Delirium is an acute, fluctuating disturbance of attention and awareness with an identifiable medical cause; Alzheimer's-type Major Neurocognitive Disorder is a gradual, progressive cognitive decline, typically memory-first, without the acute fluctuation or the same requirement of an identifiable precipitating medical trigger.
Delirium develops over hours to days; Alzheimer's-type decline develops over months to years, so a genuinely new, rapid change in cognition should always raise delirium first, especially in a hospitalized or medically ill person.
Delirium is, in principle, reversible once its cause is treated, though recovery can take time and full return to baseline isn't guaranteed, especially in older or already-cognitively-impaired patients; Alzheimer's-type decline is progressive and not reversible with current treatments.
Delirium's attention/awareness disturbance fluctuates, sometimes within the same day; Alzheimer's-type NCD's attention is more stable day-to-day, with memory impairment (especially for recent events) typically the earliest and most prominent domain.
Delirium carries acute safety and medical risk (associated with increased mortality if untreated); Alzheimer's-type NCD's risk is more about long-term functional decline and safety in daily living over a much longer timeframe.
The decisive question: is the course acute and fluctuating, or gradual and stable? And is there an identifiable medical trigger (infection, medication, surgery)? Delirium can be superimposed on existing Alzheimer's-type NCD, so a sudden worsening in a person with known dementia should always prompt a delirium workup, not be assumed to be the dementia progressing.
Don't assume new confusion in an older adult with known dementia is 'just the dementia getting worse' -- a delirium workup is warranted for any acute change, since delirium superimposed on dementia is common and often has a treatable cause.
Students sometimes treat 'confusion in an older adult' as a single undifferentiated category. Course shape (acute/fluctuating vs. gradual/stable) is the single most useful differentiator, more so than symptom content alone.
If a case describes a sudden change over days, fluctuating alertness, and a plausible medical trigger (infection, surgery, new medication), lean Delirium. If it describes a slow decline over a year or more with stable attention, lean Alzheimer's-type NCD.
| Feature | Delirium | Neurocognitive Disorder Due to Alzheimer's Disease |
|---|---|---|
| Onset speed | Hours to days | Months to years |
| Course | Fluctuating, often within the same day | Gradual and relatively stable day-to-day |
| Reversibility | Often reversible once cause is treated | Progressive, not reversible with current treatment |
An acute, fluctuating disturbance in attention and awareness, developing over a short period, with an additional cognitive disturbance, that is a direct physiological consequence of an underlying medical condition, substance, or other cause.
A progressive decline in one or more cognitive domains (most characteristically memory and learning), caused by Alzheimer's disease pathology, at either a Major (interferes with independence) or Mild (independence preserved) level.