Background Becker’s nevus (BN) presents as a hairy patch or plaque with or without proliferation of the dermal smooth muscles. BN has been described as acquired as found in a similar entity, congenital smooth muscle hamartoma (CSMH). This study was aimed at evaluating the clinicopathological aspects of BN in Korean cases in differential diagnosis with CSMH. Methods We performed a retrospective study of 103 patients histopathologically diagnosed as having BN or CSMH. The cases included 40 cases diagnosed with BN or CSMH before the age of 10 years who had clinical monitoring and a second skin biopsy after puberty to determine the disease course. Results Among cases of children to adolescents (<18 years), we observed a slight male predominance. Among children aged <14 years, sex ratio converged at 1:1. Early-onset BN showed a female predominance and hyperpigmented skin lesions. All BN cases showed hyperpigmentation, and face and neck involvement tended to make severe cosmetic concerns. In contrast, hypertrichosis was more frequent in CSMH. Either skin-colored lesion or pseudo-Darier’s sign was not seen in early-onset BN. BN showed less dermal smooth muscle than CSMH. Conclusions Androgens themselves do not seem to be related to the development of BN but play only an aggravating role especially in male patients. Considering high occurrence in exposed areas, BN may distress patients severely. As early laser treatment may be helpful in some patients with BN, early-onset BN in comparison to CSMH should be diagnosed appropriately.
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