Clonus, Hyperthermia, and a New Antibiotic in a Patient on Sertraline and Tramadol

This is serotonin syndrome. The discriminating features are all present:

• Time course: hours to a day after adding a serotonergic drug - matches the two-day linezolid course. NMS evolves over days and follows antipsychotic initiation or dose change; there is no antipsychotic here.
• Clonus - inducible ankle clonus is the single most useful sign, and ocular clonus is nearly pathognomonic. NMS shows rigidity and bradyreflexia, not clonus.
• Hyperreflexia disproportionate in the lower extremities - characteristic of serotonin syndrome.
• Hyperactive bowel sounds and mydriasis - serotonin excess. Anticholinergic toxicity gives absent bowel sounds, dry skin, and urinary retention.
• CK only 480 U/L - consistent with agitation and tremor. NMS typically gives CK in the thousands; malignant hyperthermia gives hypercapnia, masseter rigidity, and hyperkalemia after volatile anesthetics or succinylcholine, none of which apply.

Hunter criteria are met: a serotonergic agent plus spontaneous clonus (or inducible clonus with agitation/diaphoresis, or tremor with hyperreflexia).

Mechanism: linezolid is a reversible, nonselective monoamine oxidase inhibitor at clinical doses - at 600 mg BID it inhibits MAO-A enough to block serotonin catabolism. Sertraline blocks SERT; tramadol blocks SERT and NET and is itself a weak μ-agonist (and a CYP2D6 substrate, with sertraline as a mild inhibitor); sumatriptan is a 5-HT₁B/₁D agonist. Four serotonergic agents converge, and the MAOI plus SERT blockade is the critical pair. Excess synaptic serotonin overstimulates 5-HT₂A and brainstem 5-HT₁A receptors, producing the neuromuscular and autonomic findings.

Lorazepam 2 mg IV is appropriate - benzodiazepines are first-line for mild to moderate serotonin syndrome. They reduce agitation (which is driving the hyperthermia and the CK) and act on GABA-A to blunt the serotonin-mediated hyperadrenergic state. Titrate to effect; repeated doses are expected.

Dantrolene: no. It blocks ryanodine-receptor calcium release from skeletal muscle and is indicated for malignant hyperthermia and used in NMS. It has no meaningful role in serotonin syndrome, whose hyperthermia is largely driven by agitation, muscle contraction from clonus, and autonomic hyperactivity. Give it and you will delay effective treatment and may add hepatotoxicity (her ALT is already 60).

Antipsychotics (e.g., haloperidol): avoid. Sometimes used for NMS; in serotonin syndrome they can worsen hyperthermia and lower the seizure threshold.

Management:

1. Stop all four serotonergic agents - sertraline, tramadol, linezolid, sumatriptan. Substitute a non-serotonergic antibiotic for the cellulitis (e.g., trimethoprim-sulfamethoxazole, doxycycline, or clindamycin per susceptibilities). Do not simply continue linezolid and hope.
2. Supportive care: IV crystalloid, cooling measures, continuous cardiac monitoring, avoid physical restraints (they worsen hyperthermia and rhabdomyolysis).
3. Escalate to intubation with a nondepolarizing paralytic and aggressive cooling if temperature exceeds 41.1°C or rigidity is uncontrolled.
4. Cyproheptadine (5-HT₂A antagonist) 12 mg orally/enterally, then 2 mg every 2 hours - reserve for moderate-to-severe cases not responding to benzodiazepines; the evidence is anecdotal.
5. Monitor for rhabdomyolysis and AKI (serial CK, creatinine, urine output), DIC, and ARDS. Recheck the CK - 480 U/L is early.
6. The syndrome typically resolves within 24 hours of withdrawal with reversible MAO inhibition; fluoxetine and irreversible MAOIs are the exceptions that require a longer washout.

Do not restart while she is still on linezolid. Linezolid's MAO inhibition is reversible but persists for its dosing interval; restarting sertraline concurrently re-creates the interaction. If the cellulitis requires 10–14 days of linezolid, hold sertraline until the course ends and confirm the antibiotic can be changed - it usually can.

Identify the drug to sacrifice. Tramadol is the least valuable of the serotonergic agents here and carries the most baggage: serotonergic, lowers the seizure threshold, is a CYP2D6 substrate with variable analgesia, and is a poor choice in a patient with depression on an SSRI. Discontinue it and address her back pain with non-opioid, non-serotonergic options (topical NSAIDs, duloxetine only if sertraline is changed, physical therapy, acetaminophen). Four serotonergic drugs is three too many.

Antibiotic stewardship: document linezolid as an interaction hazard in the allergy/intolerance field. If a future MRSA infection requires linezolid, the plan should be to hold the SSRI, use the lowest effective dose, monitor for clonus and hyperreflexia daily, and stop at the first sign.

Counseling points for the patient:

• Symptoms to report immediately: muscle twitching, jumping muscles, shivering, sweating, fast heartbeat, confusion, fever.
• Sumatriptan plus sertraline alone is a low but real risk; she should not stack serotonergic over-the-counter products (dextromethorphan, St. John's wort, some sleep aids).
• Do not restart tramadol or any MAOI without the prescriber checking her medication list.

Documentation and handoff: notify the prescribing clinician of the interaction, place a note in the pharmacy record, and ensure the discharge medication list does not silently reintroduce tramadol. Reassess her depression regimen at follow-up rather than assuming the pre-admission dose was optimal.