Dementia vs. Delirium: What’s the Difference?

Understanding the difference between dementia and delirium is crucial when supporting an older adult who is experiencing changes in memory or behaviour. Although both conditions affect brain function, they are very different in their causes, symptoms, and treatment approaches.

What is Dementia?

Dementia is a gradual decline in memory, thinking, and reasoning that interferes with daily life. It’s caused by damage to brain cells from conditions such as Alzheimer’s disease, vascular dementia, or Lewy body dementia. Over time, dementia leads to ongoing cognitive decline and changes in mood, language, and behaviour.

• Onset: Slow, progressing over months or years

• Symptoms: Memory loss, confusion, language difficulties, personality changes

• Course: Progressive and generally not reversible

  
  

What is Delirium?

Delirium is a sudden and serious change in mental abilities that causes confusion and reduced awareness of surroundings. It often happens quickly — over hours or days — and can fluctuate in intensity. Delirium is usually caused by a medical issue such as infection, medication side effects, dehydration, or hospitalization.

• Onset: Sudden and abrupt

• Symptoms: Disorientation, inability to focus, hallucinations, agitation

• Course: Temporary and often reversible if the underlying cause is treated

  
  

Key Differences

• Speed of Onset: Delirium appears suddenly; dementia develops slowly

• Attention: People with delirium often can’t focus or follow conversations; those with dementia usually maintain attention until later stages

• Fluctuation: Delirium symptoms can change quickly during the day; dementia symptoms are more stable

• Awareness: Delirium commonly affects alertness and awareness; dementia does not affect awareness in early stages

  
  

Can They Happen Together?

Yes — someone with dementia is at higher risk of developing delirium, especially during illness, injury, or hospital stays. In these cases, delirium may be mistaken as a sudden worsening of dementia. That’s why identifying delirium quickly is important — because it can often be reversed.

Final Thoughts

Knowing the difference between dementia and delirium can help families and caregivers act quickly and get the right support. If a loved one is suddenly more confused or behaves differently, don’t assume it’s “just the dementia.” It may be something treatable.

Source: Alzheimer Society of Canada