Delirium vs Dementia for NEET PG: Onset, Attention, Course and Reversibility Compared

Delirium is an acute, fluctuating disturbance of attention and awareness with a medical, toxic or withdrawal cause, and it is usually reversible. Dementia is a chronic, usually progressive decline in cognition that erodes independence, with attention preserved until late. The Indian Psychiatric Society (IPS) calls delirium a psychiatric emergency (IPS delirium guideline, 2018).

The distinction matters because delirium is the one you cannot miss. In a 2026 cohort of 2,556 hospitalised adults aged 65 or over, delirium occurred in 37% and predicted higher 90-day mortality (adjusted HR 3.45; 95% CI 2.83-4.20) (Age and Ageing, 2026).

Key Takeaways

• Onset separates them first: hours to days in delirium, months to years in dementia.

• Attention is the core test. It is impaired in delirium and usually preserved in dementia until late stages.

• Across a single day, delirium fluctuates and tends to be worse at night, while dementia stays steady (sundowning aside) unless it is dementia with Lewy bodies (DLB).

• Reversibility is the contrast to remember: treating the cause usually clears delirium, whereas dementia usually progresses, and a treatable cause is no guarantee of full reversal.

• When the picture is unclear, assume delirium first: after age, dementia is the second most frequent risk factor for it.

• A 4AT total of at least 4 flags possible delirium. Haloperidol stays a last resort, kept for severe agitation or risk.

Delirium vs Dementia: The Comparison Table

This table follows Table 1 of Martins and Fernandes (Frontiers in Neurology, 2012) and agrees with the IPS delirium and dementia guidelines.

Delirium can persist in some patients, so "always reversible" is too strong. Evening worsening, or sundowning, is a descriptive label rather than a diagnosis, and it occurs in dementia and in delirium alike. Delirium is often worse at night (Martins and Fernandes, Table 1), while sundowning behaviours can reflect either dementia or delirium (Khachiyants et al., 2011).

Onset and Course in Delirium vs Dementia

DSM-5 delirium develops over hours to a few days and fluctuates during the day (European Delirium Association and American Delirium Society, 2014). An ICD-10 delirium episode can last up to 6 months in total (IPS delirium guideline).

Dementia is the slow counterpart. ICD-10 asks for decline present for at least six months before the diagnosis is confident, a rule meant to avoid mislabelling reversible states such as subdural haemorrhage and normal pressure hydrocephalus (IPS dementia guideline).

A memory aid of our own, not a standard mnemonic: delirium is ABC (Acute onset, Broken attention, Changing course through the day). Dementia is SSS (Slow onset, Steady course, attention Spared until late). DLB breaks the "steady" rule because its cognition fluctuates.

Attention and Arousal: The Core Difference Between Delirium and Dementia

Impaired attention is the defining deficit of delirium, whereas attention stays normal in dementia except in late stages (Martins and Fernandes). DSM-5 pairs it with reduced awareness of the environment, and a fully alert patient can still be profoundly inattentive, but the reverse does not hold (European Delirium Association and American Delirium Society).

Perception differs too: hallucinations are frequent and mainly visual in delirium, and often absent in dementia until advanced stages or DLB. Schizophrenic hallucinations are persistent and usually auditory (Martins and Fernandes); see our schizophrenia notes for first-rank symptoms.

EEG is not routine in delirium (IPS delirium guideline), but studies report generalised theta or delta slowing when it is recorded (Wiegand et al., 2022).

Diagnostic Criteria for Delirium and Dementia

DSM-5 Criteria for Delirium

The five DSM-5 criteria, paraphrased from the 2014 European and American Delirium Association paper, are:

1. Impaired attention and reduced awareness of the environment.
2. Development over hours to a few days, a change from baseline, and fluctuation during the day.
3. Disturbance in a further cognitive area, for example perception, visuospatial ability, language, orientation or memory.
4. Not better explained by a pre-existing or evolving neurocognitive disorder, and not occurring with severely reduced arousal such as coma.
5. History, examination or laboratory findings showing that the disturbance arises directly from another medical condition, substance intoxication or withdrawal, or toxin exposure, or from several of these causes together.

Acute confusional state is a synonym; recent literature writes "acute confusional state, or delirium" (Brain Sciences, 2026).

Criteria for Dementia

DSM-5 major neurocognitive disorder needs substantial impairment in at least one of six domains (complex attention, executive function, learning and memory, language, perceptual-motor function, social cognition) that interferes with independence and does not occur only during delirium (Eramudugolla et al., 2017). In mild neurocognitive disorder, independence is preserved.

The ICD-10 criteria (IPS dementia guideline) are:

• G1: decline in memory (new learning) plus decline in other abilities such as judgment and planning.
• G2: preserved awareness of the environment; if delirium is superimposed, defer the dementia diagnosis.
• G3: at least one of emotional lability, irritability, apathy or coarsened social behaviour.
• G4: G1 present for at least six months for a confident diagnosis.

Delirium Subtypes and Causes

Delirium is subtyped by psychomotor activity (Stollings et al., 2021). Hyperactive delirium brings restlessness, agitation and hypervigilance. Hypoactive delirium is the most difficult subtype to detect (Stollings et al.). It brings lethargy, apathy and slowed answers, so it is easily mistaken for fatigue, depression or advanced dementia, and it carries a poorer prognosis (IPS delirium guideline). Mixed delirium alternates between the two.

Think of causes as vulnerability plus insult. Predisposing factors include advanced age, dementia, depression, sensory impairment, polypharmacy and alcohol misuse; after age, dementia is the second most frequent (Martins and Fernandes). The table lists common precipitating insults.

This grouping is our own study aid, built from Martins and Fernandes and Stollings et al.; it is not a published mnemonic.

Delirium Superimposed on Dementia and Lewy Body Dementia

Nearly half of hospitalised patients with dementia develop delirium superimposed on dementia, or DSD (Age and Ageing, 2026). Any sudden deterioration in sensorium, activity level or behaviour in a person with dementia should raise the possibility of DSD (IPS dementia guideline). Delirium also raises the risk of later dementia, by as much as sixfold at 3-year follow-up (Martins and Fernandes).

DLB is the main mimic of delirium. Fluctuating alertness, visual hallucinations and REM sleep disorder look like delirium, and delirium preceded the DLB diagnosis in about 25% of cases versus 7% in Alzheimer disease (Nitchingham and Caplan, 2021). Whenever there is uncertainty, the safest first move is to assume delirium, investigate and manage accordingly (same review).

Bedside Tools to Separate Delirium From Dementia

Sources: Bellelli et al., 2014 (4AT), Martins and Fernandes (CAM) and Maduro et al., 2025 (MMSE and MoCA cut-offs).

A low MMSE or MoCA score does not say whether the cause is acute or chronic, so use onset, attention and fluctuation for that. The 4AT works with or without dementia (Bellelli et al.).

Dementia Types Including Alzheimer Disease

WHO estimates that Alzheimer disease may contribute to 60-70% of dementia cases (WHO dementia fact sheet). The patterns below come from the IPS dementia guideline and, for DLB, a 2025 Journal of Neurology review.

Reversible Dementia: What Can Actually Be Treated

Only a minority of dementias have a treatable cause. The reported frequency of potentially reversible dementia ranges from 0 to 23%, and an Indian study attributed 18% of dementias to reversible causes, mainly neuroinfections and vitamin B12 deficiency (Tripathi and Vibha, 2009). A 2003 meta-analysis in the same review found 9% potentially reversible, but only 0.31% fully reversed and 0.29% partly reversed. A treatable cause therefore does not guarantee full reversal.

The same review offers the mnemonic DEMENTIA for reversible causes:

• Drugs (any drug with anticholinergic activity)
• Emotional (depression)
• Metabolic (hypothyroidism)
• Eyes and ears declining
• Normal pressure hydrocephalus
• Tumour or other space-occupying lesion
• Infection (syphilis, AIDS)
• Anaemia (vitamin B12 or folate deficiency)

Normal pressure hydrocephalus presents with gait disturbance (usually first), dementia and urinary incontinence, although only about 60% of patients show the full triad. Improvement after removing 30-60 mL of CSF predicts a shunt response, but a negative tap does not rule one out (Nassar and Lippa, 2016).

Depression can mimic both hypoactive delirium and dementia. It tends to be insidious and non-fluctuating, with unaffected consciousness, earlier episodes and predominant mood symptoms (Martins and Fernandes). The IPS dementia work-up includes thyroid function, calcium, VDRL, vitamin B12 and folate, with CT or MRI as indicated.

Treatment Differences Between Delirium and Dementia

Managing Delirium

The IPS guideline gives five steps: diagnose and subtype, find and treat the cause, control symptoms, prevent complications and inform the family. Non-drug care comes first: reorientation, clocks and calendars, glasses and hearing aids, sleep hygiene, hydration, early mobilisation and family presence. Multicomponent non-drug programmes lowered the odds of hospital-acquired delirium (pooled OR 0.47; 95% CI 0.38-0.58) (Nitchingham and Caplan).

Treat haloperidol as a last resort. The IPS delirium guideline reserves antipsychotics for severe agitation, distress or risk, or symptoms that persist despite non-drug measures, and it prefers haloperidol. It gives a low starting dose of 0.25-0.50 mg every 4 hours in the elderly, with ECG monitoring for QT prolongation.

The APA 2025 delirium guideline advises against antipsychotics to prevent or shorten delirium, and ICU PADIS guidance suggests against routine antipsychotics because no large trials show they treat delirium (Stollings et al., 2021). If de-escalation fails and an antipsychotic is still needed, keep the course as short and the dose as low as possible.

Antipsychotics should be avoided in DLB and Parkinson disease (Martins and Fernandes, reporting NICE 2010), and DLB in particular carries severe neuroleptic sensitivity (IPS dementia guideline). For wider drug notes, see our psychiatry revision post on antipsychotics and lithium.

Benzodiazepines can worsen cognition and cause excess sedation, so they are not first line. The IPS delirium guideline allows them when delirium is due to sedatives or alcohol withdrawal or comes with seizures, and contraindicates them in hepatic encephalopathy.

Alcohol Withdrawal Delirium

Benzodiazepines are the mainstay, aiming for a calm but awake patient. Intravenous diazepam is usual, and lorazepam suits the elderly and patients with hepatic or pulmonary disease because of over-sedation risk. Give at least 250 mg of parenteral thiamine daily for the first 3-5 days, and 500 mg a day for 3-5 days if Wernicke encephalopathy is suspected.

Delirium usually shows a reversed sleep-wake cycle. Alcohol withdrawal delirium is typically hyperactive, with about 8% mortality (Kattimani and Bharadwaj, 2013).

Managing Dementia

Approvals for cognitive impairment in Alzheimer disease differ by stage (IPS dementia guideline). Donepezil is approved at every stage; rivastigmine and galantamine, for mild to moderate disease; and memantine, alone or with donepezil, for moderate to severe disease.

There is no clear evidence for these drugs in vascular or frontotemporal dementia, and they are not recommended for mild cognitive impairment. Antipsychotics in dementia are associated with more cerebrovascular events and higher mortality, so keep them for distressing or dangerous symptoms.

Common Mistakes When Comparing Delirium and Dementia

• Calling new confusion "dementia". A sudden change in an older patient is delirium until proven otherwise.
• Missing hypoactive delirium. A quiet, sleepy patient is easily read as depressed or "just dementia".
• Using sundowning to decide. Evening worsening occurs in both conditions.
• Using MMSE or MoCA to separate them. They grade cognition, not onset or attention.
• Treating haloperidol as the cure. The cause still needs treatment.
• Calling every dementia irreversible, or every treatable cause fully curable. A minority have a treatable cause, but full recovery is not guaranteed.

A practical drill: draw the comparison table from memory, then ask of any case stem: how fast did it start, is attention impaired, does it fluctuate, and is there a medical, drug or withdrawal cause?

Conclusion

Delirium and dementia differ most on onset and attention. Delirium starts within hours to days, fluctuates, disturbs attention and usually reverses when the cause is treated. Dementia starts slowly, spares attention until late and is usually progressive, though a minority have a treatable cause. Because dementia raises the risk of delirium, always look for a superimposed acute change. Practise the distinction with the organic and cognitive disorders question set.