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Difficulty: HardMedical History Evaluation

A 45-year-old man is brought to the emergency department by his spouse due to acute-onset confusion, agitation, and profound diaphoresis. His medical history includes major depressive disorder, for which he has taken citalopram for three years, and gastroesophageal reflux disease, managed with daily omeprazole. Ten days ago, he was discharged from the hospital after receiving treatment for a complicated methicillin-resistant *Staphylococcus aureus* (MRSA) cellulitis; he is currently completing a 14-day oral course of linezolid. Over the past 48 hours, he developed severe nausea and began taking ondansetron, which he had left over from a prior prescription. He also reports taking several doses of ibuprofen for a mild headache. On physical examination, his temperature is 39.1°C (102.4°F), blood pressure is 168/96 mm Hg, and pulse is 122/min. Neurologic examination reveals spontaneous ocular clonus, generalized hyperreflexia, and inducible ankle clonus that is most pronounced in the lower extremities. Which of the following agents from this patient's pharmacological history synergistically interact to precipitate his current toxidrome? (Select all that apply)

  1. CitalopramAnswer
  2. LinezolidAnswer
  3. OndansetronAnswer
  4. D
    Omeprazole
  5. E
    Ibuprofen

Answer

The medications that synergistically interact to precipitate this patient's serotonin syndrome are citalopram, linezolid, and ondansetron.
The patient's clinical presentation of hyperthermia, autonomic instability, altered mental status, and prominent clonus is characteristic of serotonin syndrome. This toxidrome is caused by excessive serotonergic activity in the central and peripheral nervous systems, typically resulting from the concurrent use of multiple serotonergic medications. In this patient's history, citalopram (an SSRI), linezolid (an oxazolidinone antibiotic with non-selective MAOI properties), and ondansetron (a 5-HT3 receptor antagonist) all directly increase serotonergic tone. Their combined use precipitated the severe toxidrome.

Step-by-Step Solution

1
Evaluate the patient's acute physical examination findings.
The patient exhibits hyperthermia, autonomic instability, altered mental status, and distinct neuromuscular hyperactivity characterized by spontaneous ocular and inducible ankle clonus.
To establish a primary syndromic diagnosis, which in this case is highly consistent with serotonin syndrome.
2
Review the patient's chronic medication history.
The patient has been on maintenance therapy with citalopram, a selective serotonin reuptake inhibitor (SSRI), for three years.
SSRIs form the foundational risk factor for serotonin toxicity, but typically require the addition of interacting agents to precipitate a life-threatening crisis.
3
Analyze recently added medications for serotonergic or interacting properties.
Linezolid is identified as possessing weak MAOI activity, and ondansetron is identified as a serotonergic 5-HT3 receptor antagonist.
To determine the exact precipitants; the combination of an SSRI with an MAOI (linezolid) and a 5-HT3 antagonist (ondansetron) causes profound serotonin accumulation, directly explaining the toxidrome.

Key Concept

Serotonin syndrome is a life-threatening toxidrome precipitated by the concurrent use of multiple serotonergic medications. A thorough medical history evaluation must identify 'hidden' serotonergic agents, such as the antibiotic linezolid (which has MAOI activity) and antiemetics like ondansetron, especially in patients already taking baseline SSRIs.
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Medical History Evaluation

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