Accidental hypothermia is common and carries significant risk of morbidity and mortality. Hypothermia, defined when the body temperature falls below 95°F (<35°C), occurs when the body loses more heat than it can absorb or generate. It is classified as mild, moderate, severe, or profound (Table). Environmental cold exposure, near drowning, and immersion are common causes, as are trauma and intoxication. Hypothermia is not always easily diagnosed, so awareness of the presentation, associated risks, and treatment is needed. Localized cold injury, of which frostbite is the most commonly reported, is often seen with hypothermia, although it can occur alone. This In Brief reviews accidental hypothermia: both generalized and local cold injuries, their definitions, risk factors, presentations, and management.Children are at increased risk for accidental hypothermia due to their greater body surface area-to-mass ratio and less subcutaneous fat, which leads to greater heat loss. Infants are particularly vulnerable due to immature thermoregulation and an inability to shiver. Children with neurologic conditions are more prone to hypothermia due to inability to avoid or escape a cold environment as well as difficulty with thermoregulation. Exhaustion, hunger, dehydration, and lack of adequate clothing can contribute to hypothermia in participants in outdoor sports, camping, and hiking. The epidemiology of accidental hypothermia in children shows that it is more common in boys, more prevalent in infants than in older children, and, in the United States, more common in the southern regions of the country.The most diagnostic sign of hypothermia is a low temperature. However, this sign may be missed because most thermometers do not read below 93°F (34°C), and oral and axillary temperatures are unreliable. Even rectal measurements may not reflect the true core temperature.Signs of mild hypothermia (89.6°F–95°F [32°C–35°C]) are shivering, pallor, acrocyanosis, and slurred speech. (Table) As the hypothermia becomes more severe, some signs may reverse, such as the skin becoming flushed instead of presenting with pallor. This can make recognition of the degree of hypothermia more difficult. Mental status worsens with more severe hypothermia and can range from clumsiness to confusion, progressing to stupor and a comatose state.A major problem in hypothermia is intravascular hypovolemia, which is caused by extravasation, cold diuresis, and cold-induced thickening of the blood. Combined with reduced cardiac output and drops in systemic vascular resistance, it can lead to circulatory collapse. Victims of hypothermia are at increased risk for ventricular fibrillation. Care to avoid triggers of ventricular fibrillation includes gentle handling and keeping the patient supine. Other potential complications include hypoglycemia or hyperglycemia, hyperkalemia, metabolic acidosis or alkalosis, and coagulopathy with and without thrombocytopenia. Complications that can be seen in patients more than 24 hours after rewarming include pulmonary injury and infection, renal failure, and neurologic injury.Prehospital management of hypothermic patients consists of evaluating the patient’s ABCs (airway, breathing, and circulation) and providing basic or advanced life support as needed. Conscious, shivering patients (mild hypothermia) can begin treatment in the field with ingestion of warm sweet drinks and active movement. Passive rewarming, that is, removal of wet clothing and use of warm blankets, should be performed. Active external rewarming by applying heat to the skin with a heating pad, warm air, or warm water baths should be used in those with mild to moderate hypothermia. In patients with impaired consciousness who are breathing and have a detectable pulse, careful movement of the patients is required on transport because they are at risk for cardiac dysrhythmias. If there is no pulse, cardiopulmonary resuscitation should be initiated and maintained while the patient is transported to a hospital with extracorporeal membrane oxygenation (ECMO) capabilities because this is often required for optimal resuscitation. Severely hypothermic patients, who experience hypothermia before hypoxemia, have a greater likelihood of surviving cardiac arrest without neurologic impairment if ECMO rewarming is used. Timely transport of patients to a hospital experienced in and capable of treating hypothermia and its complications has greatly reduced its morbidity and mortality.Localized hypothermia (cold injury) can be classified as frostbite (the most common type of freezing injury), frostnip, pernio (or chilblains), and immersion foot (trench foot). Frostbite involves severe localized injury caused by freezing of the tissues, leading to cell death and tissue destruction. Frostnip is defined as local cold injury where skin becomes white and numb yet reverses on warming. Pernio includes rare localized inflammatory lesions due to damp cold exposure above freezing temperatures. Trench foot, also a nonfreezing cold injury, is associated with prolonged exposure to damp cold conditions, leading to injury of the sympathetic nerves and vasculature of the feet.Environmental risk factors for frostbite are similar to those for hypothermia. Frostbite tends to occur more commonly in younger children than in infants, yet in teens it is frequently related to intoxication, and overall it can be associated with lack of supervision. Wind chill and damp clothing are contributing factors to cold injury. Extremities are most at risk; 90% involve the hands and feet, and other common sites are the nose, cheeks, and ears.The damage to tissues in frostbite occurs in the freezing of extracellular fluid and ice crystal formation, which damages the cellular membrane, causing intracellular dehydration and leading to cell death. The body attempts to warm the extremity by alternating cycles of vasoconstriction and vasodilatation, the “hunting reaction.” This partial thawing and refreezing causes a thrombotic phase, leading to progressive dermal ischemia in capillary beds. Ischemia leads to increasing inflammatory mediators associated with edema, endovascular injury, and arrested blood flow to the area.In patients with a history of exposure to extreme cold, numbness of the affected area is the first sign of frostbite. Because frostbite injuries appear similar on presentation, staging is conducted only after rewarming. Superficial frostbite (first and second degree) is generally above the vascular plexus of the dermis and heals without substantial tissue loss. In deep frostbite (third and fourth degree) the injury is full thickness of skin and subcutaneous tissue and can involve muscle, tendons, and bone. Signs favorable for a superficial injury are normal skin color, clear blisters, and skin that compresses with pressure. Hard, noncompressible skin, cyanosis, and hemorrhagic blisters suggest deep injury.Initial management of frostbite involves protecting the extremity or affected tissue from mechanical trauma in transport to a hospital. Rewarming in the field can begin with immersion in warm water (98.6°F–102.2°F [37°C–39°C]) if there is no risk of refreezing before presentation to a hospital. It is important to avoid thawing before definitive warming can be done because refreezing can cause further damage. Once in-hospital care is initiated, rewarming is best performed in a water bath at 104°F (40°C) for 15 to 30 minutes. Rewarming can be painful, so attention to analgesia is important. Ibuprofen is recommended for control of pain and inflammation, although opioids might be needed for severe pain. Daily hydrotherapy to promote debridement and movement over the first few days is also needed. White blisters are debrided and treated with aloe vera, as an inhibitor of thromboxane, to reduce tissue necrosis. Hemorrhagic blisters should be left intact. Surgical care might be needed for debridement, although more aggressive debridement or amputation might not occur until final demarcation, usually after 1 to 3 months.Hypothermia and cold injury can be common, and children are at increased risk. It is not restricted to geographic areas with extreme cold but can occur in more temperate climes, especially when caught unprepared for low overnight temperatures in damp clothing or prolonged immersion in cool waters. Therefore, a high index of suspension is needed to appropriately diagnose and treat.This In Brief addresses a fascinating topic and one that is important for us as clinicians to know how to handle. We may encounter patients with hypothermia at work at our respective health-care facilities but also “in the field” while hiking in nature, at a swimming pool, or at the scene of a motor vehicle collision. Knowing what to do emergently and when to refer for more specialized care is key. Amazing advances have been made in rewarming techniques for severe hypothermia. One study provided data that the rates of survival after ECMO for patients who later are found to not have significant neurologic impairment ranged from 47% to 63%, which reinforces the point that one cannot predict the eventual outcome until the appropriate interventions and rewarming have been accomplished. One quote I read was particularly poignant: “No one is dead until they are warm and dead.” It is an important guideline to maintain cardiopulmonary resuscitation for prolonged periods in the field for patients with significant hypothermia, unless there is clearly a lethal injury or a do not resuscitate order for the patient. The science of interventions for hypothermia is evolving, and continued research is needed to optimize care and outcomes for our patients.