Prickly heat (miliaria) is an itchy 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 causes irritation (prickling), itching, and a rash of small bumps or very tiny blisters. Occasionally the bumps can cause pain.
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.
Doctors diagnose prickly heat based on the appearance of the rash and whether the person has been in a hot environment or overdressed.
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 can be used, sometimes with a bit of menthol added. However, these treatments are not as effective as keeping the skin cool and dry.
Last full review/revision June 2013 by David M. Pariser, MD