This 4-week-old boy was clinically diagnosed with urticaria pigmentosum following a generalized eruption of pigmented macules soon after birth. The biopsy results confirmed the presence of numerous mast cells within the interstitium. This patient had only cutaneous disease, but some patients progress to systemic mastocytosis. The differential diagnosis, without history, may include mastocytoma or telangiectasia macularis eruptiva perstans (TMEP).
Histologic sections of skin show epidermal acanthosis with increased pigmentation in the basal keratinocytes. Within the dermis, there is an infiltrate of spindle cells in an interstitial pattern, extending deep within the reticular dermis.