1. Definition and clinical relevance
Older patients presenting with confusion, memory complaints, withdrawal or altered behaviour pose one of the most common and consequential diagnostic challenges in clinical practice.
Three conditions dominate the differential:
- Delirium: acute organic brain syndrome with disturbed consciousness, fluctuating attention and cognition; marker of acute physiological stress demanding search for underlying cause
- Dementia: chronic organic brain syndrome with long-term, insidious cognitive decline
- Alzheimer disease: insidious onset with progressive forgetfulness
- Frontotemporal dementias: personality and behaviour change first
- Dementia with Lewy bodies: fluctuation, visual hallucinations, Parkinsonism
- Vascular dementia: sudden onset with focal neurological signs
- Depression: can present with prominent cognitive impairment ("pseudodementia"), apathy, social withdrawal, poor concentration; mimics dementia and hypoactive delirium
Why this matters clinically:
- Delirium is common in hospital, especially in patients aged 65 years or older (10 to 18 percent of older Australians have delirium at admission)
- Associated with increased risk of death, longer length of stay, increased falls, discharge to higher-dependency care, higher chance of subsequently developing dementia
- Depression that masquerades as dementia is treatable and should be addressed first
- Missing delirium superimposed on dementia is one of the most frequent and dangerous errors in old-age care
2. Key values, thresholds and decision rules
The classic distinguishing framework compares onset, course, consciousness and attention.
| Feature | Delirium | Dementia | Depression |
|---|---|---|---|
| Onset | Acute, often dramatic, over hours to days | Insidious, over months to years (vascular may be sudden) | Weeks; may date to a life event |
| Course over 24 hours | Fluctuating; worse at night; review evening and night shift notes | Stable, slowly progressive | Relatively stable, often diurnal mood variation |
| Consciousness / arousal | Impaired, fluctuating | Usually clear until late | Clear |
| Attention | Markedly impaired | Relatively preserved early | May be reduced with poor effort |
| Reversibility | Often reversible with treatment of cause | Usually irreversible / progressive | Reversible with treatment |
| Memory | Impaired, inconsistent | Progressive short-term loss; poor delayed recall | Effortful, "don't know" answers; recall may be intact |
Key decision rules
- The rapidity and duration of disorientation help separate delirium from dementia. Disorientation of rapid onset and fluctuating character favours delirium.
- Hypoactive delirium is easily mistaken for depression. In depression there is typically no change in consciousness level and a lack of the minute-to-minute fluctuation seen in delirium.
- Do not diagnose dementia during an active delirium. Defer the diagnosis until the delirium clears and reassessment has occurred after an appropriate interval (often several months). Serial assessment helps determine whether dementia coexists.
- Depression may present as pseudodementia and should be treated first; serial assessment then clarifies whether true dementia coexists.
Interpreting cognitive testing
| Point | Implication |
|---|---|
| Education and English proficiency affect scores | Low education or non-native English speakers may score poorly on some components, but memory is affected less by these factors |
| Delayed recall is discriminating | A patient scoring 25/30 who recalls 3/3 objects at 5 minutes is less likely to be developing dementia than one scoring 28/30 who recalls only 1/3 |
Depressive symptoms occur in 20 to 40 percent of dementia cases and are more common in earlier-stage dementia and vascular cognitive impairment.
3. Approach: presentation and differential
History
Obtain a collateral history from a relative, carer or bystander who knows the person well. This is essential both to detect subtle change in affect or behaviour and because the patient may be unable to give a coherent account. Tools such as MAPLE-V can structure collateral history taking.
Clarify:
- Onset and duration (crucial for discriminating acute delirium from chronic confusion of dementia).
- Precipitating factors: recent fall, new or multiple medicines, infection, constipation, urinary retention.
- Fluctuation: ask specifically about worse periods, especially at night.
- Baseline function: what could the person do a week, a month, a year ago.
- Mood, sleep, appetite, anhedonia and any suicidal thoughts to screen for depression.
- Behaviour and personality change (may point to frontotemporal dementia), visual hallucinations, Parkinsonism and fluctuation (Lewy body dementia).
Examination
- Mental state: level of consciousness and arousal, attention, orientation. Assess thought form (flight of ideas in mania and delirium; poverty of ideas in depression, schizophrenia and dementia) and content (delusions, hallucinations, suicidal or paranoid ideation).
- Cognitive screen with a validated instrument. Weight delayed recall heavily.
- Physical examination including vital signs; look for focal neurological signs (vascular dementia), gait abnormality and early incontinence, Parkinsonism, and signs of infection or dehydration.
- Consider deafness and sensory impairment, which can masquerade as confusion.
Differential
Delirium in early stages can mimic anxiety, depression, hallucinatory states, agitated schizophrenia (rarely), extreme mania, complex partial seizures and dementia. Consider anticholinergic delirium from drugs with anticholinergic properties or illicit substances: features include hyperactivity, marked thought disorder, vivid visual hallucinations and very disturbed behaviour. Substance intoxication and withdrawal states are also key differentials.
4. Investigations
Investigations are directed at finding the cause of delirium and excluding reversible contributors to cognitive decline.
Bedside
- Vital signs including temperature.
- Blood glucose level to exclude hypoglycaemia or hyperglycaemia.
- Oxygen saturation to exclude hypoxic causes of behaviour change.
- Urine for microscopy, culture and sensitivity, and a drug screen where relevant.
Bloods
A screen to exclude common metabolic, endocrine and nutritional contributors:
- Full blood count
- Urea and electrolytes (including sodium; abnormal serum sodium predisposes to delirium)
- Calcium and parathyroid hormone
- Thyroid function tests
- Vitamin B12, folate, niacin and thiamine
- Broader biochemistry panel (CHEM20) including liver and renal function.
Imaging
- Neuroimaging (CT or MRI brain) is considered when focal signs, sudden onset, head injury or an unexplained picture warrant it, guided by clinical findings.
- Neuropsychological testing is useful for diagnosing dementia in its early stages and where cognitive screening leaves the diagnosis in doubt. Its utility in distinguishing dementia subtypes or in diagnosing depression in patients with dementia is less well established.
Screening scales for depression such as the Geriatric Depression Scale or the Cornell Scale for Depression in Dementia may help when depression is suspected within a cognitive presentation.
A formal diagnosis of dementia should be made only after comprehensive assessment: history from the patient, history from an informant, cognitive and mental state examination with a validated instrument, physical examination, medication review to identify and minimise agents that impair cognition, and explicit consideration of other causes including delirium and depression.
5. Management
Management follows the diagnosis.
Delirium
- Identify and treat the underlying cause. Treat the cause as you would in a younger person, weighing benefits and harms and respecting the patient's wishes where these can be ascertained. Address infection, constipation, urinary retention, hypoxia, glycaemic and sodium derangement, and offending medicines.
- Non-pharmacological measures first: reorientation, familiar carers, correction of sensory impairment (glasses, hearing aids), hydration, sleep-wake normalisation and avoidance of unnecessary catheters.
- Pharmacological sedation is a last resort. Antipsychotics should be reserved for patients with another indication for antipsychotic use or an existing prescription. Over-sedation risks dehydration, falls, respiratory depression, pneumonia and death.
- Avoid benzodiazepines in delirium: complications are common and long-acting benzodiazepines worsen delirium.
- Extreme caution with antipsychotics in Parkinson disease and Lewy body dementia, who are at increased risk of severe adverse reactions.
Depression (including pseudodementia) Treat depression in the first instance where it is suspected as the primary cause of cognitive impairment. Use antidepressant therapy guided by current Therapeutic Guidelines (Psychotropic), together with psychological support and social measures. Reassess cognition serially after treatment to determine whether dementia coexists.
Dementia
- Comprehensive, ideally multidisciplinary assessment and care planning.
- Provide a clear, honest and supportive explanation of the diagnosis.
- Regularly assess for medical comorbidities and key psychiatric features including depression and psychosis.
- Review and simplify medications, minimising drugs (including over-the-counter products) with anticholinergic properties, which are associated with cognitive decline.
- Monitor risk factors and remain alert to the increased risk of depression and delirium in people with dementia.
6. Australian-specific considerations
- The Delirium Clinical Care Standard (Australian Commission on Safety and Quality in Health Care) underpins Australian inpatient practice: screen at-risk older patients, use non-drug strategies first, restrict antipsychotics and avoid benzodiazepines.
- Timely diagnosis of dementia is a national priority, reflecting the central role of the primary care physician supported by access to specialists and multidisciplinary teams through defined care pathways. Timely diagnosis allows planning for finances and lifestyle.
- Use Therapeutic Guidelines (Psychotropic) for drug selection and dosing in depression and behavioural symptoms.
- Rural and remote clinicians can apply the Queensland Health Primary Clinical Care Manual approach to distinguishing delirium, dementia and depression, including the recommended bedside and blood test panel.
- GPs can coordinate collateral history, medication review, cognitive screening and depression screening within Medicare-supported chronic disease and mental health care arrangements, referring to geriatric, old-age psychiatry or memory services as needed.
Clinical pearls
- Acute onset plus fluctuation plus impaired attention equals delirium until proven otherwise. Read the evening and night nursing notes to detect fluctuation.
- Never diagnose dementia during an active delirium. Wait for the delirium to clear and reassess after an appropriate interval.
- Hypoactive delirium is the great mimic of depression. Preserved, unchanging consciousness favours depression; fluctuating consciousness favours delirium.
- Treat depression first when pseudodementia is possible; it is reversible, and serial assessment then reveals any underlying dementia.