## Neuroleptic Malignant Syndrome (NMS): Acute Pharmacotherapy **Key Point:** Dantrolene sodium is the first-line pharmacological agent for acute NMS, acting as a skeletal muscle relaxant by inhibiting calcium release from the sarcoplasmic reticulum, thereby reducing muscle rigidity and heat production. ### Pathophysiology of NMS NMS results from dopamine antagonism (antipsychotics) leading to: - Hypothalamic thermoregulatory dysfunction - Increased muscle tone and metabolic heat production - Rhabdomyolysis with myoglobinuria - Acute kidney injury risk ### Dantrolene: Mechanism & Dosing **Mechanism:** Directly relaxes skeletal muscle by blocking Ca²⁺ release from sarcoplasmic reticulum via ryanodine receptor inhibition—independent of neurotransmitter systems. **Dosing:** - **Loading:** 2.5 mg/kg IV bolus over 5–10 minutes - **Repeat:** Every 5–10 minutes up to maximum 10 mg/kg or until symptoms resolve - **Maintenance:** 1 mg/kg IV every 4–6 hours for 24–48 hours post-crisis - Oral continuation: 4–8 mg/kg/day in 3–4 divided doses for 5–7 days ### NMS Management Flowchart ```mermaid flowchart TD A[NMS Suspected: Fever + Rigidity + Altered MS + Elevated CK]:::outcome --> B[STOP antipsychotic immediately]:::action B --> C[ICU admission + supportive care]:::action C --> D[IV hydration, active cooling, monitor CK/renal function]:::action D --> E{Severity/Progression?}:::decision E -->|Severe or worsening| F[Dantrolene 2.5 mg/kg IV bolus]:::action E -->|Mild, stable| G[Supportive care alone may suffice]:::action F --> H[Repeat Q5-10 min, max 10 mg/kg]:::action H --> I[Maintenance: 1 mg/kg IV Q4-6H x 24-48 hrs]:::action I --> J[Monitor for resolution; switch to oral if stable]:::outcome G --> J ``` ### Comparative Pharmacology of NMS Agents | Agent | Mechanism | Onset | Role in NMS | |-------|-----------|-------|-------------| | **Dantrolene** | Skeletal muscle Ca²⁺ release inhibitor | 5–30 min IV | **First-line**; directly reverses muscle rigidity | | Bromocriptine | Dopamine agonist | 30–60 min | Second-line; restores dopaminergic tone | | Amantadine | NMDA antagonist + dopamine agonist | 1–2 hrs | Adjunctive; slower onset | | Benztropine | Anticholinergic | 15–30 min | Symptomatic only; does not address core pathology | **High-Yield:** Dantrolene is the **only agent that directly addresses muscle rigidity and heat production** independent of dopamine restoration. It is preferred in severe or rapidly progressive NMS because it works fastest and most reliably. **Clinical Pearl:** Bromocriptine (dopamine agonist) was historically used but is slower and less reliable than dantrolene. Modern guidelines favor dantrolene as first-line, with bromocriptine or amantadine reserved for adjunctive use or when dantrolene is unavailable. ### Concurrent Supportive Measures 1. **Discontinue antipsychotic** immediately 2. **Aggressive cooling:** Ice packs, cooling blankets, cold IV saline 3. **Hydration:** 1–2 L/hour IV to maintain urine output >200 mL/hour (prevent myoglobinuric AKI) 4. **Monitoring:** CK, creatinine, potassium, urine myoglobin every 6–12 hours 5. **Seizure prophylaxis:** Lorazepam if needed 6. **Rechallenge:** Avoid the same antipsychotic; consider atypical agents only after full recovery (weeks to months) **Warning:** Do NOT use anticholinergics (benztropine, diphenhydramine) as primary therapy—they may worsen hyperthermia by impairing sweating. [cite:Harrison's Principles of Internal Medicine 21e Ch 384; Kaplan & Sadock's Synopsis of Psychiatry 11e Ch 28]
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