Orbital Fat Involvement by Metastatic Multiple Myeloma Mimicking Involutional Lower Eyelid Fat Herniation: A Case Report

Purpose To report an unusual presentation of orbital extramedullary plasmacytoma in a patient with multiple myeloma, clinically mimicking involutional lower eyelid fat herniation. Observations A 57-year-old man with a history of multiple myeloma in remission presented with a three-month history of progressive fullness of the right lower eyelid. Clinical examination suggested minimal right-sided hyperglobus and asymmetric lower eyelid fat herniation without signs of inflammation. Orbital computerized tomography revealed a hyperdense anterior inferior orbital mass distinct from surrounding fat. Excisional biopsy demonstrated a plasmacytoma confirmed by immunohistochemistry showing CD138 and kappa light chain positivity. The lesion represented metastatic involvement of multiple myeloma. Conclusions and Importance Orbital metastases of multiple myeloma are rare. This case highlights anterior orbital involvement presenting as apparent involutional lower eyelid fat herniation, a potential diagnostic pitfall. Awareness of this presentation is important when evaluating new eyelid or orbital findings in patients with a history of systemic malignancy.