History
A 28-year-old Quarter Horse gelding was evaluated at the Texas A&M University Veterinary Medical Teaching Hospital because of an episode of colic of 1 day's duration. The horse had a history of trauma; 1 week previously, the horse was reported to have pulled back on the crossties and injured its neck.
On initial evaluation, the horse had a high heart rate (60 beats/min) and high respiratory rate (42 breaths/min). The horse's rectal temperature was within reference limits. The horse held its neck in an extended and horizontal position and was reluctant to move its head in any direction. The horse was ataxic in all 4 limbs, and its ataxia was graded as grade 4 of 5 in the hind limbs and grade 3 of 5 in the forelimbs. The horse had a hypermetric gait in the forelimbs, proprioceptive deficits in all 4 limbs, and weakness in the right forelimb. The horse was extremely reluctant to make any tight turns. There was a diffuse soft tissue swelling over the left side of the neck, particularly over C2 through C5. The horse was evaluated for the admitting complaint of colic. No remarkable findings were present on abdominal palpation per rectum or on transabdominal ultrasonography. The PCV was 35% (reference range, 32% to 53%), serum total protein concentration was 7.2 mg/dL (reference range, 5.7 to 7.9 mg/dL), and blood lactate concentration was 3.1 mmol/L (reference range, 1.1 to 1.7 mmol/L). The horse had pink and moist mucous membranes and a capillary refill time < 2 seconds. Radiographs of the cervical portion of the vertebral column were obtained while the horse was standing (Figure 1).
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Radiographic Findings and Interpretation
There are several well-circumscribed osteolytic vertebral lesions with short zones of transition and little to no sclerotic bone surrounding the lesions. The largest lesion is located in the dorsal aspect of the spinous process of C2. There is destruction of both cortical and medullary bone (Figure 2). There are 2 smaller lesions, one located in the caudal ventral aspect of C3 and the other in the cranial aspect of the vertebral body of C4. The lesion in C3 extends to the endplate. There are several smaller osteolytic lesions associated with the C2-C3 and C3-C4 articular processes. Radiographic findings are indicative of polyostotic osteolytic vertebral lesions. Differential diagnoses include round cell neoplasia (eg, plasma cell myeloma or lymphoma) or metastatic neoplasia.
Treatment and Outcome
Because of a rapidly declining clinical status and the likelihood of neoplasia, the owner elected to have the horse euthanatized. After euthanasia, the dorsal spinous process of C2 was biopsied. On midsagittal section, an ovoid, sharply demarcated and unencapsulated, tan- to cream-colored soft fibrous mass measuring 2 × 1.5 × 1.8 cm had replaced cancellous bone that normally fills this area. This unencapsulated ovoid fibrotic mass was easily dissected and removed from the cavitary defect in the cancellous bone of the spinous process. On histologic examination, tumor cells consisted of poorly differentiated but not very anaplastic epithelial cells. They closely resembled squamous epithelial cells except for the absence of intercellular bridges and absence of keratin formation. Immunohistochemical stains were used in an attempt to further characterize the neoplastic cells. The tumor cells were strongly positive for the epithelial marker cytokeratin. The connective tissue stroma forming the interstitial matrix entrapping the tumor cells stained strongly for the mesenchymal connective tissue stain vimentin. Results of stains used to identify plasma cells, B lymphocytes, T lymphocytes, MUM-1, CD79, and CD3 were negative. The immunohistologic staining, which confirmed the epithelial nature of the tumor cells, and the polyostotic distribution of the lesions supported a metastatic origin of the bone lesions. The histopathologic diagnosis was metastatic carcinoma. At the request of the owner, only a biopsy specimen of C2 was obtained and a necropsy was not performed. Therefore, a primary tumor was not identified. There was no additional information in the history to indicate a primary tumor.
Comments
Radiographically, the polyostotic vertebral lesions of this horse had both aggressive and nonaggressive features. The well-circumscribed nature, short zone of transition, and preservation of the endplates were nonaggressive features. The predominantly lytic nature, cortical lysis, and multifocal distribution were aggressive features. Neoplastic invasion of the vertebral bodies may cause osteomalacia and pathological fractures. No pathological fractures or fractures due to the recent trauma were identified on survey radiographs of this patient. The polyostotic distribution was consistent with a metastatic or multifocal tumor.1 An infectious etiology was considered less likely because the lesions were well-defined and had no irregular margins. Additionally, hematogenous osteomyelitis is uncommon in adult horses and infectious osteomyelitis is often due to a penetrating wound,2 for which there was no evidence in this patient. The smaller osteolytic lesions of the articular processes were thought to represent regions of subchondral bone lysis secondary to osteoarthritis or, less likely, additional neoplastic lesions.
The reported signs of colic in this patient were most likely due to referred pain. The blood lactate concentration was most likely high because of decreased tissue perfusion. As a result of the severe neurologic signs and history of trauma, radiographs of the cervical portion of the vertebral column were obtained. The presumed neoplastic lesions were an unexpected finding on the radiographs. It is unlikely that the osteolytic lesions were solely responsible for the acute neurologic signs but more likely that the traumatic event elicited an acute manifestation of a chronic process.
Bone neoplasms in horses are rare. When they do occur, tumors involving the axial skeleton are more common than those of the appendicular skeleton.3 These tumors may be primary or metastatic. Examples of primary bone neoplasms in horses include osteosarcomas, plasma cell myelomas,4 and chondrosarcomas. Examples of tumors with metastasis to bone include hemangiosarcomas, melanomas, fibromas, lymphosarcomas, squamous cell carcinomas,5 adenocarcinomas, and perirenal carcinomas.6 Lymphosarcoma is one of the more common neoplastic findings in horses3 and therefore may be considered more likely when considering metastatic neoplasia in the vertebral column. However, in this patient, because immunohistologic staining revealed an epithelial nature of the tumor cells, a primary carcinoma would be considered higher on the list of differential diagnoses.
1. Thrall DE. Textbook of veterinary diagnostic radiology. 5th ed. St Louis: Saunders Elsevier, 2007.
2. O'Brien TR. O'Brien's radiology for the ambulatory equine practitioner. Jackson, Wyo: Teton NewMedia, 2005.
3. Schooley EK, Hendrickson DA. Musculoskeletal system neoplasia. Vet Clin North Am Equine Pract 1998; 14:535–542.
4. Drew RA, Greatorex JC. Vertebral plasma cell myeloma causing posterior paralysis in a horse. Equine Vet J 1974; 6:131–134.
5. Perrier M, Schwarz T, Gonzalez O, et al. Squamous cell carcinoma invading the right temporomandibular joint in a Belgian mare. Can Vet J 2010; 51:885–887.
6. Young AC, Hoffmann KL, Begg AP, et al. Acute lameness associated with osseous metastasis of a peri-renal carcinoma in a horse. Aust Vet J 2010; 88:346–350.