ERYTHEMA AB IGNE
Erythema ab igne is an unusual rash that can develop secondary to exposure to an exogenous heat source. The name is derived from the Latin phrase meaning “redness from the fire.” It has a clinically characteristic pattern. The differential diagnosis is limited. For unknown reasons, not all persons exposed to heat sources develop the rash of erythema ab igne. Many patients develop the rash without even knowing of its existence. Reported causes have included hot water bottles, heating blankets, heaters, and computer laptops. Essentially any exogenous heat source can cause this reaction. Erythema ab igne has also been called the “roasted skin” or “toasted skin” syndrome. The exact temperature needed for the reaction to occur is unknown, and for some reason it does not occur from hot tub use, most likely because the causes of erythema ab igne are dry heat or temperatures higher than those of most hot tubs.
Clinical Findings: This condition can be seen in individuals of any race and gender. The initiating factor is an exogenous heat source that is applied to the skin. The heat source exposure is typically chronic and repetitive. Patients often notice a fine, lacy, red, reticulated macule or patch. Occasionally, no inflammatory phase is noticed, only a reticulated hyperpigmentation of the skin. Some patients do not realize that the rash is located on skin in direct approximation to a heat source. The lower back is a commonly affected area, secondary to the use of heating blankets or bottles to help treat chronic lower back pain. There have been many reports of erythema ab igne from exposures to all sorts of heat sources. Laptop computers can release a large amount of energy as infrared radiation; if someone is chronically using a laptop computer in direct approximation to their skin (e.g., anterior thighs), the rash of erythema ab igne may develop. The diagnosis is typically made by clinical examination and historical information. Patients often need to be asked whether they have been using a heating device or consistently using a laptop computer, because the correlation is not evident to them. The development of actinic keratosis or squamous cell carcinoma within the areas of erythema ab igne has rarely been reported.
Pathogenesis: Erythema ab igne is caused by the direct effects of heat on the skin. The temperature required has not been precisely defined, but the range of 43°C to 47°C seems to be most likely. In any case, there must be repeated exposure to subthermal burning temperatures. More frequent exposures and longer exposures seem to increase the risk of development of erythema ab igne. The exact mechanism by which the rash develops is unknown.
Histology: The skin may be slightly atrophic, and elastotic tissue is seen within the dermis. The rete ridges may be thinned. Some areas may show evidence of changes such as those seen in actinic keratosis. Vacuolar degeneration of the basal layer can be seen.
Treatment: The goal of therapy is to discover and remove the exogenous heat source. Once the heat source is removed, most of these rashes slowly fade away over months. Some of the hyperpigmented areas may persist, however. Use of emollient creams or Kligman’s formulation has been reported. Kligman’s formulation includes a retinoid, a steroid, and a skin-lightening cream. Laser therapy has also been used to decrease the pigmentary disturbance.