden change in behaviour, family or friends who know the person well should seek medical help so the person can be assessed.

If a senior suddenly becomes significantly more confused, unusually sleepy, less responsive, agitated, or noticeably different from their usual behaviour, seek prompt medical advice. If the person appears seriously unwell or it is an emergency, seek urgent medical care.

Why Familiarity and Consistent Care Matter

Family members often notice subtle changes because they know the person well. A consistent caregiver can develop that familiarity too.

Someone who regularly supports the same senior becomes familiar with their normal communication, appetite, mobility, alertness, routines, and dementia-related behaviours.

Caregivers do not diagnose delirium and dementia. Qualified healthcare professionals make medical assessments and diagnoses.

But a familiar caregiver may recognize an important change and communicate it to the family or healthcare team.

This is one of the important benefits of continuity when supporting someone with dementia at home. A familiar caregiver isn’t simply seeing symptoms—they know what is typical for that individual.

Sometimes the first indication that something needs medical attention isn’t knowing exactly what is wrong.

It’s recognizing:

“This isn’t like them.”