Serotonin Syndrome and NMS — Viva

Long Case Viva Defence

Candidate's opening statement (model answer)

"Mrs Margaret Chen is a 68-year-old retired teacher presenting with acute agitation, confusion and hyperthermia, 18 hours after linezolid was added to her longstanding sertraline and regular tramadol for MRSA osteomyelitis of her diabetic foot. She has type 2 diabetes, major depressive disorder, and chronic back pain. [1]

Her main problems are:

1. Severe serotonin syndrome from the triple serotonergic combination — sertraline, tramadol and linezolid — confirmed by the Hunter criteria (inducible clonus, ocular clonus, lower-limb hyperreflexia, agitation, autonomic instability)
2. MRSA osteomyelitis requiring an alternative antibiotic plan now linezolid is contraindicated
3. Type 2 diabetes with a diabetic foot ulcer
4. Major depressive disorder requiring a safe antidepressant re-challenge plan
5. Chronic back pain requiring a non-serotonergic analgesic strategy [1]

Her immediate management has been to stop all serotonergic agents, give benzodiazepines for agitation, active cooling, intravenous fluids, and oral cyproheptadine as the 5-HT2A antagonist. She has been admitted to the high-dependency unit. I have substituted vancomycin for linezolid and am working with infectious diseases on her osteomyelitis, and with consultation-liaison psychiatry on her depression management." [1]

Examiner probing questions and model answers

Q1: "How did you confirm this was serotonin syndrome and not sepsis or NMS?" [1]

"I made a clinical diagnosis using the Hunter Serotonin Toxicity Criteria, which require recent serotonergic drug exposure plus any one of five clinical findings — in her case, inducible clonus with agitation and diaphoresis. The Hunter criteria have a sensitivity of 84 per cent and specificity of 97 per cent, and were developed by Isbister and colleagues in Australia from over 2,000 toxicology admissions — they are now the international diagnostic standard and superior to the older Sternbach criteria [2]. Sepsis remains a differential that I have not dismissed — I drew blood cultures, sent urine and swab cultures, and I am monitoring for a source — but her mydriasis, clonus and lower-limb hyperreflexia are not features of sepsis encephalopathy. NMS is excluded by the onset (18 hours, not days to weeks), the presence of clonus and hyperreflexia (NMS has bradyreflexia), and the absence of lead-pipe rigidity and a markedly elevated CK — her CK is 480, whereas NMS typically produces values into the thousands [5]."

Q2: "Why did this combination cause the syndrome when each drug alone might not?" [1]

"The three drugs raise synaptic serotonin by two different mechanisms. Sertraline blocks serotonin reuptake via SERT, and tramadol also inhibits reuptake. Linezolid is a weak reversible monoamine oxidase inhibitor that blocks serotonin metabolism by MAO-A. Combining a reuptake inhibitor with a metabolism inhibitor produces a multiplicative, not additive, rise in synaptic serotonin — this is the classic lethal pharmacodynamic interaction [1]. The absolute risk of linezolid-associated serotonin toxicity is low — a 2023 meta-analysis found a pooled prevalence of about 0.013 per cent with a single serotonergic agent, but the odds ratio rises to 5.18 when multiple serotonergic agents are combined, which is exactly her situation [8]. The lesson is that linezolid should be used cautiously in any patient on more than one serotonergic agent, and an alternative antibiotic chosen where possible."

Q3: "Walk me through your pharmacological management and the doses." [1]

"First, I stopped all three serotonergic agents. Second, I gave diazepam 10 mg intravenously, repeated to a total of 20 mg, titrated to control her agitation and reduce her muscle tone — benzodiazepines are first-line because they enhance GABAergic inhibition and reduce the muscle-generated heat driving her hyperthermia. Third, I gave cyproheptadine 12 mg via nasogastric tube as a loading dose, then 2 mg every 2 hours — it is a 5-HT2A receptor antagonist and the specific antidote, though it is only available orally and the evidence is limited to case series [1]. Fourth, active cooling with ice packs and fans — I did NOT give paracetamol for the fever because the hyperthermia is muscle-generated, not prostaglandin-mediated. I deliberately avoided propranolol, which is a classic trap — it can prolong toxicity via 5-HT1A antagonism and worsen hypotension — and I avoided antipsychotics for her agitation, which would add dopamine blockade. If she deteriorates with rigidity, seizures or temperature above 41 degrees, my plan is to intubate and paralyse with rocuronium — not succinylcholine, because of the risk of hyperkalaemia in rhabdomyolysis."

Q4: "She recovers. How will you manage her depression and her pain going forward?" [1]

"For her depression, I have involved the consultation-liaison psychiatry team. After full resolution — usually 24 to 72 hours — a serotonergic antidepressant can be re-introduced cautiously as a single agent at low dose with monitoring. The risk of recurrence with a single SSRI at standard dose is low [1]. I would avoid MAOIs permanently and avoid combining agents. For her pain, tramadol must be replaced with a non-serotonergic option — I would use regular paracetamol plus oxycodone as needed, and involve the acute pain service. For her MRSA osteomyelitis, I have substituted vancomycin and am working with infectious diseases on a six-week plan; linezolid is now contraindicated and documented as an adverse reaction in her medication record. I have counselled her and her family about the danger of combining these drugs and given her a medic alert for serotonergic drug interactions."

Q5: "What is the mechanism of cyproheptadine, and what are its limitations?" [1]

"Cyproheptadine is a 5-HT2A receptor antagonist with additional antihistamine (H1) and anticholinergic activity. It is the specific antidote for moderate-to-severe serotonin syndrome. The dose is a 12 mg oral loading dose, then 2 mg every 2 hours to clinical response, to a maximum of 32 mg in 24 hours, then maintenance 8 mg every 6 hours [1]. The key limitations are: first, it is only available as an oral tablet, so it cannot be given intravenously and absorption in the critically ill or vomiting patient is unpredictable; second, there are no randomised controlled trials — the evidence is from case series and expert consensus; third, its anticholinergic effects (sedation, dry mouth, urinary retention) can complicate the picture. For these reasons it is adjunctive, not a substitute for supportive care, benzodiazepines, and paralysis in severe disease."

Q6: "Explain why atypical antipsychotics can still cause NMS." [1]

"NMS results from dopamine D2 receptor blockade, and atypical antipsychotics — risperidone, olanzapine, quetiapine, clozapine, aripiprazole — all antagonise the D2 receptor, though with lower affinity and faster dissociation than typical agents like haloperidol [7]. The risk of NMS with atypicals is lower than with typicals, but it is not zero, and any atypical can precipitate it, especially with rapid dose escalation, high doses, dehydration, or in patients with catatonia or organic brain disease. A particular subtlety is clozapine-induced NMS, which often lacks the classic lead-pipe rigidity because clozapine's anticholinergic and serotonergic activity can mask the extrapyramidal features — so the diagnosis must be considered even when rigidity is absent [7]. The message for the exam is: never dismiss NMS simply because the patient is on an atypical antipsychotic."

Short Case Discussion

Bedside neurological examination of the febrile rigid patient

Examiner instruction: "You are called to see a 55-year-old man on the ward who has become febrile and rigid. The nursing staff are concerned. Describe your focused bedside examination to distinguish serotonin syndrome from neuroleptic malignant syndrome." [1]

Candidate's model answer: [1]

"I would approach this as a medical emergency and examine systematically, focusing on three questions: is this a drug-induced hyperthermia syndrome, which syndrome is it, and is there an alternative explanation. [1]

First, I would take an ABCDE approach to ensure the patient is stable — airway, oxygen saturation, blood pressure, and level of consciousness — because severe cases can deteriorate to airway compromise, seizures, and cardiovascular collapse. [1]

Second, I would establish the medication history — this is the single most powerful discriminator. I would ask specifically about antidepressants (SSRIs, SNRIs, TCAs, MAOIs), analgesics (tramadol, pethidine, fentanyl), antiemetics (metoclopramide, prochlorperazine), antipsychotics (typical and atypical), antibiotics (linezolid), triptans, St John's wort, and recreational drugs. I would ask about recent dose changes and drug additions. [1]

Third, I would perform a focused neurological examination aimed at the neuromuscular domain: [1]

- Tone — I would assess limb tone. Lead-pipe rigidity (velocity-independent, uniform resistance) that is greater in the upper limbs suggests NMS. A tremulous, clonic rigidity greater in the lower limbs suggests serotonin syndrome. - Reflexes — I would assess deep tendon reflexes. Hyperreflexia, especially in the lower limbs, with sustained clonus, points to serotonin syndrome. Bradyreflexia points to NMS. - Clonus — I would specifically test for clonus. At the ankles, I would dorsiflex the foot briskly and hold it; sustained rhythmic contractions indicate clonus. I would test for ocular clonus by asking the patient to look to the extreme lateral gaze and holding it — rhythmic horizontal or vertical eye movements indicate ocular clonus. Inducible or ocular clonus are the most specific signs of serotonin syndrome and meet the Hunter criteria. - Myoclonus — I would observe for sudden, shock-like muscle jerks. - Pupils, skin, abdomen — mydriasis, diaphoresis, diarrhoea and active bowel sounds support serotonin syndrome; dry skin and urinary retention support anticholinergic toxicity. - Mental state — agitation and pressured speech suggest serotonin syndrome; mutism and catatonia suggest NMS. [1]

Fourth, I would examine for a source of sepsis — chest, urine, lines, skin, abdomen — because sepsis can mimic or coexist with a drug reaction, and blood cultures, urine culture and inflammatory markers are mandatory in any febrile patient. [1]

Based on this examination, I would apply the Hunter criteria for serotonin syndrome (serotonergic drug plus clonus or tremor with hyperreflexia) or the Gurrera consensus criteria for NMS (dopamine exposure plus hyperthermia plus rigidity plus altered mental status), and I would initiate the appropriate management immediately — the diagnosis is clinical and must not wait for blood results." [1]