A 72-year-old man with a 3-year history of progressive memory loss and difficulty recognizing family members is brought to the emergency department by his son. Over the past 6 hours, the patient has become acutely agitated, is unable to focus attention, and is seeing people in the room who are not present. His temperature is 38.5°C, heart rate 110/min, and blood pressure 145/90 mmHg. Urinalysis shows pyuria and positive nitrites. The son reports that the patient's baseline cognitive decline has been slow and steady, but this acute change occurred suddenly after he developed dysuria yesterday. What is the most likely diagnosis?

Explanation

## Clinical Reasoning This case presents a classic scenario of **acute delirium superimposed on pre-existing dementia**. ### Key Distinguishing Features **Key Point:** The temporal relationship is critical — the patient has a *known baseline* of slow, progressive cognitive decline (dementia) over 3 years, but an *acute change* (hours) with hallucinations, agitation, and inability to focus attention (delirium). ### Delirium vs Dementia Comparison | Feature | Delirium | Dementia | |---------|----------|----------| | **Onset** | Acute (hours to days) | Insidious (months to years) | | **Course** | Fluctuating throughout day | Steady, progressive | | **Attention** | Severely impaired | Relatively preserved early | | **Consciousness** | Altered (hypoactive/hyperactive/mixed) | Normal until late stages | | **Reversibility** | Often reversible if cause treated | Usually irreversible | | **Hallucinations** | Common, often visual | Less common, late in disease | | **Autonomic signs** | Tachycardia, hypertension, fever | Usually absent | **High-Yield:** In older adults with dementia, delirium is frequently triggered by **infection** (UTI, pneumonia), **medication**, **metabolic derangement**, or **hypoxia** — the "DELIRIUM" mnemonic. ### Clinical Pearl **Clinical Pearl:** The presence of fever (38.5°C), tachycardia, pyuria with nitrites, and dysuria strongly suggests **urinary tract infection (UTI)** as the precipitant. UTI is one of the most common causes of delirium in elderly patients with dementia, and treatment of the infection often reverses the delirium. ### Why This Is Not the Other Options - **Rapidly progressive dementia** would show a more gradual cognitive decline over weeks to months, not acute hallucinations and agitation within hours. - **Lewy body dementia** can present with hallucinations, but the acute temporal relationship to infection and the presence of autonomic instability point to delirium, not primary LBD. - **Vascular dementia** would not explain the acute onset of hallucinations and the clear infectious trigger (UTI). **Mnemonic: DELIRIUM** — **D**rugs, **E**lectrolytes, **L**ack of oxygen, **I**nfection, **R**enal failure, **I**ntracranial, **U**reteral (UTI), **M**etabolism.