Persistent Erythrocytosis in a Dog With a Spinal Sclerosing Paraganglioma

A 7-year-old, male, castrated, mixed-breed dog presented with right pelvic limb lameness and pain on posturing to defecate that began 1.5 years prior to presentation. Radiographs taken 8 months prior to presentation showed a small lytic lesion of the L6 vertebral body, and CBCs showed a persistently increased hematocrit (HCT) for at least 2 years. Lumbar magnetic resonance imaging (MRI) revealed a right-sided L5-L7 intradural-extramedullary mass. Cytologic evaluation of a fine-needle aspirate of the mass was consistent with a diagnosis of malignant neoplasia, with a primary differential of paraganglioma. Additional testing revealed an increased normetanephrine-to-creatinine ratio and low serum erythropoietin (EPO). Histologic evaluation, immunohistochemistry, and electron microscopy were consistent with a diagnosis of spinal sclerosing paraganglioma. No mutations were found on DNA sequencing that are associated with polycythemia vera in humans, a dog with primary erythrocytosis, or in humans with polycythemia-paraganglioma syndrome (PPS). The patient was treated with palliative radiation, after which the neurologic deficits markedly improved and the erythrocytosis resolved. The patient continued to do well with an HCT within the reference interval. Two years later, the patient's neurologic signs recurred, and MRI revealed the paraganglioma had progressed in size. The patient continued to have an HCT within the reference interval and underwent repeat palliative radiation with no complications. The concurrent erythrocytosis and paraganglioma in this patient resemble reports of PPS in humans.