Heat of sufficient intensity and duration causes coagulation necrosis and cell death, but the cell damage due to heat of lesser intensity is potentially reversible. The region of immediate cell death caused by a burn is the zone of coagulation. Areas of progressively less damaging cell injury are the zones of stasis, in which the initially compromised blood flow improves with time, and the zone of hyperemia, in which there is marked increase in blood flow as a consequence of burn-induced inflammation. In a full-thickness (third-degree) burn, the zone of coagulation comprises the entire thickness of the dermis. In a partial-thickness (second-degree) burn, a variable portion of the dermis is involved, and in a first-degree burn, only the epidermis is affected.

PARTIAL-THICKNESS BURN

A first-degree burn is the most superficial form of a partial-thickness burn, sunburn being the most common. The skin is pink or light red; the surface is usually dry, although small blisters may form. The skin remains soft with minimal edema, with subsequent exfoliation of the superficial epidermis. Such injuries are hypersensitive but heal in 3 to 6 days. They require little treatment other than administration of analgesics and oral anti-inflammatory medications to minimize inflammation. Cool showers help lessen postural hypotension and provide some pain relief. At the time of exfoliation, antipruritic treatment may be necessary.

Second-degree burns, also partial-thickness injuries, can be subdivided into superficial second-degree burns, which heal within 21 days, and deep second-degree burns, which take longer to heal. Second-degree burns are caused by limited exposure to a hot liquid, flash, flame, or chemical agent. Superficial second-degree burns appear pink or bright red with profuse serous exudation from the surface and may form blisters between the epidermis and the dermis. These injuries are hyperesthetic—even a draft of air can cause pain. Deep partial-thickness, or second-degree, burns extend to the lower layers of the dermis. The surface of a deep second-degree burn is moist and mottled in various hues of red. Sensation to pinprick is reduced, but pressure sensation remains intact. If protected from infection, these injuries heal spontaneously in 3 to 9 weeks, although often with scarring. When occurring around joints, they may impair function and require excision and grafting.

FULL-THICKNESS BURN

Third-degree burns result from prolonged exposure to a flame, hot object, or chemical agent or from contact with high-voltage electricity. They involve all layers of the dermis as well as underlying adipose and connective tissue. If deeper structures such as muscle, tendon, ligament, and bone are involved, the injury is considered a deep full-thickness or fourth-degree burn and may involve more extensive reconstruction or amputation. The surface of the burn appears pearly white, charred, translucent, or parchment-like; thrombosed superficial vessels are often visible. In young children, the initial appearance of third-degree burns can be misleading: they are initially dark red, slightly moist, and pliable; then the wound desiccates and becomes unpliable and dark reddish brown. Chemical burns cause injury by coagulation necrosis, which may take 12 to 24 hours for the full extent to be seen. Strong acid burns produce deep gray to brown coloring of the skin, which may be confused with suntan. Exposure to strong alkali may result in soap tissue necrosis. High-voltage electric injury typically causes loss of tissue and dense charring at contact sites. The majority of the injury occurs to deeper structures so the initial wound/burn on the skin may be deceiving. The wound surface of all full-thickness burns is insensate and always requires skin grafting for closure.