Neuroendocrine tumor (carcinoid) metastatic to orbital extraocular muscle: Case report and literature review

Purpose: To report a patient who showed neuroendocrine tumor (carcinoid) metastasis to the medial rectus muscle and to review patients' characteristics of carcinoid metastases to the extraocular muscles. Case: A 72-year-old woman, who initially presented with spindle-shaped enlargement of the right medial rectus muscle, was followed for 3 years with a diagnosis of orbital myositis. Initial biopsy of the medial rectus muscle showed inflammation only. She showed remission and exacerbation of right proptosis and eyelid swelling, which responded to oral and intravenous steroids. On the occasion of abdominal computed tomography for ischemic colitis, a large retroperitoneal mass was detected and diagnosed as well-differentiated neuroendocrine tumor. The gradual increase of the medial rectus muscle with optic nerve compression, and hence, visual reduction, prompted a second excisional biopsy of the medial rectus mass, which proved to be neuroendocrine tumor metastasis. Whole body 2-[ ¹⁸F]fluoro-2-deoxy-D-glucose (FDG) positron emission tomography fused with computed tomography revealed abnormal uptake only in the right orbit (maximum standardized uptake value: SUVmax3.83), and the patient underwent radiation to the right orbit with the subsidence of the residual mass. Results: The literature review found 15 patients, including this patient, with neuroendocrine tumor metastases to the extraocular muscles. Frequent symptoms and signs were diplopia, proptosis, and ocular motility limitation. Conclusions: Neuroendocrine tumor appears to have propensity to extraocular muscle metastases and its slow growth might pose difficulty in differential diagnoses of orbital myositis.