How long after anesthesia can you get malignant hyperthermia?

October 12, 2025 •

3 min read

While most malignant hyperthermia (MH) crises occur during or immediately following surgery, in extremely rare cases, a reaction can be delayed, presenting up to 24 hours after anesthesia exposure. This life-threatening, inherited disorder highlights the importance of continued vigilance and prompt action for at-risk individuals.

Quick Summary

The onset of malignant hyperthermia typically occurs during or shortly after surgery, most often within the first hour. However, rare instances of delayed presentation, up to 24 hours later, have been documented, especially with potent inhalational anesthetics.

Key Points

Peak Onset Time: The majority of malignant hyperthermia cases occur during surgery or within the first hour following the discontinuation of triggering anesthetics.
Delayed Reactions are Rare: While rare, delayed-onset malignant hyperthermia has been documented, with some cases presenting several hours, or even up to 24 hours, after anesthesia exposure.
Distinguishing Postoperative Fever: A fever in the postoperative period is common and should be distinguished from MH; a true MH crisis will typically involve other hypermetabolic signs like tachycardia and muscle rigidity.
Monitoring is Crucial: Due to the possibility of delayed onset, vigilance and monitoring for MH symptoms should continue for at least 24 hours post-operatively in susceptible patients.
Dantrolene is the Antidote: The life-saving treatment for an MH crisis, whether immediate or delayed, is the prompt administration of the drug dantrolene.

Malignant hyperthermia (MH) is a rare, inherited disorder of the skeletal muscles characterized by a hypermetabolic state that can be triggered by certain anesthetic agents in susceptible individuals. This can lead to a life-threatening crisis involving severe muscle rigidity, a dangerously high fever, and a rapid heart rate. While the reaction is most common during surgery, understanding the timeline for both immediate and delayed onset is crucial for patient safety.

The Typical Timeline of a Malignant Hyperthermia Crisis

The majority of MH crises begin while the patient is still in the operating room or shortly after. This is because triggering agents like volatile anesthetics and succinylcholine are administered during this time.

Intraoperative Onset: Reactions can occur within minutes of succinylcholine administration or around 45 minutes with inhalational anesthetics. Early signs include unexplained rises in end-tidal carbon dioxide ($ETCO_2$), tachycardia, and masseter muscle rigidity.
Immediate Postoperative Onset: MH can also manifest after the triggering agents are stopped and the patient is in the PACU. Approximately 1.9% of cases occur within the first hour after anesthesia. Monitoring in the recovery period is vital.

Delayed-Onset Malignant Hyperthermia

Delayed-onset MH is uncommon but requires consideration. Reactions can occur several hours post-surgery, with some reports noting events up to 24 hours after anesthesia.

Case reports have documented delayed MH events occurring hours after surgery. While postoperative hyperthermia alone is not typically associated with delayed MH, other signs of hypermetabolism would be expected if MH is the cause. More gradual reactions from inhalational anesthesia may have a longer latency compared to succinylcholine. Differentiating a true MH crisis from other causes of postoperative fever is important.

Triggers, Signs, and Treatment

MH is caused by a genetic mutation affecting a calcium channel in muscle cells, commonly the RYR1 gene, leading to uncontrolled calcium release and the symptoms of a crisis.

Common Triggering Agents

Volatile Anesthetics: Such as halothane, isoflurane, sevoflurane, and desflurane.
Succinylcholine: A muscle relaxant used for rapid intubation.

Symptoms of a Malignant Hyperthermia Crisis

Symptoms can vary and include both early and late signs.

Early signs:

Rapid heart rate (tachycardia)
Rapid breathing (tachypnea) or increased end-tidal carbon dioxide ($ETCO_2$)
Tight jaw muscles (masseter muscle rigidity)
Acidosis

Late signs:

Rapidly rising body temperature (hyperthermia)
Generalized muscle rigidity
Mottled skin
Muscle breakdown (rhabdomyolysis) and dark urine
Irregular heart rhythm

Management of a Malignant Hyperthermia Crisis

Immediate recognition and treatment are critical. The mortality rate has significantly decreased with the use of dantrolene.

Key steps include:

Stopping triggering agents.
Calling the MHAUS 24/7 hotline for expert guidance.
Administering the antidote, dantrolene sodium, intravenously.
Implementing cooling measures.
Hyperventilating with 100% oxygen.
Managing other symptoms like acidosis or arrhythmias.
Transferring to the ICU and monitoring for 24-48 hours with continued dantrolene to prevent recurrence.

Comparison: Immediate vs. Delayed MH Onset


Feature	Immediate Onset MH	Delayed Onset MH
Timing	During or within the first hour of anesthesia.	More than 1-2 hours after stopping triggers, up to 24 hours later.
Frequency	More common.	Very rare.
Diagnosis	Often based on intraoperative monitoring and classic signs.	More challenging, requires ruling out other causes of postoperative issues.
Severity of Reaction	Can be rapid and severe.	May be less overt but still life-threatening.
Key Indicator	Sudden increase in end-tidal carbon dioxide is often first sign.	Hyperthermia is prominent in delayed cases, but other hypermetabolic signs are necessary.

Conclusion

While most malignant hyperthermia cases occur during or shortly after anesthesia, delayed reactions are possible, sometimes up to 24 hours later. Continued monitoring is crucial for susceptible individuals. Awareness of the signs and symptoms, regardless of when they appear, is vital for timely diagnosis and administering the life-saving antidote, dantrolene. Family members of affected patients should also be aware of their potential risk and consider genetic testing. The Malignant Hyperthermia Association of the United States (MHAUS) is a valuable resource for more information.

Frequently Asked Questions

A malignant hyperthermia reaction typically starts during a surgical procedure involving triggering anesthetics or within the first hour after the patient has left the operating room.

Yes, although very rare, delayed-onset malignant hyperthermia has been reported to occur up to 24 hours after anesthesia. Isolated cases have been documented even more than 10 hours after exposure.

Early signs often include an unexplained rapid heart rate (tachycardia) and a sudden increase in end-tidal carbon dioxide ($ETCO_2$). Masseter muscle rigidity can also be an early indicator.

No, a fever alone is not a reliable indicator of malignant hyperthermia. Many patients experience postoperative fevers for other reasons. An MH crisis is distinguished by other hypermetabolic signs, such as muscle rigidity and metabolic changes.

If delayed-onset MH is suspected, triggering anesthetics must be stopped, and the specific antidote, dantrolene, should be administered immediately. The patient requires transfer to an intensive care unit (ICU) for close monitoring.

Delayed reactions appear to be more often associated with inhalational anesthetics compared to succinylcholine, possibly due to a more gradual effect. However, combining both inhalational agents and succinylcholine is known to increase the risk of a more rapid and severe reaction.

Recurrence of symptoms after initial treatment is a known risk, occurring in up to 25% of patients. For this reason, continuous administration of dantrolene is recommended for 24-48 hours after the initial reaction.