Hypothermia: Level 3

Definition

Hypothermia is when the body becomes too cold. It is defined as a core body temperature less than 35C/95F^1,2. This typically occurs in cool, wet environments but hypothermia can occur at ambient temperatures if heat loss exceeds the heat generated by the body³.

Cause

When environmental temperatures are cool the body adapts to maintain core temperature within a narrow range. This involves reducing blood supply to the skin to reduce heat lost to the environment and inducing involuntary responses such as shivering to generate heat⁴. A healthy individual will be able to maintain core body temperature with these responses and the heat generated via muscle contractions during running. Injury, exhaustion, or inactivity will limit this heat generation and hypothermia may set in⁴.

Symptoms

Initially runners will report feeling cold and they may shiver. If core body temperature continues to drop runners may feel confused and sleepy, with possible slurred speech³. In severe cases, hypothermia can cause collapse. Although rare, hypothermia can be fatal⁴.

In significant cases of hypothermia runners may also experience other cold injuries including   chilblains or frostbite³. Frostbite typically occurs at exposed skin sites, such as the nose and ears, and initially presents with numbness³.

Risk Factors

Hypothermia is more likely when conditions are wet and windy or environmental temperatures are below 12C degrees^1,2. Changes in the weather over the course of a run, especially if unexpected, increase the risk as runners may not be dressed appropriately¹.

Clothing plays an important role. If clothing is wet, due to rain or sweat, this will accelerate heat loss from the body as water has significantly greater thermal conductivity than air so heat is rapidly dissipated³.

Hypothermia occurs if heat loss exceeds heat production by the body. Running more slowly will therefore increase the risk as less heat is generated by the muscles and the runner will be exposed to the cool conditions for longer. This will include faster runners who are forced to slow down due to injury², exhausted runners who stop or slow down, and unfit or inexperienced runners who may need to stop or walk¹. Longer races are more likely to result in slower paces and therefore less metabolic heat generation. 

To reduce the risk of hypothermia, runners should check weather forecasts ahead of the event and wear appropriate clothing. This includes waterproof layers especially if undertaking higher-risk events (i.e., mountain ultramarathons). Such events usually have mandatory kit lists.

Treatment

A cold runner can often be managed conservatively by moving to a warmer environment, removing wet clothes, using blankets and having a warm drink⁵. Ideally, the runner should be warmed slowly².  If the runner is more significantly unwell, such as with signs of confusion, they need to be assessed in a medical tent⁵.