Average Onset for Delirium
Delirium develops acutely over hours to days, not weeks or months, with symptoms that fluctuate within minutes to hours throughout the day. 1
Temporal Characteristics
The 2025 Alzheimer's Association guidelines clearly define the temporal profile of typical delirium:
- Acute changes in cognition, behavior, attention, and level of consciousness develop on timescales of hours to days 1
- Symptoms may fluctuate within minutes to hours depending on the delirium subtype (hyperactive, hypoactive, or mixed) 1
- Cognitive status often varies substantially within a day due to fluctuations in arousal, attention, and psychomotor state 1
Key Diagnostic Distinction from Dementia
The time course is the most critical feature distinguishing delirium from dementia:
- Delirium: hours to days onset with fluctuating course throughout the day 2
- Dementia: months to years onset with stable presentation throughout the day 2
Clinical Recognition Points
The foundation of delirium diagnosis rests on establishing the patient's previous baseline level of cognition, function, and behavior through interviewing a knowledgeable informant to determine the exact timeline of when confusion started. 1, 2
Specific Features to Document:
- Ask specifically about when symptoms began (hours vs. days ago, not weeks) 2
- Identify fluctuations in confusion, such as worsening at night (sundowning) 2
- Determine if there are lucid intervals during the day when the patient seems more alert 1
Critical Pitfall in Elderly Patients with Dementia
Hypoactive delirium is the most commonly missed subtype in elderly patients, particularly those with underlying dementia, and carries higher mortality risk than hyperactive delirium. 3, 2 This subtype may present with:
- Cognitive and motor slowing 1
- Sedated appearance 1
- Nighttime confusion that is mistakenly attributed to baseline dementia 2
Never attribute acute confusion to known dementia without investigating for reversible causes - delirium superimposed on dementia is common (22-89% prevalence in hospitalized elderly with dementia) and represents a medical emergency. 2, 4
Assessment Frequency
Because delirium fluctuates substantially throughout the day, repeated assessments every 8-12 hours (at least once per shift) are necessary using validated tools like the Confusion Assessment Method (CAM). 3, 5