Congenital smooth muscle hamartoma. A report of six cases and a review of the literature.

Congenital smooth muscle hamartoma (CSMH) represents a proliferation of randomly oriented dermal smooth-muscle bundles. Six patients with CSMH were observed, the largest series to date, and the literature was reviewed. Congenital smooth muscle hamartoma has presented as congenital patches or slightly indurated plaques with prominent overlying hair (88% of cases), or rarely as patches with perifollicular papules without prominent hair (12% of cases). Most lesions (61% of cases) have been somewhat hyperpigmented, but 39% of cases have been flesh colored. Congenital smooth muscle hamartoma has occurred on the torso and proximal extremities, except for one case on the eyebrow and eyelid (present study). A positive pseudo-Darier's sign (temporary induration or piloerection after rubbing) helped to differentiate CSMH from congenital hairy nevo-cellular nevus. Congenital smooth muscle hamartoma is a distinct entity that is at one end of a spectrum that includes Becker's nevus, and should be considered in the differential diagnosis of any congenital hairy lesion.