A 48-year-old man with schizophrenia has been stable on haloperidol 10 mg daily for 6 years. Over the past 48 hours, he develops high fever (40.2°C), severe muscle rigidity, altered mental status, and profuse diaphoresis. Laboratory findings: CK 3800 U/L, WBC 14,500/μL, urine myoglobin positive. He was last given haloperidol 12 hours ago. Which of the following best distinguishes this presentation from serotonin syndrome?

Explanation

## Diagnosis: Neuroleptic Malignant Syndrome (NMS) **Key Point:** NMS and serotonin syndrome are both life-threatening drug-induced syndromes, but they differ fundamentally in **onset timeline**, **pathophysiology**, and **clinical features**. ### Comparative Table: NMS vs. Serotonin Syndrome | Feature | NMS | Serotonin Syndrome | | --- | --- | --- | | **Causative Agent** | Antipsychotics (D~2~ blockade) | Serotonergic drugs (SSRI, MAOI, tramadol) | | **Onset** | Insidious; 24 hrs to weeks | Acute; hours to 1–2 days | | **Peak Severity** | 3–5 days | 6–24 hours | | **Muscle Rigidity** | "Lead pipe" (uniform) | Variable; may be absent | | **Clonus** | Absent | **Pathognomonic** (present in 60–70%) | | **Hyperreflexia** | Normal or decreased | Increased | | **CK Elevation** | Marked (often >1000 U/L) | Mild to moderate | | **Fever Pattern** | Sustained, high (>39°C) | Variable; may be absent | | **Resolution Timeline** | 7–10 days (even after drug cessation) | 24–72 hours | | **Treatment** | Dantrolene + bromocriptine | Discontinue drug + benzodiazepines | **High-Yield:** **Clonus is the single best discriminator** — present in serotonin syndrome, absent in NMS. This patient has NO clonus, pointing to NMS. ### Pathophysiology ```mermaid flowchart TD A[Antipsychotic Use]:::outcome --> B[D2 Dopamine Blockade]:::action B --> C[Loss of Dopaminergic Thermoregulation]:::action C --> D[Sustained Hyperthermia]:::action D --> E[Muscle Rigidity + Rhabdomyolysis]:::action E --> F[NMS: Insidious, Sustained]:::outcome G[Serotonergic Drug Excess]:::outcome --> H[5-HT Receptor Overstimulation]:::action H --> I[Acute Hyperexcitability]:::action I --> J[Clonus + Autonomic Storm]:::action J --> K[Serotonin Syndrome: Acute, Self-Limited]:::outcome ``` ### Clinical Features of NMS in This Case 1. **Insidious onset over 48 hours** — characteristic of NMS (vs. serotonin syndrome's acute hours) 2. **Marked CK elevation (3800 U/L)** — indicates severe muscle breakdown; more typical of NMS 3. **Myoglobinuria** — evidence of rhabdomyolysis from sustained rigidity 4. **Lead-pipe rigidity** — uniform resistance throughout movement (classic NMS pattern) 5. **NO clonus** — clonus is absent in NMS, present in serotonin syndrome 6. **Recent antipsychotic exposure** — haloperidol is a high-potency typical antipsychotic with high NMS risk **Clinical Pearl:** NMS is a **medical emergency** with mortality 5–20% if untreated. Early recognition and treatment with dantrolene (inhibits calcium release in muscle) and bromocriptine (dopamine agonist) are critical. ### Management of NMS 1. **Discontinue antipsychotic immediately** 2. **Dantrolene 1 mg/kg IV bolus**, repeat every 5–10 min up to 10 mg/kg (inhibits excitation-contraction coupling) 3. **Bromocriptine 2.5 mg PO/NG** three times daily (restores dopaminergic tone) 4. **Supportive care:** cooling, IV fluids, urinary alkalinization (prevent myoglobin precipitation in renal tubules) 5. **Monitor CK, electrolytes, renal function** — rhabdomyolysis can cause acute kidney injury **Mnemonic for NMS: FEVER** - **F**ever (high, sustained) - **E**xtrapyramidal rigidity (lead-pipe) - **V**egetative instability (diaphoresis, tachycardia) - **E**levated enzymes (CK, transaminases) - **R**habdomyolysis (myoglobinuria) [cite:Harrison 21e Ch 385; Kaplan & Sadock's Synopsis of Psychiatry 11e Ch 36]