🧭 When to suspect

Delirium is an acute, fluctuating disturbance of attention and awareness, accompanied by a change in cognition, that develops over hours to days and is driven by an underlying physical illness, drug or metabolic disturbance. It is one of the commonest medical emergencies in older people, yet it is frequently missed – particularly the hypoactive form.

Suspect it in any patient with a sudden change in mental state from their baseline. Symptoms characteristically fluctuate and are often worse in the evening or at night (commonly called sundowning). Three motor subtypes are described: hyperactive (agitation, restlessness, hallucinations), hypoactive (drowsiness, withdrawal, slowed responses – the commonest and most easily overlooked, with the worst outcomes), and mixed.

Delirium matters because it is independently associated with falls, longer hospital stays, new institutionalisation, an increased risk of dementia and higher mortality. The two practical skills in primary care are to recognise it against the person's baseline and to find and treat the reversible cause.

Feature	Delirium	Dementia
Onset	Acute – hours to days	Insidious – months to years
Course	Fluctuating, often worse at night	Slowly progressive, stable day to day
Attention	Markedly impaired (the core deficit)	Usually preserved until late
Conscious level / alertness	Altered – drowsy or hyper-alert	Normal until late disease
Duration	Days to weeks (occasionally longer)	Permanent and progressive
Reversibility	Usually reversible if cause treated	Irreversible

The important caveat: dementia with Lewy bodies also fluctuates and can mimic delirium, and the two commonly coexist as delirium superimposed on dementia. Where the diagnoses are hard to separate, manage the delirium first.

Source: NICE CG103 · SIGN 157

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