The severity of thermal injury in an individual depends on:
• The extent of the burned area
• The severity of the burn
• The presence of inhalation injuries
In living individuals, the extent of the burn is indicated as the percentage of total body surface area involved by the thermal injury. This is determined by the "rule of nines." If one considers the total body surface as 100%, then the head is 9%, the upper extremities are each 9%, the front of the torso is 18%, the back is 18%, each lower extremity is 18%, and the perineum is 1%.
Burns can be described as being first-, second-, third-, or fourth-degree; superficial, partial-thickness, or full-thickness burns; or a combination of both systems of nomenclature. In first-degree (superficial) burns, the skin is erythematous without blisters. Microscopically, there are dilated congested vessels in the dermis. The epidermis is intact, but there is some injury of the cells. There is subsequent desquamation of necrotic epidermal cells, e.g., peeling in sunburns. First-degree burns can be caused by prolonged exposure to low-intensity heat or light (e.g., sunburn), or a short-duration exposure to high-intensity heat or light.
Second-degree (partial-thickness) burns are subdivided into superficial and deep. Classically, in second-degree burns, the external appearance is a moist, red, blistered lesion. In superficial second-degree (partial-thickness)
burns, there is destruction of the striatum granulosum and corneum, with the basal layer not totally destroyed and edema at the dermal-epidermal junction. This injury heals without scarring. In deep second-degree (partial-thickness) burns, there is complete disruption of the epidermis and destruction of most of the basal layer. There might be blistering. The dermal appendages (the hair and sweat glands) are spared and act as the source of regenerating epidermis. Second-degree burns heal without scarring.
In third-degree (full-thickness) burns, there is coagulation necrosis of the epidermis and dermis with destruction of the dermal appendages. Externally, the lesions usually have a dry white leathery appearance. There are no blisters. The lesions might also be brown or black, caused by charring and eschar formation. This wound heals as a scar. In fourth-degree burns, there are incinerating injuries extending deeper than the skin.
It should be understood that the surface appearance of a burn does not necessarily indicate the depth of injury. The extent of necrosis or degree of burn can be diagnosed only in retrospect if the victim survives. Thus, a person who has been in contact with a hot surface might have a pale lesion with a white leathery appearance that seems to be a third-degree or full-thickness burn. It will subsequently be found, however, to be only a deep, second-degree (partial-thickness) burn.
The thickness of the skin in the area in which the injury is inflicted can have an effect on the appearance of the wound. Thus, in thick skin, such as the palm, what appears to be a third-degree burn may be only a second-degree (partial-thickness) burn, while, in a thin area of skin, what appears to be a second-degree burn could turn out to be a third-degree (full-thickness) burn.
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