Exertional Heat Illness

My interest in exertional heat illness

My own interest in exertional heat illness started in 1989. Shortly after I joined the MH Unit at St James’s Hospital in Leeds, two military recruits, who had sustained repeated episodes of exertional heat illness during military training, were referred to my colleague, Professor Richard Ellis. The recruits had been training in weather conditions that would not normally be associated with heat illness.

Military physician colleagues wondered whether the two recruits were prone to exertional heat illness in a similar way to patients with genetic susceptibility to malignant hyperthermia under anaesthesia.

The link between exertional heat illness and MH

Within a few years of the first reports of malignant hyperthermia, there was speculation that MH may explain cases of otherwise unexplained sudden death and also heat “stress”.

A human stress syndrome encompassing MH was postulated on the basis that in the version of MH that affects pigs, the animals develop heat stress in response to a variety of triggers, such as a warm environment, crowded conditions, transport, or sexual activity, as well as general anaesthesia.

One of the reasons that people have wondered whether there was a relationship between heat illness and MH under anaesthesia is because of the similar clinical pictures:

• Both conditions involve a dangerous rise in body temperature, muscle breakdown or rhabdomyolysis and tissue acidosis consequent on the metabolic activity.
• Just as in life-threatening stages of malignant hyperthermia, people with exertional heat illness can develop widespread muscle rigidity which often leads to death with profound acidosis, high serum potassium concentration and disseminated intravascular coagulation.
• Indeed, MH can be considered to be a form of heat illness occurring during general anaesthesia.

MH needs to be classified as a separate condition because the otherwise correct classification of heat illness requires an assessment of the patient’s cerebral or mental function. This is clearly not possible in the anaesthetized patient.

Diagnostic tests for exertional heat illness and MH

At the time the two initial military personnel with recurrent heat stroke came to the Leeds MH Unit, I was working on a potential new testing compound for the in vitro contracture test. This compound was ryanodine, after which is named the ryanodine receptor skeletal muscle calcium channel. Early results had indicated that ryanodine could be used to distinguish between normal and abnormal muscle. We undertook MH diagnostic tests and the prototype ryanodine contracture test on muscle biopsy samples from the two men and found abnormal muscle responses.

Subsequent investigations of the family of these soldiers demonstrated that the trait was inherited. We published our findings in The Lancet as the first demonstration of an inherited muscle abnormality in individuals with a history of recurrent heatstroke.

Working with the Institute of Naval Medicine

In the early 2000s the Institute of Naval Medicine (INM) developed a standardised heat tolerance test for British military recruits who had a history of significant heat illness.

Howard Oakley and subsequently Dan Roiz de Sa identified an abnormal heat tolerance test response as an inability to achieve a plateau core body temperature during standardised exercise in a controlled heat chamber environment.

Individuals who fail this test on two or more occasions are referred to the MH Unit in Leeds for muscle biopsy and contracture testing. Approximately one third of these individuals have an abnormal muscle contracture response. In collaboration with the INM we investigated the genetic basis of this response and published our initial findings in Anesthesiology in 2015.

Developing military guidelines

As a result of my collaboration with the INM I was invited to be a member of the Surgeon General’s Heat Illness Working Group tasked with updating the military guidelines for exertional heat illness.

These military guidelines were published in 2009 as Joint Service Protocol 539 (JSP539).

This document provides a clear and pragmatic description of the risk factors for exertional heat illness, the symptoms, signs and management protocols.

While episodes of exertional heat illness will inevitably occur during deployment of military personnel on active service in hot environments, the availability and dissemination of JSP539 should help prevent unnecessary cases of exertional heat illness, for example during training exercises.

In the meantime, the MH Investigation Unit at St James’s Hospital in Leeds continues to pursue genetic markers for susceptibility to exertional heat illness, with an aim of identifying those at risk before they are accepted for military service.