Winter has arrived, bringing with it cold injuries including chilblains (pernio), frostnip, and of course, the dreaded frostbite.


Frostnip is a superficial nonfreezing cold injury due to vasoconstriction. The skin will be pale and may have paresthesias or numbness. It is common on exposed skin including the cheeks, ears, and nose. The skin is still pliable. This is a major difference between frostnip and frostbite. If rewarmed at this point, there is no permanent tissue damage. If you identify a body part with frostnip, warm it immediately. Frostnip is a prescursor to frostbite and once recognized, appropriate precautions should be taken to treat it and prevent progression and recurrence.

Frostbite, unlike chilblains and frostnip, involves tissue freezing and can lead to permanent tissue damage. In this post, we review the pathophysiology and presentation of frostbite.

The pathophysiology involves a progression through four phases:

  1. Prefreeze – At this point there is tissue cooling associated with vasoconstriction and ischemia, but no ice crystals yet.
  2. Freeze-thaw – Ice crystals form intracellularly (if rapid cooling) and extracellularly (if slower cooling) which causes changes in osmolarity leading to fluid and electrolyte shifts, cellular dehydration, and eventually cell lysis and death.
  3. Vascular stasis – In this phase there is thrombosis in small vessels and blood vessels may fluctuate between vasospasm and vasodilation, which causes vascular shunting and leakage of plasma.
  4. Ischemic – The hallmark of the ischemic phase is tissue infarction due to a combination of inflammation, vasoconstriction, and coagulation leading to destruction of the microcirculation and further cell death.

Similar to a burn, frostbite is classically divided into 4 degrees based on level of tissue damage. These can be difficult or impossible to distinguish in the field. If a patient presents for medical care early or before an injury is rewarmed, the extent of injury and typical signs (blisters, erythema, edema, necrosis) may not be present yet. Complete classification often requires time and/or imaging studies. It can be more practical in the austere setting to classify frostbite as either superficial or deep.

  1. First degree (superficial) – The skin will be firm and if the patient is Caucasian it will be white or yellow in colour. Different skin tones will have different presentations, but all skin tones will show ischemic changes. The skin will be numb and may be edematous.
  2. Second degree (superficial) – Injuries will have blisters filled with clear or milky fluid. These are associated erythema and edema.
  3. Third degree (deep) – This involves hemorrhagic (blood-filled) blisters, indicating that the injury has extended deeper into the dermis.
  4. Fourth degree (deep) – These injuries involve deeper structures including muscles, tendons, and bones. There is necrosis of tissues and after 9-15 days, the skin forms a black, dry, hard eschar. This will eventually demarcate and mummify in 22-45 days.

A patient with frostbite may have multiple or all of the degrees of frostbite on different areas of the skin so careful assessment and management is vital to preserve as much tissue as possible.

Stay tuned for our next post on frostbite management and prevention including new research and some potential new treatment options for the austere environment.



Alana Hawley, M.D.
Wilderness Medicine Fellowship, University of Utah
PGY-5 Emergency Medicine, McMaster University



Read more cold weather wilderness medicine articles:

Case study about about winter first responder guidelines for remote vehicle accidents.

Hypothermia fundamentals and treatment concerns.

NEW! Online CME Modules are also available. Check out the course offerings. , including our exciting Winter Wilderness Medicine bundle!


Auerbach, P.S. (2011). Wilderness Medicine. Edinburgh: Mosby.

Ingebretsen, R. and Della-Guistina, D. (2013). Advanced Wilderness Life Support: Prevention – Diagnosis – Treatment – Evacuation. Salt Lake City: AdventureMed LLC.

Mcintosh, S. E., Opacic, M., Freer, L., Grissom, C. K., Auerbach, P. S., Rodway, G. W., . . . Hackett, P. H. (2014). Wilderness Medical Society Practice Guidelines for the Prevention and Treatment of Frostbite: 2014 Update. Wilderness & Environmental Medicine, 25(4). doi:10.1016/j.wem.2014.09.001