Urticaria pigmentosa is the name given to a type of mastocytosis, in which there are brown patches on the skin due to abnormal collections of mast cells.
Mast cells are normally widely distributed in the skin. They contain granules that contain histamine and other chemicals. When the mast cell is disturbed, these chemicals are released into the surrounding skin. The chemicals make the blood vessels leaky, resulting in localised itching, swelling and redness.
Urticaria pigmentosa most often affects infants, with the first patches appearing at a few months of age. They are often confused with insect bites at first, but persist and gradually increase in number for several months or years. They can appear on any part of the body including the scalp, face, trunk and limbs.
In young children, it is common for the patches to blister when rubbed. If many patches are activated at the same time the infant may become irritable but is uncommon for severe symptoms to arise.
Over the next few years the urticaria pigmentosa becomes less irritable and eventually the patches fade away. By the teenage years, most patches will have gone.
One can demonstrate the presence of mast cells by rubbing one of the brown patches. Within a few minutes, the rubbed area becomes reddened, swollen and itchy. This is known as Darier sign, and confirms the presence of mastocytosis.
Sometimes urticaria pigmentosa develops for the first time in an adult. Few or many lesions appear and can be unsightly as well as itchy. Unfortunately, in adults urticaria pigmentosa tends to persist long term. It is also more likely to be associated with internal symptoms.
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