Prickly heat (miliaria) is an itchy skin rash caused by trapped sweat.
Prickly heat develops when the narrow ducts carrying sweat to the skin surface get clogged. The trapped sweat causes inflammation, which produces irritation (prickling), itching, and a rash of very tiny blisters. Prickly heat also can appear as large, reddened areas of skin.
Prickly heat is most common in warm, humid climates, but overdressed people in cool climates can also develop prickly heat. It tends to occur on areas of the body where skin touches skin, such as under the breasts, on the inner thighs, and under the arms.
The condition is controlled by keeping the skin cool and dry. Use of powders and antiperspirants often helps. Conditions that increase sweating should be avoided, thus an air-conditioned environment is ideal.
Once the rash develops, corticosteroid creams or lotions are used, sometimes with a bit of menthol added. However, these topical treatments are not as effective as keeping the skin cool and dry.
Last full review/revision October 2007 by Daniel W. Collison, MD