Lateral rectus muscle biopsy as diagnosis of unknown metastatic breast cancer

The case concerns a 58 year-old female with no medical history of interest who consulted due to binocular diplopia of one week onset. It was associated with costal pain, dyspnoea, intense asthenia and weight loss of 2 months onset. In the blood analysis in the Emergency Department it showed hypercalcaemia, renal failure, and bicytopenia. The chest x-ray showed lytic bone lesions that initially lead to multiple myeloma with extra-osseous involvement. In addition to the corresponding study, in case of horizontal diplopia, a cranial CT scan without contrast was performed where an adjacent lesion to the lateral wall of the left orbit is observed. This was of soft tissue density, and included the external rectus muscle that exerts a mass effect on the optic nerve by displacing it medially. Many lytic bone diffuse lesions with salt and pepper pattern were found in the calotte. A rectus lateral muscle and bone biopsy of the sacral wing was performed, resulting in metastasis of carcinoma compatible with mammary origin. An atypical case is presented of horizontal diplopia in the context of a patient with a severe constitutional picture with no established diagnosis, in which the biopsy of the lateral rectum was key to the confirmation diagnosis.