In miliaria, sweat flow is obstructed and trapped within the skin, causing skin lesions.
(See also Introduction to Sweating Disorders.)
Miliaria most often occurs in warm humid weather but may occur in cool weather in an overdressed, hospitalized, or bedridden patient. Lesions vary depending on the depth of tissue at which the sweat duct is obstructed.
Miliaria crystallina is ductal obstruction in the uppermost epidermis, with retention of sweat subcorneally. It causes clear, droplike vesicles that rupture with light pressure.
Miliaria rubra (prickly heat) is ductal obstruction in the mid-epidermis with retention of sweat in the epidermis and dermis. It causes irritated, pruritic papules (prickling).
Miliaria pustulosa is similar to miliaria rubra but manifests as pustules rather than papules.
Miliaria profunda is ductal obstruction at the entrance of the duct into the dermal papillae at the dermo-epidermal junction, with retention of sweat in the dermis. It causes papules that are larger and more deeply seated than those of miliaria rubra. Papules are frequently painful.
Diagnosis is by clinical appearance in the context of a hot environment or skin occlusion (eg, hospitalized or bedridden patients who lie with their back against the hospital bed for prolonged periods).
Treatment is cooling and drying of the involved areas and avoidance of conditions that may induce sweating; an air-conditioned environment is ideal. Once the rash develops, corticosteroid creams or lotions can be used, sometimes with a bit of menthol added.