Gwendolyn J. Levine
Introduction
Tumors originating from the vertebral bodies or epidural space are relatively uncommon in veterinary medicine, while neoplasia developing from the intervertebral disc itself has not been reported. Tumors arising from vertebrae or within the epidural space may mimic intervertebral disc herniation (IVDH) on currently available imaging and will be discussed in this chapter.
Patients afflicted with neoplasia of the vertebral column or structures within the epidural space may have similar clinical signs compared to those with IVDH. Often the course is progressive in dogs with these neoplasms, with clinical signs reported from months to years prior to presentation. Some animals with vertebral neoplasia may have acute onset dysfunction or rapid deterioration after a slowly progressive course of disease due to pathologic fracture, vascular events, or rapid tumor growth. Animals with vertebral neoplasms mimicking IVDH will exhibit levels of neurologic dysfunction ranging from paraspinal hyperesthesia only to paralysis with no deep pain perception. Patients with neoplasia may or may not exhibit pain on palpation of the affected area of the vertebral column. Occasionally, paraneoplastic symptoms may be present, such as polyuria and polydipsia secondary to humoral hypercalcemia of malignancy as a result of lymphoma, or seizures due to hyperviscosity syndrome arising from markedly elevated protein concentrations in the blood from multiple myeloma.
Neoplasia sharing embryonic origin within the intervertebral disc: Chordoma
Although no chordomas in the veterinary literature have been reported to arise from the intervertebral disc itself, these tumors share a common origin with the nucleus pulposus [1]. The nucleus pulposus is the surviving structure of the embryologic notochord in the adult animal. Chordomas are thought to arise from notochordal rests or remnants outside of the nucleus pulposus. These tumors are rare in dogs, with fewer than 10 cases reported in the veterinary literature since 1933 [1–7].
In humans, chordomas are most commonly found at the extreme cranial and caudal ends of the vertebral column [8]. In dogs, locations range from the skin over the parietal bone on the skull [4], intramedullary or extradural masses in the cervical spinal cord [3, 7], to the lumbar and coccygeal vertebrae [5]. A chordoma was also reported to arise within the meninges adjacent to the cerebellar fossa, while another was found between the trachea and ventral muscles of the vertebral column, ventral to C1 [4, 7].
Although a male sex predilection was initially reported in dogs [4], the sample size available in peer-reviewed literature is too small to make such a generalization, as females have also been described with this neoplasm. Breeds ranging from Shetland Sheepdogs to Labrador retrievers have been affected, with no two reports sharing a common breed thus far.
In humans, the imaging features of chordoma often include vertebral body lysis, infiltration of the intervertebral disc space, and extradural compression of neuroparenchyma [8]. There are limited data in veterinary medicine concerning the radiographic, computed tomographic, and magnetic resonance imaging (MRI) appearance of chordoma. In dogs, chordomas can appear very similar to herniated disc material on radiographs (if located within the vertebral canal) and will occasionally contain multifocal areas of mineralization [9]. In two dogs, computed tomography suggested the presence of soft tissue dense masses with areas of mineralization that were either extradural or intramedullary; bony lysis was not identified [7, 9]. In one dog with chordoma, MRI revealed a T2 hyperintense lesion within the spinal cord parenchyma; the lesion was initially interpreted as spinal cord contusion secondary to disc herniation or neoplasia [3].
In human patients, chordomas are characterized by local invasiveness, aggressive regrowth at the site of resection, and late metastasis [8]. In the veterinary literature, there has been a single paper describing regrowth following incomplete surgical resection and no reports of metastasis [4].
Neoplasms mimicking IVDH: Tumors of the surrounding vertebrae
These neoplasms originate in the vertebral column and can extend into the epidural space, mimicking the appearance of a disc extrusion or protrusion. Reported vertebral tumors include osteosarcoma, chondrosarcoma, myeloma, fibrosarcoma, and hemangiosarcoma. These tumors often compromise the structural integrity of the vertebral body, resulting in pathologic fracture and severe neurologic dysfunction.
Osteosarcoma is the most common tumor of bone in the dog [10]. It arises from malignant primitive bone cells, which produce osteoid, and is most commonly seen in the appendicular skeleton of dogs. Approximately 25% of osteosarcomas are found in the axial skeleton [11]. Of these, only 15% originate in the vertebral column, and osteosarcoma is the most common extradural spinal neoplasm. In one report, 46% of dogs presenting with axial osteosarcoma had documented metastatic disease [10]. While large-breed dogs account for a majority of cases of osteosarcoma, small-breed dogs have a relatively higher incidence of axial compared to appendicular osteosarcoma [12]. Affected vertebrae do not have a consistent or specific radiographic or other imaging appearance. Biopsy with cytology and/or histopathologic examination is recommended for diagnosis of this tumor.
