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Difficulty: HardFormulating a Differential Diagnosis

Paramedics bring a 72-year-old man to the emergency department due to a sudden change in mental status. His daughter reports that over the past 24 hours, he has become increasingly agitated and started conversing with people who are not in the room. He has a 5-year history of Parkinson disease and was recently evaluated for urinary frequency, for which a new oral medication was prescribed 4 days ago. Other medical history includes hypertension and benign prostatic hyperplasia. His home medications include carbidopa-levodopa, lisinopril, and the newly prescribed medication. Vital signs are temperature 37.9∘C37.9^\circ\text{C} (100.2∘F100.2^\circ\text{F}), blood pressure 138/86 mmHg, heart rate 115/min, and respiratory rate 16/min. Physical examination shows flushed, warm, and completely dry skin. His pupils are 7 mm bilaterally and sluggishly reactive to light. Abdominal examination reveals a distended, palpable mass in the suprapubic region and absent bowel sounds. Neurologic examination shows a confused, picking behavior with a resting tremor in the right hand. Which of the following is the most likely diagnosis explaining this patient's acute presentation?

  1. Anticholinergic toxicityAnswer
  2. B
    Dopamine agonist-induced psychosis
  3. C
    Delirium secondary to urinary tract infection
  4. D
    Dementia with Lewy bodies
  5. E
    Serotonin syndrome

Answer

The correct answer is anticholinergic toxicity. The patient's presentation of acute delirium, tachycardia, mydriasis, dry skin, absent bowel sounds, and urinary retention shortly after starting a new medication for overactive bladder is highly characteristic of this toxidrome.
This patient's acute presentation of agitation, visual hallucinations, tachycardia, mydriasis, dry/flushed skin, absent bowel sounds, and acute urinary retention (suprapubic mass) represents the classic anticholinergic toxidrome. This is often remembered by the mnemonic 'mad as a hatter, hot as a hare, blind as a bat, red as a beet, dry as a bone, and the bowel and bladder lose their tone.' The patient was recently prescribed a new medication for urinary frequency (overactive bladder). Medications used for this indication, such as oxybutynin or tolterodine, are antimuscarinic agents. Older adults, particularly those with underlying benign prostatic hyperplasia, are highly susceptible to anticholinergic adverse effects, which can rapidly precipitate acute urinary retention and subsequent delirium.

Step-by-Step Solution

1
Analyze the patient's acute symptoms and physical examination findings.
The patient presents with acute delirium, tachycardia, low-grade fever, mydriasis, completely dry skin, absent bowel sounds, and a palpable suprapubic mass indicating urinary retention.
Identifying the constellation of physical signs is necessary to recognize a specific systemic toxidrome.
2
Correlate the physical findings with the patient's recent medical history.
The patient recently started a new medication for urinary frequency. Medications for overactive bladder are typically antimuscarinic agents (e.g., oxybutynin, tolterodine).
Establishing a temporal relationship between a new medication and symptom onset points to an adverse drug event.
3
Synthesize the data to formulate the most likely differential diagnosis.
The combination of antimuscarinic use and the classic peripheral signs (dry skin, dilated pupils, urinary retention, ileus) confirms anticholinergic toxicity, overriding other overlapping causes of delirium.
Distinguishing this toxidrome from other causes of altered mental status in the elderly (such as infection or progression of Parkinson's disease) is critical for appropriate patient management.

Key Concept

Formulating a differential diagnosis for acute altered mental status by identifying classic toxidrome patterns and recognizing anticholinergic toxicity in a geriatric patient with polypharmacy.
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Formulating a Differential Diagnosis

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