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Case of the Week #344
A 2 year old boy presented with itchy brown skin macules (which were biopsied), but no associated systemic lesions.
Urticaria pigmentosa, also called maculopapular mastocytosis, is the most common form of cutaneous mastocytosis. It typically presents in children with the onset of multiple red-brown macules on the extremities. It has an unknown cause, and familial cases are rare.
Histologically, there is mast cell infiltration of the papillary and upper reticular dermis, and reactive basal hyperpigmentation of the overlying epidermis. The basophilic granules in the mast cells are metachromatic (red-purple) with toluidine blue, and are often immunoreactive, as in this case, for mast cell tryptase and kit / CD117 (images not available). The differential diagnosis includes chronic dermatitis, if the mast cells are not identified.
In children, cutaneous mastocytosis is generally a benign disease that may present at birth, and is often associated with mast cell mediator-related symptoms including pruritus, flushing, and abdominal pain with diarrhea. These symptoms may be triggered by physical stimuli, temperature changes, anxiety, medications and exercise, and symptoms usually respond to topical and systemic anti-mediator therapy including antihistamines and cromolyn sodium (Pediatr Allergy Immunol Pulmonol 2013;26:175). Most cases undergo remission at puberty. Urticaria pigmentosa is associated with mast cell infiltration of the bone marrow in up to 60% of cases, but this does not seem to predict an adverse clinical course (Am J Clin Pathol 1998;109:279).
The major concern is progression to systemic mastocytosis. In adolescents or adults, many of these cases may represent cutaneous involvement by systemic mastocytosis, which requires aggressive treatment (Mod Pathol 2014;27:19). Bone marrow studies are recommended if there is suspicion of progression of disease to an adult form, if cytoreductive therapy is contemplated, or if skin lesions remain present or tryptase levels remain elevated after puberty (Am J Clin Dermatol 2011;12:259).
Nat Pernick, M.D., President
and Shivani Thakore, Associate Medical Editor
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