A 28-year-old man with schizophrenia is started on haloperidol for acute agitation. Within 48 hours, he develops severe muscle rigidity, hyperthermia (39.8°C), altered mental status, and markedly elevated creatine kinase (CK 4200 U/L). What is the most common causative antipsychotic for neuroleptic malignant syndrome (NMS)?

Explanation

## Most Common Antipsychotic Causing NMS **Key Point:** Haloperidol, a typical (first-generation) antipsychotic, is the most common antipsychotic associated with neuroleptic malignant syndrome (NMS). ### Epidemiology of NMS by Antipsychotic Class | Antipsychotic Class | NMS Incidence | Risk Level | |---------------------|---------------|------------| | Typical (haloperidol, chlorpromazine) | 0.5–1.4% | **Highest** | | Atypical (risperidone, olanzapine) | 0.01–0.02% | Intermediate | | Atypical (quetiapine, aripiprazole) | <0.01% | **Lowest** | **High-Yield:** Haloperidol carries the highest risk of NMS among all antipsychotics, particularly when used in high doses or parenterally (IM injections). ### Mechanism of NMS with Haloperidol 1. **Dopamine D~2~ receptor blockade** in the basal ganglia → loss of dopaminergic inhibition of motor neurons 2. **Hypothalamic dopamine antagonism** → impaired thermoregulation 3. **Muscle rigidity** → increased heat production and rhabdomyolysis 4. **Elevated intracellular calcium** in muscle → sustained contraction ### Classic Tetrad of NMS 1. **Hyperthermia** (>38.5°C, often >39°C) 2. **Muscle rigidity** ("lead pipe" or "waxy flexibility") 3. **Altered mental status** (confusion, delirium, coma) 4. **Autonomic instability** (tachycardia, hypertension, diaphoresis) **Mnemonic:** **FEVER** = **F**irst-generation antipsychotics, **E**levated CK, **V**igorous muscle rigidity, **E**levated temperature, **R**habdomyolysis ### Why Haloperidol Is Most Common 1. **Potent D~2~ antagonism** — stronger dopamine blockade than atypicals 2. **Lipophilicity** — crosses blood–brain barrier readily, concentrating in basal ganglia 3. **Irreversible binding** to dopamine receptors (longer duration of effect) 4. **High-potency typical agent** — used for acute agitation (as in this case) 5. **IM formulation** — haloperidol decanoate and IM haloperidol have higher NMS risk than oral formulations **Clinical Pearl:** NMS typically develops within 24–72 hours of antipsychotic initiation or dose escalation. Early recognition is critical — mortality is 10–20% if untreated. ### Management of Suspected NMS 1. **Immediate:** Discontinue antipsychotic 2. **Supportive:** IV fluids, cooling measures, monitor urine output 3. **Pharmacologic:** Dantrolene (0.5–1 mg/kg IV q4–6h) or bromocriptine (dopamine agonist) 4. **Monitor:** CK, electrolytes, renal function, coagulation profile **Warning:** Do NOT rechallenge with the same antipsychotic. If antipsychotic is essential, use an atypical agent (e.g., quetiapine) with careful monitoring after a 2-week washout period. [cite:Kaplan & Sadock's Synopsis of Psychiatry 11e Ch 29; Harrison 21e Ch 385]