Flesh-Colored Papule in the Nose of a Child

The Diagnosis: Striated Muscle Hamartoma

Histopathologic evaluation revealed a dome-shaped papule with a center composed of mature striated muscle bundles, vellus hairs, sebaceous lobules, and nerve twigs (Figure) consistent with a diagnosis of striated muscle hamartoma (SMH).

A, Scanning magnification revealed a dome-shaped papule with dense bundles of skeletal muscle in its core and pilosebaceous units (H&E, original magnification ×20). B, Bundles of striated muscle extended through the reticular dermis into the papillary dermis and surrounded the adnexal units (H&E, original magnification ×100). C, Skeletal muscle with peripheral nuclei and striations (H&E, original magnification ×400).

Striated muscle hamartoma was first described in 1986 by Hendrick et al1 with 2 cases in neonates. Biopsies of the lesions taken from the upper lip and sternum showed a characteristic histology consisting of dermal striated muscle fibers and nerve bundles in the central core of the papules associated with a marked number of adnexa. In 1989, the diagnosis of rhabdomyomatous mesenchymal hamartoma was described, which showed similar findings.² Cases reported since these entities were discovered have used the terms striated muscle hamartoma and rhabdomyomatous mesenchymal hamartoma interchangeably.³

Most commonly found on the head and neck, SMH has now been observed in diverse locations including the sternum, hallux, vagina, and oral cavity.^1-15 Many reported cases describe lesions around or in the nose.^4,7,8 Multiple congenital anomalies have been described alongside SMH and may be associated with this entity including amniotic bands, cleft lip and palate, coloboma, and Delleman syndrome.^1,3,4 Almost all of the lesions present as a sessile or pedunculated papule, polyp, nodule, or plaque measuring from 0.3 cm up to 4.9 cm and typically are present since birth.^3,5,15 However, there are a few cases of lesions presenting in adults with no prior history.^5,6,15

Microscopically, SMH is defined by a dermal lesion with a core comprised of mature skeletal muscle admixed with adipose tissue, adnexa, nerve bundles, and fibrovascular tissue.¹ There are other entities that should be considered before making the diagnosis of SMH. Other hamartomas such as accessory tragus, connective tissue nevus, fibrous hamartoma of infancy, and nevus lipomatosis may present similarly; however, these lesions classically lack skeletal muscle. Benign triton tumors, or neuromuscular hamartomas, are rare lesions composed of skeletal muscle and abundant, intimately associated neural tissue. Neuromuscular hamartomas frequently involve large nerves.¹⁶ Rhabdomyomas also should be considered. Adult rhabdomyomas are composed of eosinophilic polygonal cells with granular cytoplasm and occasional cross-striations. Fetal rhabdomyomas have multiple histologic types and are defined by a variable myxoid stroma, eosinophilic spindled cells, and rhabdomyocytes in various stages of maturity. Genital rhabdomyomas histopathologically appear similar to fetal rhabdomyomas but are confined to the genital region. The skeletal muscle present in rhabdomyomas typically is less differentiated.¹⁷ TMature skeletal bundles should be a dominant component of the lesion before diagnosing SMH.

Typically presenting as congenital lesions in the head and neck region, papules with a dermal core of mature skeletal muscle associated with adnexa and nerve twigs should prompt consideration of a diagnosis of SMH or rhabdomyomatous mesenchymal hamartoma. These lesions are benign and usually are cured with complete excision.