Miliaria

A common disorder of the sweat glands that often occurs during hot, humid weather and is believed to result from blocked sweat ducts, so that sweat leaks into the skin. The three types of miliaria are classified according to the level at which obstruction of the sweat duct occurs:

• miliaria crystallina In this case, ductal obstruction is least severe, producing tiny, fragile, clear blisters.
• miliaria rubra (PRICKLY HEAT) In this case, obstruction occurs deeper within the epidermis, causing extremely itchy erythematous papules.
• miliaria profunda Ductal obstruction is at the dermal-epidermal junction, leaking sweat into the papillary dermis and producing subtle fleshcolored papules.

Miliaria occurs in individuals of all races, although some studies have shown that Asians, who produce less sweat than whites, are less likely to develop miliaria rubra. Miliaria crystallina and miliaria rubra can occur at any age but are most common in infancy. Miliaria profunda is more common in adulthood.

Cause
High heat and humidity that triggers excess sweating is the primary cause for the development of miliaria. Binding the skin with clothing or bandages can further contribute to pooling of sweat on the skin surface and overhydration of the top layer of the epidermis. If hot, humid conditions persist, the individual will continue to produce excessive sweat but will be unable to secrete the sweat onto the skin surface due to the blocked ducts. This leads to leakage of sweat en route to the skin surface, either in the dermis or in the epidermis. When the point of leakage is in the top layer of the epidermis or just below (as in miliaria crystallina), there is little inflammation. In miliaria rubra, however, leakage of sweat into the subcorneal layers produces blisters. In miliaria profunda, escape of sweat into the papillary dermis leads to white or red papules that usually do not itch.

Normal skin bacteria, such as Staphylococcus epidermidis, are thought to play a role in the development of miliaria. Patients with miliaria have three times as many bacteria as healthy people.

Symptoms
Miliaria crystallina is a common condition that is seen in infants, with a peak at one week of age, and in individuals with fever or who have recently moved to a hot, humid climate. Miliaria rubra is also common in infants who have moved to a tropical environment, occurring in up to 30 percent of people exposed to such conditions. Miliaria profunda is more rare, only seen in a minority of those who have had repeated bouts of miliaria rubra.

Miliaria crystallina is usually quite mild and gets better without complications over a period of days, although it may recur if hot, humid conditions persist. Miliaria rubra also tends to improve spontaneously when patients are removed to a cooler environment. Unlike patients with miliaria crystallina, however, those with miliaria rubra tend to have a lot of itching and stinging. They develop a lack of sweat in affected sites that may last weeks and, if generalized, may lead to heat exhaustion. Secondary infection is another possible complication of miliaria rubra, either as IMPETIGO or as multiple discrete abscesses known as periporitis staphylogenes.

Miliaria profunda is itself a complication of repeated episodes of miliaria rubra. The lesions of miliaria profunda do not cause symptoms, but patients may develop compensatory facial excess sweating, and a widespread inability to sweat elsewhere resulting from eccrine duct rupture; this is known as tropical anhidrotic asthenia and predisposes patients to heat exhaustion during exertion in warm climates.

Treatment
There is no reason to treat miliaria crystallina, as this condition is will go away on its own and does not cause unpleasant symptoms. However, miliaria rubra can be very uncomfortable and miliaria profunda may lead to heat exhaustion, so treatment of these two conditions is necessary.

Prevention and treatment of miliaria consist primarily of controlling heat and humidity so that the patient is not stimulated to sweat. This may mean treating a fever, reducing tight clothing, limiting activity, providing air conditioning, or (as a last resort) moving the patient to a cooler climate.

Topical treatments may include lotions containing calamine, boric acid, or menthol, cool wet-todry compresses, frequent showering with soap (although some doctors discourage excess soap), topical corticosteroids, and topical antibiotics. Topical application of anhydrous lanolin has resulted in dramatic improvement in patients with miliaria profunda. Anhydrous lanolin is believed to prevent ductal blockage, allowing sweat to flow to the skin surface. Calamine lotion provides cooling relief.

It may be possible to prevent miliaria by using oral antibiotics. Patients have also been treated with oral retinoids, vitamin A, and vitamin C with variable success. However, there have been no controlled studies to demonstrate the effectiveness of any of these treatments.

Since increased exertion leads to sweating, which greatly worsens miliaria, patients should limit activity (especially in hot weather) until the miliaria is cured. Patients with miliaria profunda are at particularly high risk for heat exhaustion during exertion in hot weather, since they have trouble dissipating heat via evaporation of sweat.