Although congenital smooth muscle hamartoma is present at birth, the diagnosis may get delayed and only made in early infancy. It is mostly asymptomatic. In some patients, history may include itching, pain, or numbness. Besides the lumbosacral area, the congenital smooth cell hamartoma can also occur on the trunk and proximal extremities. Many other atypical locations were reported, such as the scalp, the eyelids, the nipple, the scrotum, and on the foot. Variable in size, it can reach up to 10 centimeters.[7][8][9]

There are three clinical aspects of congenital smooth muscle hamartoma described: classic, papulo-follicular, and mixed.

The classic form presents as a well-limited skin-colored or pigmented plaque covered with prominent overlying hypertrichosis. The papulo-follicular form is less common. It presents as large patches with multiple perifollicular skin-colored papules. These papules may coalesce to form irregularly-shaped plaques. Unusual clinical presentations were reported, such as linear atrophic plaques, micro-papular plaques [1]. It can also appear as an erythematous macula with geographic contours and slight depression of the underlying subcutaneous tissue[10]]. Over time, the congenital smooth muscle hamartoma grows up in size, proportional to the child's growth. Hyperpigmentation and hypertrichosis contrarily tend to diminish. Congenital smooth cell hamartoma usually presents as a solitary lesion. There are rare reports of multiple lesions.[3]

The characteristic clinical hallmark of congenital smooth muscle hamartoma is the positive pseudo-Darier sign (rubbing of the skin results in transient piloerection associated with erythema and induration of the skin). It requires differentiation from authentic Darier sign characteristic of cutaneous mastocytosis. However, the pseudo-Darier sign only presents in 50% of congenital smooth muscle hamartomas. Occasionally, vermiform movements at the surface of the lesion occur after the rubbing of the skin. Both signs result from the contraction of the smooth muscle cells of the hamartoma.[11]

No evaluation is necessary for the diagnosis of the congenital smooth muscle hamartoma.

Dermoscopy is a non-invasive tool that has been used by some authors to assess the hair density of the congenital smooth cell hamartoma. Reports exist that hair density is proportional to the number of smooth muscle bundles of the hamartoma.[12]

The congenital smooth cell hamartoma does not display any malignant transformation risk. Therefore, no treatment is necessary. However, some measurements may serve for cosmetic reasons.[1]

Surgical excision is a possible first-line treatment, performed in different locations such as the scrotum with excellent results. In certain areas such as the face or large hamartomas, reconstruction is challenging and complicated. Therefore, other therapeutic options merit consideration.[5]

Laser therapy may also be a consideration. Hair removal laser can be indicated to treat hypertrichosis. In congenital smooth muscle hamartomas that present as an erythematous plaque, pulsed dye laser showed interesting results in decreasing redness. This superficial laser acts by destroying blood vessels and does not have direct effects in the hamartoma.[10]

The diagnosis of congenital smooth cell hamartoma is clinical. However, histopathological examination is sometimes required to exclude resembling entities.

Becker hamartoma displays overlapping clinical and histological features with congenital smooth muscle hamartoma. It is an acquired lesion usually diagnosed at puberty. It presents as a well-defined infiltrative and a pigmented plaque with hypertrichosis, Histopathology shows the same epidermal features. Occasionally, a proliferation of dermal smooth muscles may present. Unlike congenital smooth cell hamartoma, pigmentation and hypertrichosis of Becker hamartoma tend to increase over time. Considering these similarities, there are suggestions that these two entities belong to the same continuous spectrum.[5]

Other differential diagnoses of congenital muscle cell hamartoma include Congenital melanocytic nevus, piloleiomyoma, café-au-lait spots, and nevus pilosus.[13] In these cases, histopathological examination is of significant help and brings diagnosis certainty. Distinguishing congenital smooth cell hamartoma from congenital melanocytic nevus is crucial because of the risk of malignant transformation of the latter.[6]

Histologically, piloleiomyoma closely resembles congenital smooth muscle hamartoma. When smooth muscle bundles are badly-defined and intermingle with collagen fibers, the diagnosis of piloleiomyoma should be a consideration.

Finally, it is sometimes hard to differentiate an acquired smooth muscle hamartoma from a congenital one that was underdiagnosed at birth. Few clinical features should favor the acquired etiology. They mainly include discrete hyperpigmentation, decreased density of villous hair, and a negative pseudo-Darier sign.[14]

There are no specific clinical criteria for congenital smooth muscle hamartoma. The diagnosis merits consideration in cases of any hairy plaque on the skin. It may resemble other skin conditions, especially Becker’s hamartoma. Because the condition presents at birth, nurses, pediatricians, and obstetricians should be familiar with the disorder. These healthcare professionals should be able to differentiate a congenital smooth muscle hamartoma from a congenital melanocytic nevus since it displays potential malignancy and requires different medical management.