Miliaria is another name for sweat rash.
Miliaria arises from obstruction of the sweat ducts. It is commonest in hot, humid conditions but may occur in desert regions. It affects up to 30% of people exposed to these climatic conditions. However, there is a striking variation in individual susceptibility. Very young infants are especially prone to it. Miliaria may begin within a few days of arrival in a tropical climate but is maximal after 2-5 months.
In New Zealand, miliaria is most common during humid summer weather. Activities which encourage sweating and the wearing of synthetic clothing against the skin are important precipitating factors. In winter, miliaria can result from swaddling up in too much clothing, sitting too close to the fire or heater and being hot in bed with a duvet and/or electric blanket.
In hospital, miliaria is typically seen on the backs of people who are lying for prolonged periods in bed, particularly when they are sweating from an infection, heart attack or chemotherapy, or have been immobilized by a stroke, head injury or orthopaedic operation.
Miliaria may be produced experimentally in susceptible subjects by injury to the epidermis (ie. surface layers of the skin). It can be reproduced regularly by occlusion of the skin under polythene for 3-4 days, following which the sweat ducts remain blocked for about 3 weeks. Prolonged exposure of the skin to sweat achieves the same effect. It is believed the first event in the production of miliaria is an increase in certain normal Staphylococcus epidermidis bacteria which live on the skin. These produce a sticky substance wich blocks the sweat ducts. Leakage of sweat through the walls of the duct behind the block is responsible for production of the miliaria spots and for further aggravation.
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