[Smooth muscle hamartoma associated with acquired Blaschkoid nevus spilus].

Smooth muscle hamartoma (SMH), described by Stokes in 1923, is a proliferation of smooth muscle bundles. It can be single or multiple and may be congenital and acquired.1 The most frequent presentation is the localized congenital form, which is characterized by a plaque with a variable degree of hyperpigmentation, hypertrichosis, and induration. It is usually less than 10 cm in diameter and appears on the trunk or proximal areas of the limbs. Rubbing the affected area can lead to pseudo-Darier sign, which consists of elevation, induration, or transient piloerection.2-4 We report the case of a patient who recently attended our hospital with localized SMH within an acquired blaschkoid nevus spilus. The patient was a 32-year-old man with no past history of interest who was seen for an asymptomatic pigmented lesion that appeared on the right shoulder when he was 6 years old. The patient and his mother described the gradual onset of darker spots within the lesion and, aged 13, the appearance of a hairy and slightly pruritic area. Physical examination showed a homogenous, light brown, macular lesion with well-circumscribed irregular margins. It had a unilateral, segmental distribution that followed the lines of Blaschko in a wide band across the upper area of the back and right shoulder and was sharply interrupted at the midline. Multiple blackish or dark brown spots were scattered over the lesion, most of which were elevated and less than 1 cm in diameter (Figure 1A). Near to the right external border there was a light brown plaque, 2 cm in diameter, firmer to the touch, and with long thick dark hair (Figure 1B); rubbing the lesion elicited transient piloerection. Histopathology of the biopsy taken from the hairy area showed lentiginous melanocytic hyperplasia in the epidermis and disorganized, irregularly shaped smooth muscle fascicles in the dermis that were not associated with the pilosebaceous unit. These fascicles were surrounded by a clear space that separated them from the surrounding collagen (Figure 2). Immunohistochemical staining for muscle-specific actin highlighted the irregular distribution and organization of the smooth muscle fascicles in the dermis (Figure 3). A diagnosis was made of blaschkoid nevus spilus associated with SMH. Smooth muscle hamartoma is sometimes associated with other skin conditions. Becker nevus is an abnormality that usually appears at the onset of puberty, presenting as a hyperpigmented area that can develop hypertrichosis. Histopathology shows a degree of acanthosis, elongated rete ridges, and hyperpigmentation of the basal layer. It is not unusual to observe smooth muscle hyperplasia in the dermis. For this reason, some authors suggest that SMH and Becker nevus represent opposite ends of a spectrum