Introduction
A7-month-old 25-kg (55-lb) neutered female German Shepherd Dog was referred for evaluation of pelvic limb ataxia of 2 weeks’ duration. The ataxia had been insidious in onset and slowly progressive. Diffuse muscle atrophy of the pelvic limbs was noted. One week prior to the referral consultation, the dog had been spayed, and treatment with meloxicam was started without improvement of the ataxia. Results of a CBC and serum biochemical profile were considered normal. Lumbosacral and hip joint radiography revealed mild signs of hip dysplasia. General physical examination findings were unremarkable. No behavior indicative of pain was reported.
What is the problem? Where is the lesion? What are the most probable causes of this problem? What is your plan to establish a diagnosis? Please turn the page.
Assessment Anatomic diagnosis
Problem | Rule out location |
---|---|
Reduced pelvic limb musculature | T3-S1 spinal cord segments or pelvic limb disuse |
Delayed proprioceptive positioning of left and right hind feet | T3-S1, C1-C5, or C6-T2 spinal cord segments |
Bilateral pelvic limb ataxia | T3-L3, C1-C5, or C6-T2 spinal cord segments |
Bilateral plantigrade stance | Breed conformation |
Likely location of 1 lesion
T3-L3 spinal cord segments
Etiologic diagnosis—A symmetric, nonpainful thoracolumbar myelopathy of insidious onset and short duration was investigated. The differential diagnoses included congenital anomalies and inflammatory, infectious, degenerative, and neoplastic disorders of the vertebral column or spinal cord and paraspinal structures. With regard to possible congenital anomalies, vertebral malformations included caudal articular facet hypoplasia, block vertebra, butterfly vertebra, wedge-shaped vertebra, and hemivertebra of the upper thoracic portion of the vertebral column, and possible spinal cord malformations included syringomyelia. Possible inflammatory or infectious causes included viral (distemper), parasitic (Neospora caninum or Toxoplasma gondii), or fungal infection or immune-mediated disease, any of which could have resulted in inflammation of the spinal cord. Possible degenerative causes included intervertebral disk herniation leading to compression of the spinal cord. Possible neoplastic causes included nephroblastoma (most likely given the neurolocalization, breed, and age), osteochondroma, multiple cartilaginous exostoses, chondroma, osteoma, and calcinosis circumscripta. The diagnostic plan included electrophysiologic studies to confirm the upper motor neuron nature of the disease, MRI of the T3-L3 spinal cord segments and associated portion of the vertebral column, and collection and analysis of a CSF sample from the lumbar portion of the vertebral canal.
Diagnostic test findings—Electromyography, conducted while the dog was anesthetized, did not reveal spontaneous electrical activity in the pelvic limb muscles; thus, a lower motor neuron disease was considered less likely. Magnetic resonance imaging was performed with a 1.5-T permanent magnet.a Multiplanar T2-weighted and T1-weighted pre- and postcontrastb (0.1 mL of gadolinium/kg [0.045 mL/lb], IV) images of the thoracolumbar portion of the vertebral column were obtained during anesthesia. At the level of T13, a right-sided extradural, well-defined, ovoid (length, 1.74 cm; height, 1.37 cm; and width, 1.03 cm), space-occupying mass was identified. The lesion was associated with focal left displacement and moderate compression of the spinal cord and was heterogeneously iso- to hyperintense on T2-weighted images, isointense on T1-weighted images, and minimally contrast enhancing on T1-weighted postcontrast images. A T2-weighted hyperintense and T1-weighted hypointense peripheral rim was observed, consistent with a uniform hyaline cartilage matrix. On T2-weighted images, hyperintense areas in the spinal cord cranial and caudal to the mass location were observed and considered most consistent with edema or gliosis (Figure 1). Few focal cortical interruptions were observed on the body of T13. No regional lymphadenopathy was detected. An extradural neoplasm was suspected, and the differential diagnoses were most likely benign neoplasia (osteochondroma or osteoma) and less likely malignant neoplasia (fibrosarcoma or, less likely, nephroblastoma) given the age and breed of the dog. Collection of a CSF sample was not performed because an inflammatory or infectious process was considered unlikely in light of the MRI findings.
Following the diagnostic investigations, surgical excisional biopsy and decompression of the spinal cord at the level of T13 were elected. A T13 right-sided hemilaminectomy was performed. The mass was identified as a hard, bone-like structure with a smooth surface that extended from the right pedicle of T13. The structure was removed by burring around it and slowly isolating it. However, the bone forming the floor of the vertebral canal on the right side and the cranial and caudal parts of the pedicle, despite being apparently separate from the mass, was friable and preserved to prevent spinal instability. The mass was outside the spinal cord and dura mater (ie, extra-medullary and extradural). Histologic examination of sections of the mass identified a mixture of medullary bone with intertrabecular spaces containing fat and hematopoietic bone marrow, together with increased amounts of atypical cartilage associated with areas of degeneration and some necrosis as well as fibrous connective tissue. The lesion was diagnosed as an osteochondroma of T13.
Ten months after surgery, the owner reported that the dog was apparently normal; on repeated neurologic examination, no abnormalities were detected. The dog was sedated, and CTc was performed with 16 × 0.6-mm scanner detectors with a helical pitch of 0.65 and rotation time of 1 second. Pre- and postcontrastd (iodine solution, 1.7 mL/kg [0.77 mL/lb], IV) images of the thoracolumbar portion of the vertebral column were obtained. At the level of T13, the vertebral canal was moderately narrowed and triangular shaped because of smooth new bone formation on the right lateral and ventral aspects of the canal. The spinal cord was located on the left side of the canal and very mildly compressed. No epidural fat was visible at this level. No contrast enhancement was observed in the postcontrast images. Therefore, regrowth of the osteochondroma had occurred, but it was not affecting the dog's gait at that stage.
Comments
For the dog of the present report, the anatomic localization of the lesion prior to diagnostic imaging was the T3-L3 spinal cord segments. Subsequent MRI confirmed a lesion at T13, and abnormal tissue obtained during surgery was histologically consistent with an osteochondroma.
An osteochondroma is an abnormal cartilaginous growth that undergoes endochondral ossification with a central mass that is replaced by bone tissue.1,2 These tumors arise from perturbations of the perichondrial ring of the metaphysis or juxta-epiphyseal regions of the bones in the axial and appendicular skeleton.1,2 Osteochondromas have a cortex and medulla that blend with those of the involved bone.2 These cartilage-capped exostoses can be solitary, as in the dog of the present report, or multiple growths may develop, which are called multiple cartilaginous exostoses if they have polyostotic skeletal involvement.1,2 The etiopathogenesis of this disease is still unknown. With regard to osteochondromatosis (multiple lesions), a mutation in the exostosin 2 (EXT2) gene has been identified in affected American Staffordshire Terriers.3
Osteochondromas in people, dogs, cats, and horses have been described.1 In dogs, the tumors typically affect young animals and have self-limiting development, ceasing at the time of skeletal maturity (12 to 14 months of age).4 The common locations of these lesions (in decreasing order of frequency) are vertebrae, ribs, long bones, carpus or tarsus, and pelvis. The most frequently affected part of a vertebra is the spinous process.1,5 When an osteochondroma does not compress the spinal cord, it remains clinically silent; however, an osteochondroma can expand into the vertebral canal, leading to a compressive myelopathy with subsequent neurologic signs, as evident in the dog of the present report.2,5 Possible malignant transformation of osteochondromas into chondrosarcomas or osteosarcomas has been described.4,6,7 For the dog of the present report, there was no histologic evidence of malignancy. Malignancy can be determined by the thickness of the cartilaginous cap.8
The prognosis for dogs with osteochondromas if surgical excision is not performed is guarded to poor. Early excision of the tumor is associated with a good prognosis for skeletally mature dogs, especially when full marginal resection is achieved. Whether full excision of the tumor from the dog of the present report was achieved was not definitively determined. In growing dogs, relapses may develop following surgical excision of osteochondromas.1
Footnotes
Achieva 1.5-T Pulsar System, Philips Medical Systems, Guildford, England.
Gadovist (1.0 mmol/mL) injection, Bayer, Berlin, Germany.
Siemens Scope, Erlangen, Germany.
Omnipaque 350 mg/mL injection, GE Healthcare, Oslo, Norway.
References
- 1. ↑
Marioni K, Hathcock JT, Simpson ST. What Is Your Neurologic Diagnosis? Osteochondroma of the dorsal process of T4. J Am Vet Med Assoc 2001;219:917–920.
- 2. ↑
Mai W. Spinal neoplasia. In: Diagnostic MRI in dogs and cats. Boca Raton, Fla: CRC Press, 2018;532–533.
- 3. ↑
Friedenberg SG, Vansteenkiste D, Olby NJ, et al. A de novo mutation in the EXT2 gene associated with osteochondromatosis in a litter of American Staffordshire Terriers. J Vet Intern Med 2018;32:986–992.
- 4. ↑
Silver GM, Bagley RS, Gavin PR, et al. Radiographic diagnosis: cartilaginous exostoses in a dog. Vet Radiol Ultrasound 2001;42:231–234.
- 5. ↑
Miwa Y, Nishimura R, Sasaki N, et al. Thoracic vertebral osteochondroma causing spinal cord compression in a young dog. Jpn J Vet Anesth Surg 2001;32:67–74.
- 6. ↑
Bhatti S, Van Ham L, Putcuyps I, et al. Atlantoaxial cartilaginous exostosis causing spinal cord compression in a mature Bernese Mountain Dog. J Small Anim Pract 2001;42:79–81.
- 7. ↑
Owen LN, Bostock DE. Multiple cartilaginous exostoses with development of a metastasizing osteosarcoma in a Shetland Sheepdog. J Small Anim Pract 1971;12:507–512.
- 8. ↑
Green EM, Adams WM, Steinberg H. Malignant transformation of solitary spinal osteochondroma in two mature dogs. Vet Radiol Ultrasound 1999;40:634–637.