Which clinical feature best distinguishes malignant hyperthermia (MH) from neuroleptic malignant syndrome (NMS)?

Explanation

## Distinguishing Malignant Hyperthermia from Neuroleptic Malignant Syndrome ### Key Clinical Differences **Key Point:** The onset timing is the most reliable discriminator between MH and NMS. MH occurs within **minutes to hours** of anesthetic exposure (typically within 30 minutes), whereas NMS develops over **24–72 hours** after neuroleptic administration. ### Comparison Table | Feature | Malignant Hyperthermia | Neuroleptic Malignant Syndrome | | --- | --- | --- | | **Onset** | Minutes to 1 hour after induction | 24–72 hours after neuroleptic exposure | | **Trigger** | Succinylcholine, volatile anesthetics | Antipsychotics (dopamine antagonists) | | **CK elevation** | Marked (often >5000 IU/L) | Marked (often >5000 IU/L) | | **Rigidity** | Present (muscle contracture) | Present (lead-pipe or cogwheel) | | **Altered mental status** | Not prominent early | Prominent feature | | **Dantrolene response** | Rapid, dramatic | Variable, slower | | **Rechallenging** | Contraindicated with same agent | Can use different antipsychotic | ### Why Onset Timing Matters **High-Yield:** In the operating room, a patient who develops hyperthermia, muscle rigidity, and hypermetabolism **within minutes** of receiving succinylcholine or volatile anesthetic is MH until proven otherwise. The acute temporal relationship to anesthetic exposure is pathognomonic. **Clinical Pearl:** NMS is a psychiatric emergency that develops insidiously in the ward or ICU setting, not acutely in the OR. A patient who is fine during induction and then rapidly deteriorates within 15–30 minutes has MH; a patient who develops symptoms a day or more after antipsychotic exposure has NMS. ### Why Other Options Are Incorrect - **Elevated CK:** Both conditions cause severe muscle breakdown and marked CK elevation; not discriminatory. - **Rigidity and altered mental status:** Both present with rigidity; NMS is characterized by prominent altered mental status, but MH can also cause confusion from hyperthermia and acidosis. - **Dantrolene response:** Both respond to dantrolene, though MH response is more dramatic and rapid. This is not a reliable discriminator in the acute setting. [cite:Harrison 21e Ch 397]