Superior Oblique Muscle Extramedullary Plasmacytoma in a Patient with Multiple Myeloma and a Review of Literature.

Introduction: Multiple myeloma (MM), a plasma cell malignancy, is a systemic disease affecting various body organs. Plasmacytoma of bone and extramedullary disease (EMD) are presentations of MM. EMD is usually the sign of a more aggressive form of the disease. Herein, we report a patient with refractory MM presenting with extramedullary plasmacytoma in the superior oblique (SO) muscle.

Case presentation: A 51-year-old female presented complaining of gradual protrusion of the left eye and ocular pain from 20 days prior. She received bone marrow transplantation 1 year prior and was on a chemotherapy regimen for MM for the past 1 year. Ocular examination revealed proptosis of the left eye and mild limitations of adduction and elevation. Orbital magnetic resonance imaging demonstrated remarkable enlargement of the left SO muscle with focal contrast enhancement. The patient underwent a biopsy and mass debulking. The histopathologic exam revealed fibromuscular tissue containing a neoplasm composed of sheets of plasmacytoid cells in a varying degree of differentiation with intervening scantly vascularized stromal components. The plasmacytoid cells were diffusely positive for a cluster of differentiation 138 (CD138), leading to a diagnosis of EMD involving the EOM and soft tissue of the orbit. The patient underwent palliative radiotherapy and a systemic workup. The PET-CT scan revealed involvement of the pelvic bone and left calf. Accordingly, the chemotherapy regimen was upgraded to reflect the aggressive nature of the disease. In the last follow-up, there was no sign of tumor reactivation in the orbital soft tissues. Unfortunately, the patient succumbed to her illness 7 months following her most recent presentation.

Conclusion: Early recognition of disease recurrence is lifesaving in MM patients; ophthalmic manifestations should be seriously considered as a sign of MM activity.