Adult-Onset Polymorphic Maculopapular Cutaneous Mastocytosis

Cutaneous mastocytosis commonly manifests with maculopapular lesions. In children, the lesions tend to be large and polymorphic, while in adults, they are typically small and monomorphic. Adults who develop cutaneous manifestations are usually found to have systemic mastocytosis (SM).  In this report, we describe the case of a 42-year-old female patient who developed polymorphic hyperpigmented cutaneous lesions scattered over her chest and abdomen. Punch biopsies of two lesions were obtained and demonstrated increased perivascular and interstitial mast cells. Over the following month, she continued to develop 1-5 mm hyperpigmented macular lesions scattered over her arms, associated with intermittent flushing episodes of the face and chest, pruritus, and a positive Darier's sign. She underwent a bone marrow biopsy, which was negative for dense infiltrates of mast cells; immature, atypical, or spindle-shaped mast cells; expression of CD2, CD25, and CD30; and the KIT D816V mutation. Peripheral blood was also negative for the KIT D816V mutation. Her basal tryptase level ranged from 2.9 ng/mL to 4.0 ng/mL, which was within normal limits. Thus, symptomatic treatment with cetirizine and montelukast was initiated.  The overall clinicopathologic correlation, including the heterogeneity of lesions, was most consistent with polymorphic maculopapular cutaneous mastocytosis (MPCM). In contrast to the typical adult presentation of monomorphic lesions and associated systemic involvement, this patient presented with polymorphic lesions and did not meet any major or minor criteria for SM. Regular allergy follow-up was indicated to monitor for potential development of systemic involvement over time.  In adult patients presenting with a polymorphic cutaneous macular eruption, it is important to keep MPCM in the differential diagnosis.