Urticaria pigmentosa (UP) is a form of cutaneous mastocytosis commonly seen in children. Mastocytosis refers to a spectrum of disorders characterized by increased mast cells in the skin — the organ most commonly involved — and/or other organs, rather than a true neoplastic process. The symptoms are due to the release of mast cell mediators.

Nearly three-quarters of cases are seen in kids, with no racial or gender predilection. Urticaria pigmentosa is the commonest cutaneous mastocytosis and usually presents before the child’s first birthday. The cause of UP is generally unknown; however, in some adults, a mutation in proto-oncogen c-KIT may play a role. Other forms of cutaneous mastocytosis include mastocytoma — solitary nodule, diffuse cutaneous mastocytosis, and telangiectasia macularis eruptiva perstans.

Clinical features

In UP, lesions are often oval to round, reddish-brown macules or papules most often found on the trunk. The head, palms and soles are generally spared. Minor trauma or rubbing a lesion can cause a wheal and flare reaction — Darier’s sign. This is due to stimulating degranulation and release of histamine of lesional mast cells. Pruritus is variable. In very young children, lesions tend to be larger and may blister, whereas in older kids and adults, lesions tend to be smaller, flat and more stable.

UP is almost always confined to the skin. Very rarely, there can be systemic symptoms such as diarrhea, gastritis, hypotension, syncope, and bone pain and osteopenia.

Differential diagnosis

Urticaria is the main differential but easily distinguished, as there’s no hyperpigmentation, and lesions last for less than 24 hours.

Investigations

Investigations are rarely necessary as UP is primarily a clinical diagnosis in kids. If in doubt, a skin biopsy can confirm the presence of increased mast cells in the upper dermis.

A further workup may be warranted in children if lesions are extensive and if there are systemic symptoms. All adults with UP require a workup. Key investigations include a skeletal survery to assess for bony lesions, bone mineral density, serum tryptase, CBC and blood film, blood chemistry, bone marrow biopsy, and abdominal ultrasound. Endoscopy and cardiac investigations are done if symptoms are present.

Treatment

Patient education is an important component of managing UP as there’s no cure to this condition. UP has a good prognosis and self-resolves in more than half of patients by adulthood. Patients should be warned to avoid agents that can degranulate mast cells.

Patients should also be educated on the signs of anaphylaxis, and how to treat with an EpiPen.

For symptomatic management, first-line therapy includes antihistamines. Consider use of both sedating first-generation antihistamines such as hydroxyzine and non-sedating antihistamines such as cetirizine, loratadine and fexofenadine. H2 receptor blockers such as ranitidine may also be added.

Ketotifen is both an antihistamine and MC stabilizer that’s also helpful in reducing symptoms of UP. The main side effects include increased appetite and weight gain.

Other less commonly used treatments include phototherapy, e.g. PUVA, topical or intralesional corticosteroids, and rarely, systemic steroids.