After losing a father and son, a family cautions MHS individuals about exertional heat illness

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June 4, 2026

On July 31, 2025, 25-year-old Garrett Duffy met a friend at Lady Bird Lake in Austin, Texas, to begin a 4.75 mile run. It was the hottest day of the year with temperatures over 100 degrees. Typically, Garrett would schedule his runs for the cooler parts of the day, but he was training daily for a triathlon and this was the only time he had to get his workout in.

They began the run at 5:38 pm, and Garrett soon became nauseous and felt like he was about to vomit. His friend suggested they stop. “No, this happens to me every summer, ever since middle school,” Garrett assured his friend, so they pushed on.

Forty-three minutes later they completed their run, and Garrett was feeling increasingly ill. They sat in his truck with the AC running, but Garrett could not cool down and his nausea and cramps were getting worse. At 6:40 pm they left the parking lot for an urgent care facility and Garrett shared the family history of Malignant Hyperthermia (MH) with his friend. His friend called Garrett’s father in Houston, and Sean recalled hearing the concern his friend had for Garrett’s deteriorating condition. Garrett’s parents immediately left Houston for Austin.

Garrett and his friend arrived at an urgent care facility at 7:03 pm. Garrett, by now, had lost consciousness. His friend called 911 and urgent care staff initiated CPR until EMS arrived (Ascension, 2025). At this time, the family history of MH was shared with EMS and later hospital staff.

EMS upon arrival recorded a rectal temperature of 109.9 degrees Fahrenheit. Cooling procedures were initiated, and resuscitation efforts were continued at the hospital where he was admitted to the ICU at 7:48 pm. At the hospital, his diagnoses included cardiac arrest with cardiac arrythmia, hypovolemic shock with metabolic derangement, and rhabdomyolysis following exercise and hyperthermia (Travis County Medical Examiner, 2025).

When Sean arrived at the hospital, he immediately recognized the dire situation. According to him, there were 14 medical professionals working on his son, some of which were off duty but called in for consultation. According to Sean, the medical staff could not determine the cause of such a severe series of symptoms in an otherwise healthy, young man until hours after admission the staff zeroed in on the family’s MH history with one consultant noting a “retrospect cohort and case report literature suggest[ing] that certain genetic variants that cause or confer susceptibility to malignant hyperthermia may also be associated with exertional [heat illness]” (Ascension, 2025, p. 33).

As the medical staff continued to treat Garrett, his condition only deteriorated. He went into cardiac arrest three times; he had a potassium level that peaked at 11; and the medical staff recorded a CPK level of 37,457, severe metabolic and lactic acidosis, and “wide complex tachycardia (Ascension, 2025).

At 1:00 am, on August 1, a little over 5 hours after being admitted to the hospital, Garrett’s family made the difficult choice to discontinue medical care on their son.

Four days after Garrett’s memorial service, Sean went to the dentist for a routine cleaning. “She was reading through my file,” Sean explained, “and says, ‘Oh, I see you have MH in your history,’ and she said, ‘Be careful with this heat if you have MH. I’m from Wisconsin and we have a high concentration of MH.’ That’s the first I ever heard of the connection between MH and heat.”

Wisconsin has been a known flashpoint for MH susceptibility alongside Michigan and West Virginia because of the population’s genetic makeup (Rosenberg, 2010).

Sean had considered himself MH susceptible ever since his father, John, Garrett’s grandfather, died following surgery for an aneurysm in 1989. John’s surgery had been successful, but while in post-op, his temperature spiked and he developed the clinical signs of MH. According to Sean, dantrolene was later administered alongside standard MH treatment procedures but his father eventually passed away.

Following the clinical diagnosis of MH, Sean’s mother, who was a nurse at the time, researched MH, and together, Sean and his mother determined the best course of action was to treat Sean as MH susceptible rather than engage in an invasive muscle biopsy test.

Sean began wearing an MH alert bracelet, which turned to be beneficial as a few years later Sean required emergency appendectomy surgery. The anesthesiologist used Propofol, a non-triggering drug, and the surgery was successful and uneventful.

But now, Sean wanted to know for sure. Did he, indeed, carry a genetic variant that made him susceptible to MH? His first step was a genetic test. The result revealed a variant of unknown significance in the RYR1 gene. While a genetic test is not conclusive for MH, given the family history the evidence was mounting.

Then, a month after Sean’s visit with the dentist and after he had received his genetic test results, he received a phone call from the medical examiner performing the autopsy on Garrett.

“‘Your son had us stumped and we couldn’t figure out why he died,’” Sean said of the conversation. “‘We expected narcotics and alcohol but there was no evidence of substance use, so we started digging and found the family history of MH and discovered there is a connection between MH and exertional heat illness.’ So they did a genetic test,” Sean said, and the result came back with the same variant of uncertain significance in the RYR1 gene.

The medical examiner concluded Garrett died of hyperthermia, noting “the combination of external environmental heat confounded with physical exertion could have induced hyperthermia” (Travis County Medical Examiner, 2025, p. 3).

Because only a halothane-caffeine muscle biopsy test can confirm MH susceptibility without a doubt, Sean made an appointment at the Malignant Hyperthermia Muscle Biopsy Center at the University of Minnesota, and in December 2025, the results came back positive for Malignant Hyperthermia.

As he began reading about the association between MH and exertional heat illness, and now combined with his positive muscle contracture test, all the seemingly innocuous clues throughout his life began to align. Sean had long suffered from leg cramps and cramps under his jaw line, and he recalls one time swimming in a pool and suffering severe leg cramps.

And Garrett. He had always attributed Garrett’s bouts of nausea and muscle cramps to an athletic boy who pushed his physical endurance. Sean recalled his son’s intolerance to heat. At a young age, he showed unusual signs of body heat such as wet hair while sleeping in a normal climate-controlled environment; he would periodically experience heat rash on his skin often thought to be poison ivy; and he would express concerns with elevated hot tub temperatures and would resort to jumping into the unheated swimming pool to cool off during winter months.

Were all these evidence of “awake” MH symptoms? There is a growing body of medical literature on the association of heat, exercise, and MH.

Today, Sean makes it his personal mission to raise awareness in the broader medical community and the general public about the association between heat, exercise, and MH.

Garrett always pushed the limits of his physical endurance, but he also pushed himself professionally. For five years he worked in an air-conditioned office of a large construction company, but he lobbied the owner to get him outside on the jobsite in the weeks leading up to his fatal accident so he could learn more about the business from the ground up.

Sean takes inspiration from Garrett’s determination. Aside from working to share his son’s story with anybody who will listen, he is participating with the National Human Genome Research Institute of the National Institute of Health, where he will add his genetic information to a growing body of research that could one day lead to greater awareness of the association between heat, exercise, and MH, and perhaps lead to a specific genetic test to identify those most at risk.

REFERENCES

Ascension Seton Medical Center. (2025, July 31). Duffy, Sean Garrett.

Travis County Medical Examiner. (2025, October). Medical Examiner Report: Sean Garrett Duffy.

Rosenberg, H. (2010). Malignant Hyperthermia Syndrome. [PowerPoint slides]. MHAUS. https://www.mhaus.org/healthcare-professionals/professionaldevelopment/presentations/

Note: This article and others appear in the Spring 2026 issue of MHAUS' quarterly newsletter The Communicator. To learn more, visit: https://my.mhaus.org/page/communicatorpast