Cold-induced cutaneous smooth muscle hamartoma

Cutaneous smooth muscle hamartomas (SMHs) are benign proliferations of smooth muscle tissue, typically congenital and characterized by hyperpigmentation, hypertrichosis, and a positive pseudo- Darier's sign. We report an unusual case of an early-onset acquired SMH in a 49-year-old woman. The lesion first appeared during adolescence and remained asymptomatic and clinically inapparent except upon exposure to cold. Cold stimulation induced a transient, linear, wrinkled, hyperpigmented papular lesion on the left thigh, which resolved spontaneously within seconds after rewarming. Histological examination revealed dermal fibrosis and well-organized lobules of smooth muscle cells, confirming the diagnosis of SMH without requiring immunohistochemical analysis. This case highlights a unique and transient presentation of SMH revealed exclusively by cold exposure and reversible with rewarming. It broadens the clinical spectrum of SMHs and the importance of external stimuli in their diagnosis.