What is delirium, and how is it different from dementia?

Delirium is an acute, fluctuating disturbance of attention and awareness, usually caused by an underlying physical problem — and distinguishing it from dementia matters enormously, because delirium is frequently reversible and is a medical emergency, while dementia is not.

The distinguishing features:

Onset. Delirium develops over hours to days; dementia over months to years.

Course. Delirium fluctuates, often markedly — someone can be lucid in the morning and severely confused by evening, which is why a brief assessment can miss it entirely and why family reports are essential.

Attention. Delirium's core feature is inability to sustain or shift attention. In early dementia attention is relatively preserved while memory is affected.

Consciousness. Reduced awareness of surroundings in delirium; generally preserved in dementia until late.

Reversibility. Delirium usually resolves when the cause is treated.

The two forms, and this is where cases are missed:

Hyperactive delirium — agitation, restlessness, hallucinations. Obvious, and the version everyone recognises.

Hypoactive delirium — drowsiness, withdrawal, reduced movement and speech. More common, associated with worse outcomes, and routinely missed because a quiet patient causes no trouble and is assumed to be tired or depressed.

What causes it: infection — especially urinary and chest — dehydration, constipation, urinary retention, pain, medication changes, alcohol withdrawal, low sodium, hypoxia and surgery. Frequently several at once.

Who is at risk: older people, anyone with existing dementia, people with sensory impairment, and those who are frail or have had recent surgery.

Why this is not a minor complication. Delirium is associated with longer hospital stays, higher mortality, and an increased risk of subsequent cognitive decline — it is not merely a temporary confusion to be waited out.

What actually helps: treating the cause; keeping glasses and hearing aids available; familiar faces and objects; good lighting and clear day-night cues; reorientation; mobility; hydration; and avoiding sedation, which usually makes it worse.

Any sudden confusion in an older person should be treated as urgent until a cause is found.

General information, not medical advice.