Prickly heat

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Prickly heat is an itchy rash caused by trapped sweat.
(See also Introduction to Sweating Disorders.)
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 and in people who sweat more, but overdressed people in cool climates and hospitalized people 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. It also occurs when the skin is covered by clothing or rests against another surface for a prolonged period of time, such as in hospitalized people who lie with their back against the hospital bed.
Doctors diagnose prickly heat based on the appearance of the rash and whether the person has been in a hot environment, overdressed, or hospitalized on bedrest.
Treatment of prickly heat includes keeping the skin cool and dry. Use of powders and antiperspirants often helps. Conditions that increase sweating should be avoided, and an air-conditioned environment is ideal. People should avoid overdressing and using thick ointments.
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.