Proptosis Caused by Rhabdomyomatous Mesenchymal Hamartomata Occurring in the Orbit

Two infants were referred for progressive orbital proptosis. MRI in both cases demonstrated a homogenous mass in the orbit adherent to and isointense with a rectus muscle. Histopathology in both cases demonstrated a bland proliferation of spindle cells with entrapped skeletal muscle. Immunochemistry demonstrated that the abnormal tissue was of skeletal muscle origin, consistent with rhabdomyomatous mesenchymal hamartoma (RMH). Observation was elected due to the reported benign nature of RMH. In contrast to RMH of the cutaneous tissues that typically follows a benign course, RMH of the orbit may present with rapid growth.