Burn Depth and Classification

Burn depth is a significant determinant of mortality and the primary determinant of the patient’s long-term appearance and functional outcome. Superficial burns (second-degree burns) do not extend entirely through the dermis and leave behind epithelial-lined dermal appendages including sweat glands, hair follicles, and sebaceous glands. When dead dermal tissue is removed, epithelial cells migrate from the surface of each dermal appendage to other epithelial cells from neighboring appendages, forming a new, fragile epidermis on top of a thin residual dermal bed. With deeper burns, fewer appendages contribute to healing so the burn takes longer to heal and scarring is more severe.

Second-degree burns that heal spontaneously within 2-3 weeks, facilitated by local wound care alone and without surgery, usually resolve without hypertrophic scarring or functional impairment, although long-term pigmentation changes are common. Burns that require longer than 3 weeks to heal routinely produce hypertrophic scars, frequently lead to functional impairment, and provide only a thin epithelial covering that remains fragile for many weeks or months.

Optimal burn care requires early excision and grafting of all burns that will produce hypertrophic scars (typically those that will not or have not healed within three weeks of the injury) so an accurate estimation of burn depth is crucial. The appearance of the wound—and the apparent burn depth—changes dramatically within the first seven to 10 days. A burn appearing shallow on day 1 may appear considerably deeper by day 3. This demarcation of the burn is a consequence of thrombosis of dermal blood vessels and the death of thermally injured skin cells. Superficial burns may convert to deeper burns due to infection, desiccation of the wound, or the use of vasoactive agents during resuscitation from shock.

Burns are classified according to the depth of tissue injury: epidermal (first-degree), partial-thickness (second-degree), or full-thickness (third-degree). Burns extending beneath the subcutaneous tissues and involving fascia and/or muscle are considered fourth-degree. For coding purposes, burns causing such deep tissue destruction to require amputation or loss of a body part, are termed fifth-degree. Nevertheless, distinguishing between deeper burns that are best treated by early excision and grafting and shallow

burns that heal spontaneously is not always straightforward, and many burn wounds have a mixture of superficial and deep burns, making precise classification of the entire wound difficult.

First Degree (Superficial or Epidermal) Burns

These burns involve only the epidermis. They do not blister, but are red and quite painful. Over 2-3 days the erythema and the pain subside. By about day 4, the injured epithelium peels away from the newly healed epidermis underneath, a process which is commonly seen after sunburn.

Second Degree (Partial Thickness) Burns

Partial-thickness burns involve the epidermis and portions of the dermis and can be clinically categorized as either superficial partial-thickness or deep partial-thickness burns. Superficial partial-thickness burns characteristically form blisters between the epidermis and dermis. Since blistering may not occur for some hours after injury, burns that initially appear to be only epidermal in depth (first degree) may be determined to be partial-thickness burns 12-24 hours later. Most superficial partial-thickness burns heal spontaneously in less than 3 weeks, and do so typically without functionalimpairment or hypertrophic scarring. Second degree burns often accumulate a layer of fibrinous exudate and necrotic debris on the surface, which may predispose the wound to heavy bacterial colonization and delayed wound healing, in addition to making more difficult the determination of wound depth by visual inspection.

Deep partial-thickness burns extend into the lower layers of the dermis. They possess characteristics that are distinctly different from superficial or mid-dermal partial-thickness burns. If infection is prevented and spontaneous healing is allowed to progress, these burns will heal in three to nine weeks. However, they invariably cause considerable scar formation. Even with active physical therapy throughout the healing process, hypertrophic scarring is common and joint function is usually impaired. These burns are best treated by excision and grafting. For the patient, a partial-thickness burn that fails to heal within 3 weeks is functionally and cosmetically equivalent to a full-thickness injury.

Third Degree (Full-Thickness) Burns

Full-thickness burns involve all layers of the dermis and often injure underlying subcutaneous adipose tissue as well. Burn eschar is structurally intact but dead and denatured dermis. Over days and weeks if left in situ, eschar separates from the underlying viable tissue, leaving an open, unhealed bed of granulation tissue. Without surgery, they can heal only by wound contracture with epithelialization from the wound margins. Some full-thickness burns involve not only all layers of the skin, but also deeper structures such as muscle, tendon, ligament and bone, and are classified as deep full-thickness or fourth-degree. Grafting may use autologous skin grafts or biologic dressings and skin substitutes or both. (Excision and grafting using biologic dressings or skin substitutes permits closure of extensive burns in stages, with autografting done at a later date; see detailed discussion elsewhere in this paper.) Deep full-thickness burns may require amputation or closure with alternative techniques (such as adjacent tissue transfer or microvascular procedures)