Hyperthermia Syndromes — NMS, Serotonin Syndrome & MH

📘 1 · Washington Manual of Critical Care

Hyperthermia Is Not Fever

It is worth being precise: fever is a regulated rise in the hypothalamic set-point and responds to antipyretics; hyperthermia is uncontrolled heat production overwhelming heat loss, with a normal set-point, and antipyretics do nothing. The drug-induced hyperthermia syndromes — neuroleptic malignant syndrome, serotonin syndrome and malignant hyperthermia — share a lethal final common pathway of extreme temperature, muscle rigidity, rhabdomyolysis and multi-organ failure, but they have different triggers, different tempos, and, crucially, different specific antidotes. Sorting out which one you are facing — from the drug history and the pattern of rigidity and reflexes — directs the right treatment.

Summarised from the Washington Manual of Critical Care — Hyperthermia Syndromes.

The shared emergency

Core temperature often >40 °C with rigidity, autonomic instability and altered mental status.
Complications — rhabdomyolysis (see Rhabdomyolysis), hyperkalaemia, AKI, DIC, seizures, arrhythmia.
Immediate priorities in all — stop the trigger, aggressive cooling, and supportive care — then the syndrome-specific antidote.

📗 2 · Marino's The ICU Book, 5th Edition (2025)

Three Syndromes, Three Fingerprints

The clinical fingerprints separate these syndromes reliably. Neuroleptic malignant syndrome comes on over days after a dopamine blocker, with "lead-pipe" rigidity, hyporeflexia and bradykinesia — it is a hypodopaminergic, sluggish state. Serotonin syndrome comes on within hours of a serotonergic drug, with hyperreflexia, clonus (especially lower-limb, inducible and ocular) and agitation — it is a hyperexcitable state. Malignant hyperthermia appears within minutes to hours of a volatile anaesthetic or succinylcholine, with masseter and generalised rigidity and a soaring end-tidal CO₂ on the ventilator. Read the history and the reflexes and the diagnosis usually declares itself.

Summarised from Marino PL. The ICU Book, 5th Ed — Malignant Hyperthermia & Drug-Induced Hyperthermia.

Feature	NMS	Serotonin syndrome	Malignant hyperthermia
Trigger	Dopamine antagonists (antipsychotics, metoclopramide); or withdrawal of dopaminergic drug	Serotonergic drugs (SSRIs, SNRIs, MAOIs, tramadol, linezolid, triptans, ondansetron, illicit) — often ≥2 in combination	Volatile anaesthetics & succinylcholine (genetic RYR1)
Onset	Days (slow)	Hours (fast)	Minutes–hours (intra/post-op)
Neuromuscular	"Lead-pipe" rigidity, hyporeflexia, bradykinesia	Hyperreflexia, clonus (lower-limb, ocular), tremor, agitation	Masseter + generalised rigidity, rising EtCO₂
Antidote	Dantrolene ± bromocriptine (± amantadine)	Cyproheptadine + benzodiazepines	Dantrolene (+ stop trigger)

📗 Marino — the diagnostic short-cuts

Reflexes are the quickest discriminator: hyper-reflexia + clonus points to serotonin syndrome; "lead-pipe" rigidity with hypo-reflexia points to NMS.
Tempo: serotonin syndrome evolves in hours, NMS over days.
Context: a soaring end-tidal CO₂ and rigidity in the anaesthetised patient is malignant hyperthermia until proven otherwise — call for dantrolene and the MH box immediately.
Consider look-alikes: sympathomimetic/anticholinergic toxidromes, sepsis/meningitis, thyroid storm, heat stroke, status epilepticus.

📋 3 · Antidotes & Cooling

Evidence-Based Management

All three — stop the trigger, cool, support STRONG

Withdraw the offending drug (or restart the withdrawn dopaminergic drug in NMS), and begin aggressive active cooling (evaporative + cold IV fluids; cooling devices; target normothermia). Give generous benzodiazepines for agitation/rigidity (first-line sedative in serotonin syndrome and useful in all), aggressive IV fluids to protect the kidney from rhabdomyolysis, and treat hyperkalaemia, seizures and arrhythmia. Secure the airway if obtunded; avoid antipyretics (ineffective — the set-point is normal) and avoid physical restraints in serotonin syndrome (isometric muscle activity worsens hyperthermia).

Malignant hyperthermia — dantrolene, now MHAUS

Stop the volatile agent and succinylcholine, hyperventilate with 100% oxygen, and give dantrolene 2.5 mg/kg IV, repeated to effect (up to ~10 mg/kg) — dantrolene uncouples excitation-contraction by blocking RYR1 calcium release and is the specific, life-saving antidote. Change to a non-triggering technique, treat hyperkalaemia, acidosis and arrhythmia, cool aggressively, and monitor for DIC and recrudescence. Refer the patient/family for MH testing and document the anaesthetic alert.

NMS — supportive care, dantrolene & bromocriptine STRONG

Stop the dopamine antagonist; the backbone is cooling, fluids and supportive care plus benzodiazepines. For moderate–severe NMS add dantrolene (for the rigidity/hyperthermia/rhabdomyolysis) and/or bromocriptine (a dopamine agonist that reverses the hypodopaminergic state); amantadine is an alternative. Reintroduce the withdrawn dopaminergic drug in NMS caused by withdrawal (e.g. Parkinson's medication). Recovery is slower than serotonin syndrome (days–weeks); watch for recurrence on antipsychotic rechallenge (use a lower-potency agent later, cautiously).

Serotonin syndrome — benzodiazepines & cyproheptadine Hunter criteria

Diagnose with the Hunter criteria (a serotonergic agent plus spontaneous clonus, or inducible/ocular clonus with agitation/diaphoresis, or tremor + hyperreflexia, or hypertonia + temperature >38 °C with clonus). Treat by stopping all serotonergic drugs, generous benzodiazepines, aggressive cooling and supportive care; for moderate–severe cases give cyproheptadine (an oral/NG 5-HT antagonist). It typically resolves within 24 hours of stopping the drug. Avoid physical restraints and avoid serotonergic antiemetics.

🇮🇳 Indian Context

Two practical realities in India: antipsychotics and metoclopramide are very widely used, so NMS is not rare — keep it in mind in any rigid, hyperthermic, obtunded patient on these drugs; and serotonin syndrome is increasingly seen with the combination of SSRIs/SNRIs, tramadol (heavily prescribed), and linezolid (a common antibiotic that is also an MAO inhibitor — a frequently missed interaction). Dantrolene is expensive and often not stocked outside major anaesthetic units — every theatre giving volatile anaesthesia should know where its nearest dantrolene supply is, because MH is a minutes-count emergency. Aggressive cooling, benzodiazepines and fluid resuscitation for rhabdomyolysis are available everywhere and are the highest-yield first steps. Distinguish these from heat stroke (see Environmental Emergencies) in the Indian summer.

💊 4 · Drug Doses

Antidotes & Supportive Agents

Drug	Indication	Dose	Notes
Dantrolene	Malignant hyperthermia; moderate–severe NMS	MH: 2.5 mg/kg IV, repeat to effect (up to ~10 mg/kg)	Blocks RYR1 Ca²⁺ release; reconstitute rapidly; watch weakness/phlebitis
Bromocriptine	NMS (restore dopaminergic tone)	2.5 mg PO/NG 6–8 hourly, titrate	Dopamine agonist; may worsen psychosis; taper slowly
Cyproheptadine	Serotonin syndrome (moderate–severe)	12 mg PO/NG then 2 mg q2h (up to ~32 mg/day)	5-HT₂A antagonist; oral/NG only
Benzodiazepines	All — agitation, rigidity, seizures	e.g. diazepam/lorazepam/midazolam titrated	First-line sedation; reduce heat generation
Active cooling	All — hyperthermia	Evaporative + cold IV fluids; cooling devices	Target normothermia; antipyretics ineffective
IV isotonic fluids	Rhabdomyolysis / AKI prevention	Aggressive, titrate to urine output	See Rhabdomyolysis
Non-depolarising NMB	Refractory rigidity/hyperthermia (intubated)	e.g. rocuronium/vecuronium	Abolishes muscle heat production; NOT succinylcholine (esp. MH-risk)

🗺 5 · Clinical Flowchart

Hyperthermia + Rigidity — Identify & Treat

Recognise hyperthermia (not fever)

Core temp often >40 °C + rigidity + autonomic instability + altered mental status
Immediately: stop the trigger, cool aggressively, benzodiazepines, IV fluids, ABC

Read the drug history + reflexes to identify the syndrome

Volatile/sux + rising EtCO₂ → malignant hyperthermia
Serotonergic drug + hyperreflexia/clonus (hours) → serotonin syndrome
Dopamine blocker + lead-pipe rigidity/hyporeflexia (days) → NMS

Give the specific antidote

MH → dantrolene (+ 100% O₂, hyperventilate, stop trigger)
Serotonin → cyproheptadine + benzodiazepines
NMS → dantrolene ± bromocriptine (restart dopaminergic drug if withdrawal)

Treat the complications

Rhabdomyolysis (fluids), hyperkalaemia, AKI, DIC, seizures, arrhythmia
Intubate + non-depolarising NMB for refractory rigidity/hyperthermia

Monitor recovery & prevent recurrence

Serotonin syndrome resolves in ~24 h; NMS over days–weeks; MH — watch recrudescence
Document drug alerts; MH testing & family counselling; cautious future drug choices

⚠️ 6 · Common Mistakes

Common Mistakes in Hyperthermia Syndromes

❌ Mistake 1 — Treating it as a fever with antipyretics

These are hyperthermia (normal set-point), not fever. Antipyretics don't work — active cooling, stopping the trigger and the specific antidote are the treatment.

❌ Mistake 2 — Not distinguishing the syndromes

Reflexes and tempo separate them: hyperreflexia/clonus + hours = serotonin syndrome; lead-pipe rigidity/hyporeflexia + days = NMS; rising EtCO₂ intra-op = MH. Each has a different antidote.

❌ Mistake 3 — Delaying dantrolene in malignant hyperthermia

MH is a minutes-count emergency. Give dantrolene immediately alongside stopping the volatile/succinylcholine and hyperventilating on 100% oxygen — know where your dantrolene is kept.

❌ Mistake 4 — Missing the linezolid + serotonergic drug interaction

Linezolid is an MAO inhibitor; combined with SSRIs/SNRIs/tramadol it precipitates serotonin syndrome — a frequently overlooked cause.

❌ Mistake 5 — Using physical restraints in serotonin syndrome

Isometric muscle contraction against restraints generates heat and worsens hyperthermia and acidosis. Sedate with benzodiazepines instead.

❌ Mistake 6 — Forgetting rhabdomyolysis and its complications

Extreme rigidity causes rhabdomyolysis, hyperkalaemia and AKI. Give aggressive fluids and monitor CK, potassium and renal function.

🎓 7 · Exam Pearls — DrNB / PDCC / IFCCM / EDIC

Exam Pearls

Q: How do NMS and serotonin syndrome differ clinically?
NMS: dopamine blocker, onset over days, lead-pipe rigidity with hyporeflexia. Serotonin syndrome: serotonergic drug, onset hours, hyperreflexia and clonus.

Q: What triggers malignant hyperthermia and how is it treated?
Volatile anaesthetics and succinylcholine (RYR1 mutation); treat with dantrolene, stopping the trigger, 100% O₂ and hyperventilation.

Q: What is the antidote for serotonin syndrome?
Cyproheptadine (a 5-HT₂A antagonist), with benzodiazepines and cooling; it usually resolves within 24 hours of stopping the drug.

Q: Which drugs are used in NMS?
Dantrolene and/or bromocriptine (dopamine agonist), plus benzodiazepines, cooling and supportive care.

Q: Why are antipyretics ineffective?
These are hyperthermia with a normal hypothalamic set-point — heat production exceeds loss, so antipyretics have no target; active cooling is required.

Q: How does dantrolene work?
It blocks calcium release from the sarcoplasmic reticulum (RYR1), uncoupling excitation-contraction and reducing muscle heat/rigidity.

Q: A commonly missed cause of serotonin syndrome?
Linezolid (an MAO inhibitor) combined with an SSRI/SNRI or tramadol.

📚 8 · References

References

Boyer EW, Shannon M. The Serotonin Syndrome. N Engl J Med. 2005;352:1112–1120.
Dunkley EJC, Isbister GK, Sibbritt D, et al. The Hunter Serotonin Toxicity Criteria. QJM. 2003;96:635–642.
Strawn JR, Keck PE, Caroff SN. Neuroleptic Malignant Syndrome. Am J Psychiatry. 2007;164:870–876.
Rosenberg H, Pollock N, Schiemann A, et al. Malignant hyperthermia: a review. Orphanet J Rare Dis. 2015;10:93.
Glahn KPE, Ellis FR, Halsall PJ, et al. (EMHG). Recognizing and managing a malignant hyperthermia crisis: guidelines from the European Malignant Hyperthermia Group. Br J Anaesth. 2010;105:417–420.
Marino PL. The ICU Book, 5th Edition. Malignant Hyperthermia & Drug-Induced Hyperthermia. Wolters Kluwer; 2025.
Washington Manual of Critical Care, 4th Edition. Kollef MH, Witt CA (eds). Hyperthermia Syndromes. Wolters Kluwer; 2023.