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.
