Rhabdomyomatous Mesenchymal Hamartoma: Report of 4 Cases With Histochemical and Immunohistochemical Findings and Emphasis on Potential Pitfalls.

Rhabdomyomatous mesenchymal hamartoma (RMH) typically presents as a congenital midline head and neck cutaneous polyp in infants. Perianal and mucocutaneous lesions have been reported, and recently, acquired adult-onset variants have been proposed. This makes the true prevalence, etiopathogenesis, and clinicopathologic distribution and classification of RMHs in children compared with those in adults uncertain. We performed a retrospective review to highlight the salient histopathologic, histochemical, and immunohistochemical features in RMHs and to emphasize their specific clinicopathologic criteria to avoid diagnostic pitfalls. We found 4 (0.3%) infants [2 female infants and 2 male infants, average age: 4 months] with mental, nasal, lingual, and perianal midline RMHs (average size: 1.0 cm) of 1303 patients with cutaneous polypoid lesions. Three were isolated, and 1 was associated with Goldenhar syndrome. The cutaneous polyps demonstrated intermixed skeletal muscle, adipose, and fibrocollagenous core stroma that extended into the dermis and around the dermal appendages. The lingual lesion demonstrated skeletal muscle and fibrocollagenous stroma with prominent nerve bundles and little adipose tissue. All showed interstitial loose mesenchyme. Masson trichome demarcated the triphasic stromal components. Alcian blue demonstrated the loose myxoid mesenchyme. Elastic van Gieson did not show elastic fibers. Desmin demonstrated the skeletal muscle bundles, S100 highlighted the adipose tissue lobules and the nerve bundles, and CD34 displayed the mesenchymal stroma. Ki67 showed a low proliferation index in the loose mesenchyme. Smooth muscle actin did not reveal smooth muscle bundles, but with CD31, they highlighted the thick blood vessels. CD117 revealed prominent mast cells. From our retrospective review series, 4 cases that originally diagnosed as RMHs were excluded. Likewise, we found some examples of the reported cases in the English literature that might have been mistaken for RMHs. This is because they did not fulfill the diagnostic clinicopathologic criteria. RMH constitutes a rare entity with specific clinicopathologic features. Most lesions are isolated. Some are associated with congenital anomalies and syndromes. Strict clinicopathologic diagnostic criteria should be applied to avoid mislabeling look-alike lesions for RMHs.