(See also Overview of Cold Injuries.)
The damage caused by frostbite results from a combination of factors. Freezing kills some cells, but others survive. Because cold causes blood vessels to narrow, tissue that is near the frozen area but not itself frozen may be damaged as a result of the decreased blood flow. Sometimes cold also causes clots to form in small blood vessels in this tissue. These clots may limit blood flow so much that the tissue dies. When blood flow returns to the affected area, the damaged tissues release a number of chemical substances that promote inflammation. Inflammation worsens the damage caused by the cold. In addition, toxic substances are released into the bloodstream as frozen tissue is warmed.
Exposure to below-freezing temperatures puts any part of the body at risk of frostbite. The risk of frostbite damage depends on how cold it is and how long the part was exposed. People at greatest risk of developing frostbite are those who have poor circulation because of diabetes or arteriosclerosis, blood vessel spasm (which may be caused by smoking, some neurologic disorders, or certain drugs), or constriction of blood flow by gloves or boots that are too tight. Exposed hands and feet and an exposed face and ears are most vulnerable. Contact with wetness or metal accelerates freezing and is particularly dangerous.
Symptoms of frostbite vary with the depth and amount of tissue frozen. Shallow frostbite results in a numb white patch of skin that peels after warming. Slightly deeper frostbite causes blisters and swelling of the affected area. Deeper freezing causes the extremity to feel numb, cold, and hard. The area is pale and cold. Blisters often appear. Blisters filled with clear fluid indicate milder damage than do blisters filled with blood-stained fluid.
Dead tissue may cause the extremity to become gray and soft (wet gangrene). If wet gangrene develops, in many cases the extremity must be amputated. More frequently, the area of dead tissue becomes black and leathery (dry gangrene).
Frostbite is diagnosed by its typical appearance and occurrence after significant exposure to cold. Sometimes frostbite appears the same as nonfreezing injuries for the first few days. After a period of time, frostbitten tissue develops characteristics that differentiate it from nonfreezing tissue injuries.
People who have frostbite should be covered with a warm blanket because they may also have hypothermia. When possible, warming of the frostbitten area should begin immediately. The area is immersed in warm water that is no hotter than can be comfortably tolerated by the caregiver (98.6 to 104° F or about 37 to 40° C). Rubbing the area (for example, with snow) should be avoided because it leads to further tissue damage. Because the area has no sensation, people cannot tell if a burn is developing. Thus, the area should not be warmed in front of a fire or with a heating pad or electric blanket.
It is more damaging to thaw and refreeze tissue than to allow it to remain frozen. Thus, if people with frostbite must be re-exposed to freezing conditions, particularly if they must walk on frostbitten feet, the tissue should not be thawed. Thawed feet are more vulnerable to damage from walking. Also, every effort should be made to protect the damaged tissue from rubbing, constriction, or further damage. The feet are usually cleaned, dried, and covered. People are kept warm and given an analgesic if possible. They are taken to a hospital as soon as possible.
In the hospital, warming is begun or continued. Full rewarming takes about 15 to 30 minutes. During rewarming, people are encouraged to move the affected part gently. The frostbitten area becomes extremely painful as it is warmed, so an injection of an opioid analgesic may be necessary. Blisters should not be broken. If blisters break, they should be covered with antibiotic ointment.
Once the tissue is warmed, the frostbitten area should be gently washed, dried, wrapped in sterile bandages, and kept meticulously clean and dry to prevent infection. Anti-inflammatory drugs, such as ibuprofen by mouth or aloe vera gel applied topically, help relieve the inflammation. Infection requires use of antibiotics, although some doctors try to prevent infection from occurring by giving antibiotics to all people with deep frostbite. Some doctors also use drugs given into a vein or artery to improve circulation to the affected area, although these forms of treatment are beneficial only in the first few days after injury. Doctors may give tetanus toxoid if the person has not been vaccinated against tetanus or is overdue for a tetanus booster.
People need to eat a healthy diet to ensure the body produces enough heat.
Whirlpool baths with warm water (about 98.6° F, or 37° C) three times a day followed by gentle drying, rest, and time are the best ongoing treatments. Most people slowly improve over several months, although amputation is sometimes necessary to remove the dead tissue. Because frostbite may appear to affect a larger area and to be more severe than it will weeks or months later, the decision to amputate is usually postponed for several months until the area has had time to heal. Sometimes an imaging test, such as radionuclide scanning, microwave thermography, or a laser-Doppler flow study, helps determine which areas may recover and which will not. Areas that will not recover require amputation. Some people develop numbness or oversensitivity to cold after frostbite heals.