A Case of Multiple Acquired Smooth Muscle Hamartoma With Depressed Appearance

Smooth muscle hamartoma (SMH) is a benign cutaneous tumor that typically presents at birth as a solitary lesion in the lumbosacral region. Acquired cases with multiple lesions and subtle depression are exceedingly rare. We report a 40-year-old male with a seven-year history of multiple pigmented macules with slight depression on the trunk and extremities. Histopathology of a back lesion revealed irregularly distributed bundles of pale eosinophilic smooth muscle fibers in the upper dermis, positive for α-smooth muscle actin (α-SMA). Basal hyperpigmentation was observed, without swelling of collagen fibers or atrophic eccrine glands. Differential diagnoses such as Becker's nevus and atrophoderma of Pasini and Pierini were excluded based on clinical and histological findings. Taken together, the patient was diagnosed with SMH. Although excision may be considered for solitary lesions, it was not performed in our patient due to multiple lesions. No progression was observed over two years of follow-up. This case illustrates a unique acquired SMH with multiple lesions and depressed appearance, emphasizing the importance of recognizing this entity in the differential diagnosis of multiple pigmented macules in adults.