Classification of Frostbite by Depth of Tissue Injury
Frostbite is classified into four degrees of severity based on the extent and depth of tissue involvement, with the classification system predicting final outcomes including the need for and level of amputation.
Four-Degree Classification System
The most clinically useful classification divides frostbite into four degrees based on initial lesion topography and early bone scan findings 1:
First-Degree Frostbite
- Involves only superficial skin layers with complete recovery expected 1
- No permanent tissue loss occurs 2
- Presents with numbness and white-cyanotic discoloration 3
- Rewarming causes hyperemia and is often painful 2
Second-Degree Frostbite
- Results in soft tissue amputation without bone involvement 1
- Blisters and edema develop after rewarming 2
- Superficial tissue damage occurs, but deeper structures remain viable 4
- Probability of bone amputation remains around 1% when lesions are limited to the distal phalanx 1
Third-Degree Frostbite
- Leads to bone amputation of affected digits 1
- When initial lesions extend to the middle phalanx, bone amputation probability rises to 31% 1
- Lesions involving the proximal phalanx carry a 67% amputation risk 1
- Hard eschar may form with potentially healthy tissue deep to the eschar 2
Fourth-Degree Frostbite
- Results in large amputation (metacarpal/metatarsal or carpal/tarsal level) with systemic effects 1
- Lesions at the metacarpal/metatarsal level have a 98% amputation probability 1
- Carpal/tarsal involvement approaches 100% amputation risk 1
- Represents the most severe form with potential systemic complications 1
Alternative Binary Classification
A simpler two-tier system is also used clinically 2:
- Superficial frostbite: No permanent tissue loss 2
- Deep frostbite: Varying degrees of permanent tissue loss, often less extensive than initial appearance suggests 2
Critical Assessment Challenges
- Initial clinical impression typically overestimates actual tissue damage 4
- Wide discrepancy may exist between skin damage and deeper structure involvement 4
- Estimating the amount of tissue loss at presentation and early in the course is difficult 2, 4
- Triple-phase bone scanning helps differentiate superficial from deep damage and predict outcomes 4, 1
- Clear demarcation between healthy and necrotic tissue may take weeks to develop 4, 3
Common Pitfall
The most common error is making premature surgical decisions based on initial appearance—surgical debridement or amputation should be postponed until clear demarcation between viable and necrotic tissue occurs 4, 3, as frozen tissue often appears worse than the actual extent of permanent damage 2.