Infantile Mastocytoma: Clinical Presentation and Diagnostic Insights

Mastocytosis is a heterogeneous group of disorders characterized by abnormal proliferation and accumulation of mast cells in one or more organs. Cutaneous mastocytosis (CM) is the most common form in children, with solitary mastocytoma representing a distinct subtype. Mastocytomas are generally benign, present at birth or in early infancy, and may manifest as pigmented macules, nodules, or plaques, occasionally associated with flushing or urtication. Histopathological confirmation is essential in atypical presentations. We report a 4.5-month-old infant presenting with a lesion on the metacarpophalangeal joint of the right hand, first noted at birth. The lesion initially appeared erythematous, then developed as bullous eruptions with serous fluid, and eventually evolved into a reddish, crusted nodule with a rough surface. The infant experienced intermittent, brief episodes of generalized flushing but had no diarrhea, systemic symptoms, or developmental delay. Physical examination was otherwise unremarkable, and laboratory tests, including complete blood count, liver and renal function, and serum tryptase, were within normal limits. A skin biopsy revealed a dense dermal infiltrate of monomorphic mast cells with round to oval nuclei and abundant eosinophilic granules. Toluidine blue staining confirmed metachromatic granules, consistent with a solitary mastocytoma. Mastocytomas typically present as solitary reddish-brown nodules or plaques, often exhibiting Darier's sign, although this may be absent or unsafe to elicit. Our case demonstrates an uncommon bullous transformation preceding the classic nodular appearance. Episodic flushing suggested mast cell mediator release; however, the absence of systemic involvement and normal tryptase levels confirmed a purely cutaneous form. Histological examination with toluidine blue or immunohistochemical stains (CD117, tryptase) remains the diagnostic gold standard. Management is usually conservative, with antihistamines for symptomatic relief. Most lesions regress spontaneously by two to three years of age, and prognosis is excellent in the absence of systemic disease. This case highlights an under-recognized presentation of infantile mastocytoma, emphasizing the importance of clinical awareness and histopathological confirmation. Accurate diagnosis allows appropriate reassurance for caregivers, while long-term monitoring ensures early detection of any rare progression to systemic mastocytosis.