A 32-year-old male undergoes elective knee arthroscopy under general anesthesia with succinylcholine and sevoflurane. Intraoperatively, he develops masseter muscle rigidity, followed by a rapid rise in end-tidal CO₂ and core temperature. Which single clinical feature best distinguishes malignant hyperthermia (MH) from neuroleptic malignant syndrome (NMS)?

Explanation

## Distinguishing Malignant Hyperthermia from Neuroleptic Malignant Syndrome ### Clinical Context Both MH and NMS present with hyperthermia, rigidity, and rhabdomyolysis, but the **timing of onset** is the most reliable discriminator in clinical practice. ### Key Discriminating Feature **Key Point:** Malignant hyperthermia develops **within minutes** (typically 5–30 minutes) of exposure to a triggering agent (succinylcholine or volatile anesthetic), whereas neuroleptic malignant syndrome develops **over hours to days** (usually 24–72 hours) after exposure to an antipsychotic. ### Comparison Table | Feature | Malignant Hyperthermia | Neuroleptic Malignant Syndrome | |---------|------------------------|--------------------------------| | **Onset** | Minutes (5–30 min) | Hours to days (24–72 hrs) | | **Trigger** | Succinylcholine, volatile anesthetics | Antipsychotics (especially typical) | | **Rigidity pattern** | Generalized, including masseter | Generalized, "lead pipe" | | **CK elevation** | Marked (often >5000 IU/L) | Marked (often >1000 IU/L) | | **Dantrolene response** | Rapid (within 5–10 min) | Slow or variable | | **Mental status** | Normal (under anesthesia) | Altered (fever, confusion, agitation) | | **Recurrence risk** | High if re-exposed to trigger | Low if antipsychotic discontinued | ### Why Onset is the Best Discriminator 1. **Intraoperative vs. Outpatient setting**: MH is an acute perioperative emergency; NMS is a delayed drug reaction in psychiatric or medical settings. 2. **Trigger exposure timing**: The anesthesiologist knows exactly when the triggering agent was administered, making temporal correlation immediate. 3. **Practical utility**: In the operating room, rapid recognition of MH's acute onset drives immediate cessation of anesthetic, hyperventilation, and dantrolene administration—a life-saving intervention. ### Clinical Pearl **Clinical Pearl:** The **masseter muscle rigidity** (trismus) seen in this case is an early sign of MH and should trigger immediate suspicion. In contrast, NMS rigidity develops gradually over hours and is not preceded by masseter involvement. ### High-Yield Fact **High-Yield:** Both conditions require dantrolene, but MH patients respond within minutes, whereas NMS patients may show delayed or incomplete response. This difference reflects the underlying pathophysiology: MH is a direct muscle membrane defect (calcium channel disorder), while NMS involves dopamine antagonism and is more complex to reverse. [cite:Miller's Anesthesia 8e Ch 33]