Urticaria pigmentosa.

A 6-year-old female, presented with multiple dark-colored lesions, which started over the scalp and further progressed to involve the face and trunk since past six months. She gave a history of elevation, redness, and itching on the lesions on application of pressure. There was no associated flushing, vomiting, diarrhoea, or wheeze. The personal and family history was not contributory.

On examination, there were multiple hyperpigmented macules over the scalp, forehead, face, and neck in addition to minimally elevated hyperpigmented plaques over the chest and the back [Figure 1a–c]. Gentle rubbing of the lesions elicited urtication and itching within 2 min and it resolved within 15–20 minutes, suggestive of the Darier's sign [Figure 1d].

Figure 1

(a) Multiple ill-defined hyperpigmented macules on the forehead, nose, and the upper lip, (b) multiple ill-defined hyperpigmented macules over the scalp, (c) minimally elevated hyperpigmented plaques on the chest, and (d) positive lesional Darier's sign

A complete blood count, liver and renal functions, urine analysis, and chest roentgenogram were normal. The histopathology of a minimally elevated lesion from the back demonstrated increased melanization of the basal layer with sheets of oval mast cells infiltrating the upper and lower dermis [Figure 2a–c]. The metachromatic granules of the mast cells were visualized using the toulidine blue stain [Figure 2d]. The diagnosis was thus established as urticaria pigmentosa.

Figure 2

(a) 4×—Scanner view: Sheets of mast cells seen occupying the upper mid and the lower dermis, (b) 10×—Basal layer hyperpigmentation with oval mast cells seen infiltrating the upper and mid-dermis without any evidence of epidermotropism, (c) 40×—Sheets of oval monomorphic mast cells, (d) Oil immersion-toulidine blue stain demonstrating the metachromatic granules of the mast cells

Urticaria pigmentosa is the most common form of mastocytosis with onset generally below two years of age. The lesions are rose colored, pruritic, urticarial, slightly pigmented macules, papules, or nodules. The nonevanescent urticated lesions persist and gradually become pigmented. The increased number of the functionally normal mast cells in the dermis degranulate in response to stroking with the release of histamine, which is responsible for the local itching, erythema, and weal formation or the Darier's sign.[1] The lesions usually disappear within a few years, usually before puberty, although in a few cases the lesions may persist into adult life. Although systemic involvement is possible, malignant transformation is exceedingly rare.

The multiplicity of lesions on the scalp is an uncommon finding in the case discussed.[2]

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