Heat rash and miliaria are umbrella terms used to describe a category of conditions related to the abnormal retention of sweat. Miliaria can be caused by several factors, including fever, prolonged bed rest, immature sweat ducts, hot and humid environment, skin occlusion from bandages or dressings, and intense physical activity. Toxic epidermal necrolysis, Stevens-Johnson syndrome, genetic diseases of the skin, radiotherapy, and adverse reaction to medication can also cause miliaria. Diagnosis of heat rash or miliaria is made with the use of a physical examination and a punch biopsy in some cases. Untreated miliaria can cause an affected individual to develop a secondary bacterial infection, hyperhidrosis, and impaired thermoregulation. The type of heat rash that develops in an individual is determined by the location of the sweat that becomes obstructed in the dermis or epidermis.
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Miliaria crystallina is a term used to describe when an individual experiences an obstruction of the sweat ducts closest to the surface of their skin. The most common group of individuals affected by miliaria crystallina are infants around one week old. Some adults who have a fever may also develop this form of miliaria as a complication of their illness. Symptoms of miliaria crystallina include clear superficial blisters that tend to burst easily. The size of the blisters that appear on the skin of an individual with this type of heat rash is between one and two millimeters. The clear blisters may look similar to beads of sweat. The most common areas for the blisters caused by miliaria crystallina to develop is the upper trunk, head, and neck. These blisters do not become inflamed, and they do not form any crusted lesions. Most cases of miliaria crystallina resolve without the necessity of any medical intervention and do not have any long term effects.
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