Background
Also known as maculopapular cutaneous mastocytosis, this is the most common type of cutaneous mastocytosis in children.
Urticaria pigmentosa is an accumulation of mast cells which have the same genetic mutation. The common mutation called D816V in the receptor called c-KIT has been found in up to 95% of adults with the condition and in some children. The reason for this mutation is still unknown. The release of histamine can cause a variety of symptoms: itching, hives (also known as nettle rash), wheezing, and diarrhoea, and uncommonly to life-threatening collapse (anaphylaxis). Certain factors may trigger the release of histamine and the other chemicals from mast cells, including drugs (codeine and morphine, some general anaesthetic agents, aspirin and other non-steroidal anti-inflammatories), alcohol, emotional stress, physical stimuli (heat, exercise and skin friction) and insect stings.
Signs and symptoms
It consists of brown and red oval-shaped patches on the skin and is most often first seen in infancy, occasionally at birth, and may appear anywhere on the body. These patches persist throughout childhood and may increase in number. Since the patches represent sites of increased mast cell proliferation, they are pruritic, may blister when rubbed and exhibit Darier sign (appearance of a wheal on affected skin on rubbing). However, pruritus and colour become less intense as the child grows and most would have resolved completely by puberty. Characteristically there would be brown lesions on the lateral aspects of the forehead.
Diagnosis
Although appearance and symptoms are characteristic, should there be diagnostic doubt a skin biopsy may be useful. Alternatively, serum tryptase may be measured, and this would be expected to be raised, however, it usually decreases beyond infancy. Rarely lesions may persist into adulthood, in which case lesions would behave more like systemic mastocystosis, which contrasts to the more common polymorphic type of maculopapular cutaneous mastocytosis
Differential diagnosis
Differentials include moles and insect bites.
Treatment
There is no cure for urticaria pigmentosa. Management is conservative as the lesions eventually self-resolve, however symptomatic treatment includes oral antihistamines, topical steroids and topical calcineurin inhibitors. Laser can sometimes help with the appearance of the lesions. Childhood urticaria pigmentosa resolves by puberty in many cases. If urticaria pigmentosa started after the age of 10, there is a greater chance of progressing to systemic mastocytosis where mast cells may accumulate at sites other than the skin.
References
https://dermnetnz.org/topics/maculopapular-cutaneous-mastocytosis/
Harper’s Textbook of Paediatric Dermatology (4th Edn)