To the Editor:
We read with great interest the newest practice guidelines offered by the Wilderness Medical Society (WMS) on prevention and treatment of heat-related illness. However, we believe that several points should be refined to reflect the more recent body of literature, modern nomenclature, and pathophysiologic understanding of the disease entities described.
Lipman et al 1 begin their definition of heat-related illness with a discussion of the spectrum of diseases related to heat. We submit that there is no objective evidence in the existing body of literature to suggest a necessary link between the development of so-called heat cramps, heat syncope, heat exhaustion, and heat stress. Further, there is no objective evidence to support the dogma that heat cramps or heat exhaustion will progress to heat stroke (exertional hyperthermia), just as there is no evidence to suggest that hypertension continues to worsen until hypertensive emergency occurs.
Rather than providing supportive objective evidence, the existing body of literature in fact refutes the concept of a spectrum of diseases related to heat. In 2006, for example, Byrne et al 2 performed continuous core temperature monitoring in 18 volunteers at a half-marathon in Singapore. All runners remained asymptomatic from any heat illness despite the fact that 56% of them had core temperatures greater than 40°C and 11%, greater than 41°C.
By the definition used in the newest guidelines, the body remains normothermic during heat exhaustion. We invite the reader to review work done by Noakes, 3 who articulates why heat exhaustion is an inaccurate description of a phenomenon that is not objectively linked to heat accumulation or hyperthermia. Alternatively, Noakes suggests that we begin to update our nomenclature to be more accurate and commensurate with known pathophysiology. Instead of labeling the condition heat exhaustion, it is more appropriately understood as “exhaustion that may or may not happen in the heat.”
We also submit that heat cramps is a confusing and inaccurate title for a disease process likely unrelated to heat. Although the theory of dehydration with dysmetabolism of sodium (dilutional hyponatremia) and potassium (wasting) ions is commonly described, the true pathophysiology of exertion-associated muscle cramps is not entirely understood. Maughan 4 studied 82 male marathoners in 2008 of which 15% suffered from cramps. There was no difference in sodium, potassium, or fluid losses between those who developed cramps and those who did not. Alternatively, the neuromuscular theory, first proposed by Schwellnus et al, 5 suggests that neuromuscular fatigue and muscle overload creates an imbalance between the inhibitory Golgi tendon organs and the excitatory muscle spindles. This concept of neuromuscular fatigue is more consistent with what is already known of the major clinical manifestations of the phenomenon. Heat stress is neither a necessary element in, nor probable contributor to, this pathophysiologic process.
With respect to heat syncope, we believe the authors’ definition of a transient loss of consciousness with spontaneous return to mentation is most consistent with exercise-associated collapse. We again refer the reader to the work of Noakes, 3 who provides an elegant description of the pathophysiologic process behind exercise-associated collapse via the Barcroft-Edholm reflex. Heat stress is not an integral part of this process. Here, we presume the authors refer to collapse during or immediately on cessation of active exertion. Had the authors intended on referring to, say, military personnel in parade, we believe their description of peripheral vasodilation and orthostatic pooling of blood is sufficient without the addition of heat stress. We can attest firsthand that troops at prolonged attention suffer syncope just as readily in cold environs.
Regarding treatment, we object to statements such as “[m]ore severe exhaustion typically has pronounced volume depletion and may require intravenous replacement of fluids as well as evaporative and convective cooling.” Heat exhaustion is a nebulous term without real objective criteria. The only criterion described in the guidelines is that it occurs in a normothermic patient. If this is the case, then why would they require cooling? May we suggest that the patient whom the authors had in mind when writing this may be suffering from simple exhaustion with or without renal insufficiency? Indeed, the prior WMS guidelines published admonished against empiric use of intravenous fluids in these patients. 6
Finally, we contest the suggestion that dehydration plays a prominent role in the development of the presented spectrum of heat illnesses, and that body weight measurement is a useful tool in the prevention of heat-related illnesses. We present for the reader’s consideration a series of references outlined in Joslin et al 7 suggesting that measurable hypohydration or weight loss does not reliably correlate with the development of cramps, collapse, exhaustion, or exertional heat stroke. Studies have suggested an average weight loss among marathon runners of 2 to 3 kg, with up to 1 kg of weight loss attributed to oxidization of fuel stores alone. 8 These data do not support the author’s recommendation for adequate hydration to prevent more than 2% body weight loss.
We submit that the WMS has a role to play in updating and progressing the science of medical concerns such as these. Although we agree on the majority of recommendations given and appreciate the visibility given to exertional hyperthermia by these guidelines, propagating the outdated dogma of a spectrum of heat illnesses is inconsistent with the modern pathophysiologic understanding of these phenomena and likely detracts focus from the one true heat illness: heat stroke. We propose that newer paradigms supported by recent literature exist, and that readers should make themselves aware of newer nomenclature and descriptions of the so-called heat illnesses. We contend that there are but two of these heat-related illnesses: 1) true heat stroke (exertional hyperthermia) and 2) other inconvenient phenomena that may or may not occur in the setting of heat.