Heat-related illness

Introduction

Heat-related disorders have a broad range of potential aetiologies and manifestations. In some the primary disorder is a failure of thermal homoeostasis, whereas in others the hyperthermia is secondary to other processes. The major heat-related illnesses to consider are exercise-associated collapse (EAC), heatstroke, and the drug-related heat illnesses neuroleptic malignant syndrome, serotonin toxicity and malignant hyperthermia. Whilst still in common use the term ‘heat exhaustion’ should be discouraged as it has no defining pathophysiology or clinical syndrome, and has become a catch-all term for any illness in the context of a thermally stressful environment.

Epidemiology and pathophysiology

EAC is the most common heat-related illnesses presenting either to medical tents at sporting events or to emergency departments (EDs). EAC manifests at the end of a race when muscle pump enhanced venous return ceases and cardiac output drops. This leads to collapse, often with a brief loss of consciousness. The primary mechanism is a failure of prompt baroreceptor responses and not haemodynamically significant dehydration. Severe heat-related dehydration is rare.

The other, more serious, heat-related disorders are all associated with, or have the potential for, significant hyperthermia which if not treated promptly results in similar pathophysiology at a cellular and organ system level. A core body temperature around or greater than 41.5°C results in progressive denaturing of a number of vital cellular proteins, failure of vital energy-producing processes and loss of cell membrane function. At an organ system level these changes may manifest as rhabdomyolysis, acute pulmonary oedema, disseminated intravascular coagulation, cardiovascular dysfunction, electrolyte disturbance, renal failure, liver failure and permanent neurological damage. Any or all of these complications must be expected in severe heat illness.

The hallmark of heatstroke is failure of the hypothalamic thermostat, leading to hyperthermia and the associated additional pathophysiological features described above. Clinically, heatstroke can be divided into ‘exertional heatstroke’ due to exercise in a thermally stressful environment, and ‘classic heatstroke’, which occurs in patients with impaired thermostatic mechanisms. Common risk factors for heatstroke are listed in Box 24.1.1 .