Congenital Smooth Muscle Hamartoma at Unusual Location

Sir,

Congenital smooth muscle hamartoma (CSMH) is a benign cutaneous lesion, defined as a disorganized proliferation of mature smooth muscle fibers of arrector pili muscle. Typically, it is located on the lumbosacral area.[1] CSMH may resemble other skin conditions with potential malignancy or different medical approach, so it is important to make a correct differential diagnosis. Herein, we report an atypical location for CSMH.

A healthy 2-year-old boy was referred for a congenital and asymptomatic cutaneous lesion. At physical examination, a slightly elevated and indurated, hyperpigmented plaque was observed on the anterior part of his right ankle with extension to the dorsum of his foot. Prominent vellus hairs were observed on the ankle area [Figure 1]. A pseudo-Darier sign was observed after rubbing over the lesion. The rest of the examination was unremarkable. No family member had a similar lesion. A skin biopsy showed a disorganized proliferation of dermal smooth muscle fibers in the perifollicular and interfollicular area [Figure 2a]. Masson's trichrome stain showed increased smooth muscles bundles between collagen fibers in the dermis [Figure 2b]. The diagnosis of CSMH was made. Due to the benign behavior, an expectant management was decided.

Figure 1.

Indurated hyperpigmented plaque located on the anterior part of his right ankle with extension to the dorsum of his foot. Prominent vellus hairs observed on the ankle area

Figure 2.

(a) Proliferation of smooth muscle bundles oriented in different directions in the dermis (perifollicular and interfollicular area) (H and E, ×200). (b) Masson's trichrome stain showed increased smooth muscles bundles between collagen fibers in the dermis (×400)

The clinical presentations of CSMH include the classic, papulo-follicular and mixed form.[2,3] The classic type presents a localized skin-colored or hyperpigmented plaque with prominent vellus hairs. It can vary in size, up to 10 cm in diameter.[2] Around 50% of these lesions can have a positive pseudo-Darier's sign, which consists of a transient erythema, induration or piloerection when the lesion is rubbed.[2] The papulofollicular form presents multiple skin-colored papules that can coalesce to form irregularly shaped plaques. CSMH is not associated with other systemic manifestations and malignant transformation has not been reported.[4,5] Usually, hyperpigmentation and vellus hairs diminish over time.

Histologically, CSMH has numerous smooth muscle bundles oriented in different directions, which sometimes can reach the subcutaneous tissue or hair follicles.[3,5] A hyperpigmented basal layer, acanthosis and hyperkeratosis can be found in the epidermis.[2,5] Histochemistry (i.e., Masson's trichrome) and immunohistochemistry (i.e., smooth muscle actin) are useful tools for revealing the nature of cell proliferation.

A few reports of CSMH located on atypical areas such as scalp and eyelid have been published.[5] Interestingly, our case is the first report of CSMH located on the foot.

The differential diagnosis of CSMH includes: Congenital melanocytic nevus, Becker's nevus, solitary mastocytoma, piloleiomyoma, café-au-lait spots, and nevus pilosus.[3] Distinguishing CSMH from congenital melanocytic nevus might be difficult by using only clinical criteria. In fact, this is made by histopathological examination. The differentiation of these two conditions is very important because congenital melanocytic nevi present a risk of malignant transformation. piloleiomyoma is another important entity in the histopathological differential diagnosis of CSMH. It closely resembles CSMH. However, piloleomyoma forms large aggregates of poorly defined, interlacing smooth muscle bundles intermingled with collagen fibers. In contrast, CSMH forms more distinct and well-defined smooth muscle bundles with a characteristic clear space that surrounds each individual bundle, separating it from collagen fibers.[2,3,4] On the other hand, Becker's nevus resembles CSMH both clinically and histopathologically. Unlike CSMH, the hyperpigmentation and hypertrichosis of Becker's nevus increases over time and it is usually an acquired lesion that is diagnosed during the adolescent period.

Besides the lumbosacral area, CSMH could be found in any other site, so it is important to include this diagnosis in a hairy plaque on the skin. Therefore, it is worth mentioning that the recognition and differentiation of CSMH from other pathologies can prevent unnecessary excision of this benign condition.