Rhabdomyomatous Mesenchymal Hamartoma: Expanding the Histological Spectrum of This Rare Congenital Entity

Introduction

Rhabdomyomatous mesenchymal hamartoma (RMH) is a rare congenital lesion characterized by the presence of mature skeletal muscle fibers interspersed with adipose tissue, adnexal elements, and vasculature within the dermis and subcutis. ¹ The entity was initially reported by Hendrick et al ² and later designated as RMH by Mills in 1989. ³

RMH typically presents as a solitary papule or polypoid lesion in midline cutaneous locations, often on the face, neck, or upper trunk of infants and young children.^1,4 Clinically, the lesion may be mistaken for skin tags, dermoid cysts, or benign fibrous growths due to its superficial appearance and location.^4,5 Diagnosis is typically established via histologic examination, which reveals disorganized skeletal muscle fibers within a stroma containing adipocytes and dermal adnexal structures.^1,3

Most reported cases conform to a consistent histologic pattern.^1,6 However, rare morphologic variation may occur. We report 4 cases of histologically confirmed RMH, including 1 with a distinct fibrocartilaginous component not previously described in the literature. These findings contribute to the understanding of RMH’s morphologic spectrum and support the continued refinement of its diagnostic criteria.

Design/Methods

A retrospective search was conducted using our institutional laboratory information system to identify cases diagnosed as rhabdomyomatous mesenchymal hamartoma between January 2011 and August 2024. Four cases were retrieved. Available clinical data, imaging, gross findings, and histologic sections were reviewed for each case. All lesions were excised and evaluated by standard histologic techniques. No ancillary molecular or immunohistochemical studies were performed, as diagnoses were rendered based on morphology.

Results

A total of 4 RMH cases were identified, each with variable anatomical site, clinical impression, and histopathologic findings (Table 1, Figure 1).

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Table 1.

Clinical Features of Rhabdomyomatous Mesenchymal Hamartoma (RMH).

Category	Case 1	Case 2	Case 3	Case 4
Age	16 months	13 months	16 months	5 months
Assigned sex at birth	Male	Female	Female	Male
Lesion site	Upper midline chest	Central chin/neck	Midline neck	Nose, midline to right nostril
Clinical impression	Skin tag	Skin tag/probable developmental remnant, differential accessory nipple, accessory tragus, dermoid cyst, bronchogenic cyst	Thyroglossal duct cyst	Benign skin tumor clinically favor fibroma
Imaging impression	N/A	N/A	US Neck: Well circumscribed skin tag/cleft, 0.6 cm × 0.2 cm × 0.2 cm	N/A
Surgical procedure	Simple excision (Plastic surgery)	Simple excision (Plastic surgery)	Sistrunk procedure (ENT Surgery)	Simple excision (Plastic surgery)
Gross findings	0.3 cm × 0.2 cm × 0.2 cm pink-tan elevated central lesion	0.4 cm × 0.4 cm raised tan-brown “nipple-like” lesion	0.4 cm × 0.4 cm × 0.3 cm exophytic skin lesion	0.6 cm × 0.5 cm × 0.4 cm tan-white skin lesion
Histology findings	Polypoid portion of skin centered by bundles of mature skeletal muscle	Dermal skeletal muscle bundles with periductal chronic inflammation	Polypoid lesion with vellus hairs on the surface, skeletal muscle, and a small nodule of fibrocartilage	Polypoid fragment of skin with haphazard benign dermal proliferation of mature-appearing skeletal muscle bundles, vascular channels, and skin adnexal structures
Mesenchymal hamartomatous components	Skeletal muscle Vascular structures Adnexal structures	Skeletal muscle Vascular structures Adnexal structures Adipose tissue Nerve tissue	Skeletal muscle Fibrocartilage Adipose tissue	Skeletal muscle Vascular structures Adnexal structures
Final diagnosis	Rhabdomyomatous mesenchymal hamartoma (Congenital midline hamartoma)	Rhabdomyomatous mesenchymal hamartoma (Original Diagnosis: Accessory Nipple)	Benign hair-bearing polyp with skeletal muscle and cartilage, Rhabdomyomatous mesenchymal hamartoma/congenital midline hamartoma	Rhabdomyomatous mesenchymal hamartoma

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Figure 1.

Histologic features of rhabdomyomatous mesenchymal hamartoma (RMH). Representative sections from the 4 cases demonstrate haphazardly arranged bundles of mature skeletal muscle fibers within the dermis, admixed with adnexal structures and vasculature (A-B, H&E original magnification ×20). In case 3 (C, H&E original magnification ×4), a discrete, well-demarcated nodule of fibrocartilage was present within the lesion (D, H&E, original magnification ×20). Case 4 (E, H&E original magnification ×4), highlights the polypoid appearance on low power, with skeletal muscle dissecting the dermis (F, H&E, original magnification ×20).

Discussion

Rhabdomyomatous mesenchymal hamartoma (RMH) is a benign cutaneous lesion composed of mature skeletal muscle fibers arranged haphazardly within a mesenchymal stroma.^1,4 It typically arises along midline sites during early infancy and may present as a papule, nodule, or skin tag–like lesion.^1,3 Its superficial location and subtle clinical presentation often lead to misdiagnosis. Common clinical differentials include acrochordons, accessory nipples, and midline cystic lesions such as dermoid cysts or thyroglossal duct remnants.^4,5

Histologically, RMH is characterized by mature skeletal muscle bundles situated within the dermis and subcutis, often accompanied by adipocytes, small-caliber vessels, and adnexal structures.^1,3 The arrangement is disorganized, and there is typically no encapsulation or significant cytologic atypia. Although the overall histologic pattern is well described in the literature,^1,6 our third case demonstrated a discrete fibrocartilaginous nodule embedded within the lesion—an element not previously reported. This finding broadens the morphologic spectrum of RMH but its clinical significance remains uncertain. The presence of fibrocartilage may suggest aberrant differentiation within mesodermal tissues and raises questions about the developmental plasticity of RMH. Further studies are needed to determine whether this component has diagnostic or prognostic implications.

Despite the presence of multiple tissue types, RMH should not be interpreted as a teratoma. Teratomas are derived from totipotent germ cells and characteristically include components from all 3 germ layers. ⁷ In contrast, RMH arises within cutaneous mesodermal tissues and lacks endodermal or neuroectodermal derivatives. Its development is hypothesized to result from aberrant migration or differentiation of mesodermal stem cells during embryogenesis. ¹² The presence of adnexal elements in RMH likely reflects dermal interaction with epithelial structures during embryogenesis rather than a true multiphenotypic origin. ⁷

RMH should also be distinguished from other muscle-containing lesions in children, including fetal rhabdomyoma, which exhibits more cellularity and immature muscle fibers; benign triton tumor, which includes neural elements; and myolipoma, which tends to be deep-seated and lacks adnexal components. ⁶ Smooth muscle hamartoma may enter the differential when fascicular bundles are prominent, but these lesions contain smooth rather than striated muscle. ⁸ In ambiguous cases, immunohistochemical stains such as desmin and myoglobin can confirm skeletal muscle differentiation. Desmin is highly sensitive but not entirely specific, while myoglobin offers greater specificity for skeletal muscle. ¹⁰ Immunohistochemical studies can be helpful in ambiguous cases but were not required in our series.

Terminologic overlap between RMH and so-called “congenital midline hamartomas” has occasionally led to confusion. While some reports use these labels interchangeably, RMH refers specifically to lesions containing mature skeletal muscle fibers arranged in a non-neoplastic, hamartomatous pattern.^3,8 Greater diagnostic consistency could be achieved by maintaining this specific nomenclature.

Although RMH is usually an isolated finding, rare syndromic associations have been reported. Cardis et al ⁹ described RMH in the context of Delleman syndrome, amniotic band sequence, and other congenital anomalies. ⁹ RMH is a benign lesion with no reported cases of malignant transformation. ¹¹ None of our patients exhibited syndromic features or extracutaneous anomalies, and no recurrence was observed in available follow-up.

Conclusions

Rhabdomyomatous mesenchymal hamartoma (RMH) remains an underrecognized congenital lesion with distinctive yet variable histologic features. Our series contributes to the expanding clinicopathologic spectrum of RMH by reporting a rare case with a fibrocartilaginous component not previously described in the literature. These findings underscore the importance of considering RMH in the differential diagnosis of midline cutaneous lesions in infants and young children, especially when clinical impressions favor benign skin tags or developmental remnants. Accurate histologic recognition is essential to prevent misdiagnosis and avoid unnecessary surgical or oncologic interventions. Clinicians and pathologists should be aware of its variable presentation and benign nature. Additional studies and larger case series may further elucidate the pathogenesis and the full histologic spectrum of this rare entity.