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Miliaria rubra, sometimes known as prickly heat, is a type of heat rash in which red spots appear and the skin prickles or itches. The condition is the most common form of a disorder known as miliaria, where blocked sweat glands result in sweat being unable to evaporate from the skin surface, with the result that it seeps into the skin. Miliaria rubra affects newborns and adults who spend time in environments which are humid and hot. It is treated by cooling the skin, which may be achieved by applying various medications, wearing different clothing, reducing activity, or using measures such as fans or air conditioning. The branch of medicine in which cutaneous conditions such as miliaria rubra are studied and treated is known as dermatology.
Three varieties of miliaria exist, known as miliaria crystallina, rubra and profunda. Each type of miliaria affects sweat ducts in a different level of the skin. Miliaria crystallina is a relatively mild condition affecting the top of the outer layer of the skin and causing tiny blisters, which may not itch at all and tend to disappear in a matter of hours or a few days. Blocked sweat ducts in the middle layer of the skin, below the outer layer, give rise to the rarer miliaria profunda, which causes larger, skin-colored lumps and is associated with a risk of heat exhaustion. Miliaria rubra is more common, is more likely to cause itching, and affects sweat ducts in the lower part of the skin's outer layer.
There are a number of miliaria rubra causes. In newborns, the sweat glands are immature in the first weeks of life and, in a hot environment or if the baby has a fever, they can rupture, leading to sweat escaping into the skin. People who become too hot due to physical activity, by covering up too much at night, or through wearing clothing made of non-breathable materials may also develop the disorder. Certain drugs are associated with the condition, such as bethanechol, a medication which causes increased sweating and which is used in the treatment of bladder disorders.
Miliaria rubra treatment involves making a variety of lifestyle changes to cool the skin, such as taking cold showers and wearing cotton clothing. Lotions and ointments applied to the skin, such as calamine, may relieve miliaria rubra symptoms of prickling and itching and can help prevent infection or heat exhaustion from developing. On moving to a country with a tropical climate some people may develop miliaria rubra, but then adjust over a number of months so that the problem resolves. In extreme cases, if the condition does not improve, it may become necessary to relocate.