Delirium is common wherever patients are old, acutely ill, recently operated on, or critically ill. Roughly 1 in 6 older adults in the emergency department meets criteria, and the share runs higher on general wards, after major surgery, and in intensive care. Up to two-thirds of cases are missed, mostly the quiet hypoactive form, which draws little attention. Unrecognised delirium means longer hospital stays, loss of independence, and worse survival.
Delirium presents as an acute mental status change with abnormal, fluctuating attention. It is a syndrome rather than a single symptom: features appear in different combinations and intensities across patients, and they wax and wane across hours in the same patient. Recognition requires observing the patient over time, because fluctuation itself is one of the most reliable hallmarks.
The central deficit is inattention. The patient is distractible: trivial stimuli capture attention while important ones are missed, and every aspect of attention suffers, including selectivity (choosing what to attend to), sustainment (keeping attention on it), and the ability to shift focus. This is the feature that most reliably separates delirium from other cognitive disorders at the bedside. Thinking becomes disorganised alongside it: the patient cannot hold a clear, coherent stream of thought, and speech turns rambling, tangential, and perseverative (returning repeatedly to the same words or ideas), with hesitations and repetitions.
Awareness is disturbed in parallel. Most patients are lethargic with reduced arousal (how alert and responsive they are), though hyperactive patients may instead look hyperalert. Disorientation typically unfolds from time to place to person, and recent memory fails before remote memory. The sleep-wake cycle reverses, with daytime drowsiness and nocturnal restlessness, and dream-like states can spill into wakefulness.
Perception, mood, and behaviour
Patients take in less of their surroundings per unit of time and misread what they do perceive. Illusions, mistaking one thing for another, are typically visual. Hallucinations, perceptions with no real stimulus behind them, are common, especially in younger patients and the hyperactive form. They are often vivid, three-dimensional, full-colour images of animals or people in motion, and they are usually frightening, so patients may try to fight or flee. Delusions and hallucinations in delirium are generally fleeting and easily reshaped by sensory input, unlike the fixed delusions of primary psychotic disorders.
Behaviour and mood shift with the confusion. Patients may grow agitated, fearful, or withdrawn, develop poorly systematised persecutory ideas, or show low mood. Writing is often among the earliest abilities to degrade: letters turn indistinct and sprawling, with errors in spelling and small grammatical words, so asking the patient to write can reveal decline that conversation hides.
The three motor subtypes
Delirium takes three outward forms, distinguished by how much the patient moves and reacts (psychomotor activity):
| Subtype | Share | Key features |
|---|---|---|
| Hyperactive | Roughly 1 in 7 | Agitation, restlessness, hallucinations, autonomic overactivity (overactivity of involuntary functions such as heart rate) |
| Hypoactive | Roughly 1 in 3 | Lethargy with slowed responses and reduced movement |
| Mixed | Roughly half | Alternation between hyperactive and hypoactive states |
These shares are rough general-ward teaching and vary by setting and case-mix: intensive-care cohorts run heavier toward mixed and hypoactive forms. The hyperactive patient is hard to ignore, so hyperactive delirium is recognised quickly and drug intoxication is a common trigger. The hypoactive patient is easy to overlook, and a quiet patient who asks for nothing receives less clinical attention than an agitated one. This is why the missed cases described earlier are mostly hypoactive.
Delirium versus dementia
| Feature | Delirium | Dementia |
|---|---|---|
| Onset | Abrupt, over hours to days | Insidious, over months to years |
| Course | Fluctuating, worse at night | Steadily progressive |
| Attention | Impaired from the start | Intact early, impaired late |
| Alertness | Disturbed, lethargic or hyperalert | Normal early |
| Sleep-wake cycle | Reversed or disrupted | Largely preserved |
| Hallucinations | Common, vivid, changeable | Usually absent early |
The key discriminator is tempo: abrupt and fluctuating against slow and steady. The two conditions are not mutually exclusive. Delirium frequently supervenes on dementia (arises on top of it) and worsens it, having delirium does not mean the patient is demented, and experiencing delirium raises the later risk of dementia. That so many different illnesses and drugs can produce this one recognisable picture raises the question of what is happening in the brain.