Understanding Heat Rash: Causes, Symptoms, Prevention, and Treatment

Miliaria, commonly known as heat rash, is a sweat retention syndrome in which the flow of sweat through its duct to the skin surface is impeded. Heat rash occurs due to increased perspiration from heat, humidity, fever, or occlusion of the skin. Infants are particularly susceptible due to immature sweat glands, although the problem can affect all age groups. Intense physical exercise in hot climates—such as Florida, where our experienced dermatologists live and work—and hospitalized adults with fever are other common settings that promote the development of heat rash.

The obstruction of the sweat duct may be due to the excessive sweating itself, as well as plugging with keratin or skin bacteria. The clinical appearance of the rash corresponds to the level of the obstruction of the sweat duct and is divided into three main subtypes:

• Miliaria crystallina is blockage of the sweat ducts that occurs at the most superficial layer of the epidermis, creating multiple, tiny fluid-filled blisters of the head, neck, and upper trunk. These blisters are quite fragile and break easily, resembling a bead of sweat.

• Miliaria rubra (prickly heat) happens when the obstruction is deeper in the epidermis and presents with larger red bumps of the chest, back, neck, as well as skin folds including armpits and groin. Miliaria rubra is more inflammatory, with background redness and itching. Small pustules may also be seen, sometimes referred to as miliaria pustulosa.

• Miliaria profunda is a rare variant of heat rash in which the sweat duct blockage occurs even deeper in the skin at the dermal layer. This creates flesh-tone papules of the trunk and extremities, usually with no symptoms. Profunda is usually seen only in tropical climates and often after repeated episodes of miliaria rubra. 

The diagnosis of heat rash is usually straightforward and based upon physical findings and patient history alone. In stubborn cases, a skin biopsy may be necessary. All forms of heat rash are generally self-limited and will resolve with relief from excessive sweating. Occasionally secondary bacterial infection can occur, requiring the use of topical antiseptics or antibiotics. Even more rarely, heat exhaustion can occur due to abnormal thermoregulation.

If you think you are suffering from heat rash, try these simple measures first:

• Get out of the heat. Move to a cooler, preferably air-conditioned environment.

• Take a cool bath or shower with a gentle soap.

• Avoid exercise with tight, occlusive clothing. Wear looser cotton or breathable synthetic fabrics instead.

• Make sure your bedroom is well-ventilated and cool at night.

• Limit use of plastic pants in children to prevent occlusion in the diaper area.

• Try over-the-counter calamine or Sarna lotions for the itch of prickly heat. Avoid ointments such as petrolatum that can further occlude the sweat duct.

• If febrile, reduce fever with acetaminophen.

Once skin temperatures are cooled, heat rash should resolve in a matter of days. If it persists, consult your dermatologist at Skin Wellness Physicians. Call 239.732.0044 or send a message online.