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NMS vs Serotonin Syndrome vs Malignant Hyperthermia vs Malignant Catatonia

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Overview / Conceptual Framework

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1. Neuroleptic Malignant Syndrome (NMS)

Pathophysiology

• Idiosyncratic reaction from dopamine D2 receptor blockade in the CNS (Clozapine Handbook)
• Abrupt withdrawal of dopaminergic agents (L-Dopa) produces a clinically identical parkinsonism-hyperpyrexia syndrome
• Possible genetic predisposition: polymorphism in the D2 receptor gene; fatigue and dehydration are precipitating factors (Adams & Victor)

Causative Agents

• All dopamine D2 antagonists: FGAs (especially high-potency - haloperidol, fluphenazine, thioxanthenes, chlorpromazine) and SGAs (lower risk but not zero)
• Common antiemetics: metoclopramide, promethazine
• Clozapine: very rare due to weak D2 antagonism; <15 well-documented cases (Clozapine Handbook)
• Abrupt withdrawal of levodopa/dopamine agonists
• Onset: usually within 1-2 weeks of starting drug; average 50 days for clozapine

Risk Factors (Rosen's)

• Male sex (50% more likely), age 20-25 years
• Recent dose changes, high-potency antipsychotics, parenteral formulations
• Polypharmacy, prolonged physical restraint, dehydration
• Prior brain injury, family history of catatonia, muscle channelopathies
• Previous NMS episode

Clinical Features - The Classic Tetrad

1. Altered mental status - confusion, stupor, coma
2. Muscular rigidity - "lead-pipe" rigidity (severe, generalized)
3. Hyperthermia - often >40°C (104°F)
4. Autonomic instability - unstable BP, diaphoresis, tachycardia, sialorrhea

Labs

• CK markedly elevated (often >1000 IU/mL, up to 60,000 units - Adams & Victor)
• Leukocytosis
• Elevated CRP, ESR
• Myoglobinuria - may cause renal failure

Onset & Course

• Gradual onset, signs may develop in any order
• Most resolve within 2 weeks of stopping offending drug; symptoms can wax and wane for months

Mortality

• Untreated: 10-20% (Goldman-Cecil); historically 15-30% (Adams & Victor)
• Avoidable with prompt diagnosis

Treatment

1. Stop the offending agent immediately
2. ICU-level supportive care: IV fluids, electrolyte correction, cooling
3. Benzodiazepines (lorazepam 1-2 mg IV q4-6h) - for catatonic features
4. Bromocriptine (2.5 mg PO/NG q6-8h, up to 20 mg tid) - dopamine agonist; may terminate the condition in hours if started early (Adams & Victor)
5. Amantadine (100 mg PO/NG daily) - alternative dopamine agonist
6. Dantrolene (1-2.5 mg/kg IV; 0.25-3.0 mg IV per Adams & Victor) - for fulminant cases with severe rigidity and rhabdomyolysis; risk of hepatotoxicity
7. ECT - for refractory cases; considered treatment of choice when NMS/malignant catatonia overlap

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2. Serotonin Syndrome (SS)

Pathophysiology

• Excess serotonergic activity in CNS and PNS - primarily via 5-HT2A receptor agonism (Rosen's)
• Usually from drug combinations with serotonergic activity by different mechanisms
• More rarely from a single agent overdose (e.g., SSRI overdose)

Causative Agents (Rosen's - Box 141.1)

• SSRIs (citalopram, fluoxetine, paroxetine, sertraline, etc.)
• SNRIs (venlafaxine, milnacipran)
• MAOIs (phenelzine, isocarboxazid, selegiline, linezolid, moclobemide)
• TCAs (clomipramine, amitriptyline)
• Analgesics: tramadol, meperidine, pentazocine
• Drugs of abuse: cocaine, amphetamine derivatives (MDMA/"ecstasy"), LSD
• Miscellaneous: dextromethorphan, lithium, metoclopramide, St. John's wort, buspirone, triptans

Clinical Features - Triad

1. Altered mental status - restlessness, agitation, confusion, hyperactivity, mood elevation, hallucinations - to lethargy, disorientation, coma
2. Neuromuscular hyperactivity - clonus (especially lower extremities/ankles), myoclonus, hyperreflexia, tremor, hypertonicity, ocular clonus
3. Autonomic instability - diaphoresis, tachycardia, hypertension, hyperthermia

Key Distinguishing Feature

• Clonus (inducible or spontaneous, most pronounced at ankles) is the hallmark finding that distinguishes SS from NMS
• Onset within hours (minutes to hours after the offending agent; nearly always within 24 hours) - much faster than NMS
• With proper treatment, symptoms typically resolve within 24 hours (vs. days-weeks for NMS)

Diagnostic Criteria

• Sternbach Criteria (1991): first widely used algorithm
• Hunter Serotonin Toxicity Criteria (Box 141.2 in Rosen's): appear to be the most accurate; no gold standard exists
• No laboratory test confirms diagnosis; requires careful medication history and physical exam (Kaplan & Sadock)

Treatment (Goldman-Cecil, Adams & Victor)

1. Stop all serotonergic agents immediately
2. Benzodiazepines (lorazepam 1-2 mg IV) for agitation
3. Cyproheptadine (5-HT2A antagonist): initial dose 12 mg PO, then 2 mg q2h (Goldman-Cecil); or 4-8 mg q4-6h crushed via NG tube (Adams & Victor)
4. Supportive care: cooling, BP control
5. Atypical antipsychotics with serotonin antagonism (olanzapine, chlorpromazine) have been used

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3. Malignant Hyperthermia (MH)

Pathophysiology

• Hypermetabolic reaction triggered by volatile inhalation anesthetics (halothane, sevoflurane, desflurane) and depolarizing neuromuscular blocking agents (succinylcholine) (Bradley & Daroff)
• Genetic basis: Mutations in RYR1 gene (skeletal muscle ryanodine receptor - calcium release channel on sarcoplasmic reticulum) and CACNA1S (L-type voltage-gated calcium channel)

• 20 disease-causing point mutations in RYR1 identified; accounts for ~50% of affected families

• Inheritance: usually autosomal dominant; genetic heterogeneity exists
• Pathophysiology: uncontrolled calcium release from sarcoplasmic reticulum → sustained muscle contraction → hyperthermia, acidosis, rhabdomyolysis
• A small proportion of NMS cases may have ryanodine receptor gene mutations (Adams & Victor); however, "there is no evidence that the occurrence of one of these syndromes confers a susceptibility to the other" (Adams & Victor)

Triggering Agents

• Volatile anesthetics: halothane, enflurane, isoflurane, sevoflurane, desflurane
• Depolarizing muscle relaxants: succinylcholine

Clinical Features

• Tachypnea, tachycardia
• Rigidity (unlike NMS, often masseter spasm is first sign)
• Metabolic/respiratory acidosis
• Rhabdomyolysis
• Rapid, severe hyperthermia (temperature rise can be extremely fast, >1°C/5 min)
• Occurs intraoperatively or in immediate post-anesthesia period

Distinguishing Features

• Setting: exclusively surgical/anesthetic context
• No psychotropic drugs involved
• No altered mental status (patient is under anesthesia)
• No autonomic lability in the same pattern as NMS
• Masseter muscle rigidity after succinylcholine is a classic early sign
• Family history of anesthetic complications

Diagnosis

• Caffeine-contracture test: Gold standard for susceptibility testing - thin strip of explanted muscle exposed to caffeine; increased contracture = positive (highly operator-dependent)
• Genetic testing (RYR1 mutations)
• CK elevation, metabolic acidosis, myoglobinuria

Treatment (Bradley & Daroff)

1. Immediate termination of anesthetic exposure
2. Dantrolene sodium (inhibitor of calcium release from sarcoplasmic reticulum) - mainstay of treatment; has significantly reduced mortality
3. Rapid active core cooling
4. Correct metabolic acidosis, support ventilation
5. Prevention in susceptible individuals: pretreat with dantrolene before every anesthetic

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4. Malignant Catatonia (MC)

Conceptual Position

• Karl Heinz Stauder described a fatal form of catatonia with hyperpyrexia in 1934 (Kaplan & Sadock)
• NMS = Malignant Catatonia - this is the position of most modern authorities; a 2000 APA symposium concluded they are the same entity
• Malignant catatonia is essentially the severe, life-threatening end of the catatonia spectrum

Definition

Spectrum of Catatonia

• Hypokinetic/stuporous catatonia: mutism, psychomotor slowing, waxy flexibility, posturing, negativism
• Hyperkinetic/excited catatonia: impulsivity, restlessness, agitation
• Malignant catatonia: either type + dysautonomia (fever, tachycardia, labile BP, diaphoresis) + severe rigidity

Clinical Features

• Hyperthermia
• Autonomic instability (labile BP, tachycardia, diaphoresis)
• Severe rigidity
• Rhabdomyolysis, aspiration, thromboembolic disease
• May occur with any underlying psychiatric or medical etiology (not exclusive to antipsychotic use)

Causes of Underlying Catatonia (Goldman-Cecil)

• Primary psychiatric: schizophrenia, bipolar disorder, depression
• Medical/neurologic: inflammatory conditions (paraneoplastic, limbic encephalopathy), seizures, CNS infections
• Drug-induced: antipsychotics can precipitate NMS/malignant catatonia; withdrawal of dopaminergic agents

Treatment (Kaplan & Sadock - comprehensive)

1. Lorazepam (IV preferred): 2 mg challenge dose (diagnostic + therapeutic); if positive response, schedule 2 mg q6h; may require up to 30 mg/day total; "Lazarus response" may be seen within 30 minutes
2. ECT - considered the definitive treatment for malignant catatonia, especially when refractory to benzodiazepines; Rosen's states: "ECT is proposed as a treatment of choice in malignant catatonia, which may be indistinguishable from NMS"
3. Supportive care: cooling, IV fluids, anticoagulation, NG tube feeds, urinary catheter, suction, autonomic control
4. Stop any dopamine antagonists (antipsychotics, metoclopramide, tetrabenazine)
5. Resume dopaminergic/GABAergic agents if withdrawal-precipitated
6. Memantine or amantadine (glutamate antagonists) as adjuncts to reduce benzodiazepine requirements

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Key Comparison Table

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Key Differentiating Pearls

1. Clonus is serotonin syndrome's signature - ankle clonus, ocular clonus, hyperreflexia. NMS has rigidity without clonus.
2. Speed of onset distinguishes SS from NMS: SS develops within hours; NMS develops over days to weeks.
3. Context is everything: intraoperative hyperthermia + rigidity + no psychotropics = malignant hyperthermia. Antipsychotic exposure + gradual onset = NMS/malignant catatonia.
4. NMS = Malignant Catatonia - the modern view. If a patient on antipsychotics develops catatonic features + dysautonomia, the label may be either; treatment is identical.
5. Lead-pipe rigidity is characteristic of NMS/malignant catatonia; masseter rigidity first is classic for MH; mild-moderate rigidity or hypotonia with clonus favors SS.
6. Dantrolene is the cornerstone of MH treatment and an adjunct in severe NMS; it works on the same final pathway (calcium release from SR).
7. ECT should be offered early in malignant catatonia and NMS refractory to benzos - it is the definitive treatment and can be life-saving.
8. The Lazarus response: a robust, dramatic improvement within 30 minutes of IV lorazepam is virtually pathognomonic of catatonia (including malignant catatonia/NMS).
9. Bromocriptine (dopamine agonist) is a specific antidote rationale for NMS - it reverses the central dopamine blockade. It has no role in SS or MH.
10. Cyproheptadine (5-HT2A blocker) is specific to SS; no role in NMS, MH, or catatonia.

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