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Urticaria pigmentosa (UP) is a skin rash that is seen in most kinds of mastocytosis.
Mastocytosis of the skin has been classified into the following forms: Mastocytoma is one or a very few individual large lesions that become red and may blister
and swell when stimulated;
maculopapular cutaneous mastocytosis denotes flat to slightly raised pink/cream/tan/brown
spots that itch, turn red, and form a welt when stimulated - a feature called Darier's sign;
and diffuse cutaneous mastocytosis involves heavy infiltration of the skin with mast cells.
Darier's sign is extreme, and blisters with bleeding are often seen.
Urticaria pigmentosa is a subset of maculopapular cutaneous mastocytosis. Pediatric mastocytosis of the skin has some important differences from adult mastocytosis, so this discussion will deal first with the two groups separately, then present observations that are appropriate to both groups. Pediatric UP
The rash of UP may be present at birth, or may appear at any age throughout childhood. The appearance of each child's UP remains consistent over months to years, although it varies between children. The spots are coloured tan to dark brown, often on a reddened background; they may be small (3 to 5 mm) round or oval spots, or larger (up to about 1 cm in the longest direction) and irregularly shaped. They are flat or slightly raised. A child may have only a few spots, or may be almost entirely covered on the trunk, arms and legs. The palms, and soles are usually clear, but some children have spots on their face. The spots of UP remain in the same place for months to years and do not appear and disappear over the short term, although children with UP may also have short-term red rashes when their symptom level is high. A child's UP rash may disappear before they reach adulthood. If the rash continues to be present after puberty, the spots usually become smaller and flat and symptoms are often reduced in severity. In general, children whose spots appear in late childhood are more likely to retain their rash as adults. Diagnosis of UP in children is often made from history, the appearance of the skin, and the presence of Darier's sign. Biopsy of a spot is appropriate when the diagnosis is not clear without it, especially for newborns with blisters and few spots - other blistering diseases in newborns require treatment in the first few days, and this is not the same as treatment for UP. Most children with UP do not have and will not develop systemic mastocytosis. Study of the mast cells in the skin beneath pediatric UP spots has shown that there often is a change, or mutation, in a surface protein, the 'kit' receptor, associated with pediatric UP. Children who have UP and systemic mastocytosis usually have a change in construction of the kit receptor, with the 816th amino acid building block being something other than the normal one at that location, and this mutation is associated with increased activation of those mast cells. Children with only UP do not have this mutation, but may have any one of several others. The relationship between these other mutations and the presence of UP and symptoms is not clear. In a significant number of children, the kit receptor is completely normal. The UP spots in adults vary from round to oval, usually 3 to 5mm across, are tan to brown, and may be sparsely sprinkled or heavily clustered so the normal skin is almost occluded. Darier's sign (see above) may or may not be present, and diagnosis is almost always made by skin biopsy if the rash has appeared in a teenager or adult. With the exception of those rare adults whose mastocytosis is inherited, adults with UP have a mutation at the 816th amino acid of the kit receptor on mast cells in the skin beneath UP spots. This mutation is associated with increased activation of those mast cells. Most people with adult-onset UP have or will progress to systemic mastocytosis. However, there is a group of adults who are unlikely to develop systemic mastocytosis in spite of UP that may be pronounced. These adults have inherited a form of a gene that is usually associated with allergic symptoms; the protective form of this gene is associated with lack of allergic symptoms in the general population, and with a lack of systemic mastocytosis (or very mild systemic disease) in adults with UP. UP spots are unlikely to disappear in adults, although with ageing they may become less prominent. Symptoms and treatment of paediatric and adult UP
Some people with UP have few or no symptoms, and require only occasional or no mediations. Other people have symptoms associated with release of chemicals, or mediators, from activated skin mast cells. Symptoms may be restricted to the skin, such as itching, hives, or flushing (turning pink or red over areas of the skin), or they may involve internal organs because of the high systemic level of mast cell mediators. These symptoms include abdominal cramping, diarrhoea, increased stomach acid, vomiting, or headache, and there may be episodes of low blood pressure causing fainting or light-headedness. Anaphylaxis may occur in people with any type of mastocytosis. Some parents of young children with UP report occasional dramatic emotional swings followed by sleepiness. Treatment of symptoms based on blocking the effects of mast cell mediators is with the use of H1 antihistamines (the ones usually associated with treatment of allergy), H2 antihistamines (usually used to reduce stomach acid, although they also help reduce skin symptoms), and blocking the formation of prostaglandin D2 using aspirin or other non-steroidal anti-inflammatory drugs (NSAIDs), although these medications can cause mast cell activation so must be started under strict medical supervision. Continued use of aspirin or NSAIDs must be evaluated often because of the possible occurrence of unwanted side-effects. Skin mast cells produce very little if any leukotriene C4, so leukotriene blockers such as Singulair may not be helpful unless the person also has respiratory symptoms that may not be associated with UP. Reduction of mast cell activity through the use of topical sodium cromoglycate is another approach to treatment. Altoderm is such a preparation, and is available in the EU. In other parts of the world, a topical cream may be produced using any available form of cromolyn, mixed in a concentration of 1% to 4%. Treatment is sometimes sought to reduce the cosmetic impact of the UP rash. The effect of all presently used cosmetic treatments is temporary, with a return of rash in a few months to a year. Dermatologists may be able to offer several possible choices of treatment using laser or ultraviolet light. Exposure to moderate amounts of sunlight may reduce the rash intensity during summer months, although this requires care because sunlight and heat may trigger mast cell activity. Strong steroid creams may reduce the colour of UP spots temporarily, but a significant amount of steroid is absorbed into the system so only very small areas should be treated; in children topical steroid treatment is rarely justified. Clinical and histopathological aspects of cutaneous mastocytosis; K Wolff, M Komar, P Petzelbauer; Leukemia Research (2001) 25:519-528. Mastocytosis: molecular mechanisms and clinical disease heterogeneity; DD Metcalfe, C Akin; Leukemia Research; (2001)25:577-582. Association of the Q576R polymorphism in the interleukin-4 receptor chain with indolent mastocytosis limited to the skin; T Daley, DD Metcalfe, C Akin; Blood (2001);98:880-882.

Source: http://www.ukmasto.org/UP_Nancy.pdf