Skull Base Metastasis As the Initial Manifestation of Occult Clear Cell Renal Cell Carcinoma: An Uncommon Cause of Acute Diplopia.

Skull base metastasis is an uncommon initial presentation of renal cell carcinoma (RCC). Diplopia caused by cranial nerve involvement may be the first and only sign of an otherwise silent malignancy. A 54-year-old woman presented with sudden horizontal diplopia that resolved with monocular occlusion. Visual acuity and optic nerve examination were normal, yet oculomotor testing showed a right-eye adduction deficit with diplopia on left gaze. Cranial computed tomography (CT) revealed an extensive osteolytic lesion involving multiple structures of the skull base, including the left lateral mass of C1, occipital condyle, foramen magnum margin, jugular foramen, clivus, petrous apex, and sphenoid body. Systemic imaging identified a large heterogeneous right renal mass and widespread osseous metastases. Biopsy confirmed stage IV clear cell RCC. High-dose corticosteroids led to rapid resolution of diplopia, and the patient was referred for systemic oncologic therapy. Although acute diplopia warrants evaluation for intracranial or skull base pathology, it is rarely the first clinical manifestation of metastatic RCC. Extensive skull base osteolysis should prompt urgent investigation for an underlying malignancy.