An 8.7-year-old 34-kg (74.8-lb) sexually intact male Labrador Retriever was referred for evaluation because of a 3-month history of signs of progressive, severe neck pain. The dog was treated with an injection of meloxicam and, after a 3-day interval of no treatment, by administration of prednisolone and omeprazole and exercise restriction for 4 weeks. One week prior to the referral evaluation, tramadol administration was also added to the treatment plan to address the exacerbated signs of pain. On evaluation, results of a general physical examination were unremarkable apart from tensing of the abdominal wall during palpation.
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 |
|---|---|
| Cervical hyperesthesia and decreased range of motion of the neck | C1-C5 or C6-T2 spinal cord segments |
| Decreased mass of the cervical epaxial muscles | C1-C5 or C6-T2 spinal cord segments |
| Decreased extensor carpi radialis reflex bilaterally | C6-T2 spinal cord segments |
| Reduced flexion of the left forelimb | Left-sided C6-T2 spinal cord segments, or possibly the C1-C5 spinal cord segments if signs are secondary to pain and not true hyporeflexia |
| Delayed proprioceptive positioning of the left hind foot | Left-sided C1-C5, C6-T2, or T3-S1 spinal cord segments |
Likely location of 1 lesion
C6-T2 spinal cord segments, possibly lateralized to the left side.
Etiologic diagnosis—For this dog, asymmetrical cervical myelopathy of a painful, chronic, and progressive nature was suspected. The main differential diagnoses included degenerative, neoplastic, inflammatory, and infectious diseases. With regard to possible degenerative causes, Hansen type II intervertebral disk disease (disk protrusion) and synovial cysts were considered. Possible neoplastic causes included metastatic disease or primary tumors (eg, meningioma, peripheral nerve sheath tumor, glial tumor, lymphoma, plasma cell tumor, osteosarcoma, chondrosarcoma, and hemangiosarcoma). In older dogs with chronically progressive clinical signs, asymmetrical cervical myelopathy caused by inflammatory or infectious conditions are generally less likely; however, radiography of the vertebral column to rule out diskospondylitis was not performed. Bacterial, viral (canine distemper virus), protozoal (Toxoplasma gondii or Neospora caninum), tick-borne (Ehrlichia canis, Rickettsia rickettsia, or Borrelia burgdorferi), and fungal (Cryptococcus neoformans, Coccidioides immitis, Blastomyces dermatitidis, Histoplasma capsulatum, or Aspergillus spp) infections were included as other possible causes of inflammation of the spinal cord and meninges (ie, meningomyelitis or meningitis). Finally, causes of sterile (likely immune-mediated) meningitis, such as granulomatous meningoencephalomyelitis and steroid-responsive meningitis-arteritis, were also considered but thought unlikely given the dog's signalment. The diagnostic plan included a CBC and serum biochemical analysis to rule out systemic signs of infectious or neoplastic diseases as well as to assess the dog's health prior to anesthesia; MRI of the C1-T2 region to rule out intervertebral disk disease, diskospondylitis, and neoplastic lesions; and possible analysis of a CSF sample to rule out inflammatory or infectious causes. Thoracic radiography was not performed.
Diagnostic test findings—Results of a CBC were unremarkable. Serum biochemical analysis revealed high activities of alanine aminotransferase (248 U/L; reference interval, 21 to 102 U/L), and alkaline phosphatase (250 U/L; reference interval, 20 to 60 U/L), mildly high cholesterol concentration (7.4 mmol/L; reference interval, 3.8 to 7.0 mmol/L), and high triglycerides concentration (2.34 mmol/L; reference interval, 0.57 to 1.14 mmol/L). The clinicopathologic findings were indicative of possible mild hepatopathy and consistent with a lipid profile expected for a dog from which food had not been withheld. Magnetic resonance imaging was performed with a 1.5-T permanent magnet.a Multiplanar T1- and T2-weighted, short-inversion recovery images and additional T1-weighted fat-saturated pre- and postcontrastb (gadodiamide, 0.2 mL/kg [0.09 mL/lb], IV) images of the cervical portion of the vertebral column were obtained. Within the vertebral canal at the level of the C2 vertebra, a right-sided extramedullary, extradural, non–contrast-enhancing, ellipsoid (length, 2.2 cm; height, 1.2 cm; and width, 0.7 cm) space-occupying cystic lesion was identified (Figure 1). The lesion was associated with focal left displacement and marked compression of the spinal cord, and it was hyperintense on T2-weighted images, hypointense on T1-weighted images, and non–contrast-enhancing. This lesion was connected dorsally and laterally to the atlantoaxial joint space by 2 tubular structures (both hyperintense on T2-weighted images and hypointense on T1-weighted images) that surrounded the cranial aspect of the body of C2 and the dens. A juxta-articular cyst of the occipito-atlanto-axial joint was suspected, and the differential diagnosis was a synovial or ganglion cyst. The cystic structure was associated with muscular hyperintensities and mild, heterogeneous contrast enhancement of the right rectus capitis dorsalis and right obliquus capitis caudalis muscles and within the fascial planes of the left digastric and pterygoideus medialis muscles, which extended caudodorsally up to the level of the left atlanto-occipital joint. These abnormalities were suspected to be secondary to synovial fluid leakage and associated myositis, and less likely a result of primary infectious or immune-mediated myositis. No regional lymphadenopathy was detected. Ultrasound-guided fine-needle aspirate samples were subsequently collected from the abnormal cervical muscle areas; these areas appeared heterogeneously hypoechoic on ultrasonographic images. Cytologic examination of the sample from the right epaxial muscle lesion was consistent with a possible synovial fluid–containing structure; narrow, small- to medium-sized, regular mesenchymal cells were present individually and in loose aggregates (possibly derived from synovial membrane) with areas containing clumps of amorphous pink material or a more diffuse granular pink background and occasional collagenlike bundles and capillary fragments.
Transverse T2-weighted (A and D) and T1-weighted fat-saturated precontrast (B and E) and postcontrast (C and F) MRI images obtained at the level of the C2 vertebra of an 8.7-year-old Labrador Retriever that was evaluated because of a 3-month history of signs of severe progressive neck pain. Images in panels D, E, and F were obtained at a level slightly more caudal to that of images A, B, and C to illustrate the MRI characteristics, extent, and shape of the lesion. In panels A, B, and C, notice the apparent presence of 2 tubular structures (arrows) surrounding the cranial aspect of the body of C2. The structures are hyperintense on the T2-weighted image (A) and hypointense and non–contrast-enhancing on the T1-weighted images (B and C). In the slightly more caudal images in panels D, E, and F, notice the T2-hyperintense (D), T1-hypointense (E), non–contrast-enhancing (F), right-sided, extramedullary, extradural, space-occupying, cystic lesion with associated displacement and compression of the spinal cord (arrowhead).
Citation: Journal of the American Veterinary Medical Association 255, 3; 10.2460/javma.255.3.295
Following the initial investigations, surgical excisional biopsy and decompression of the spinal cord at the level of C2 was proposed; however, the dog's condition deteriorated drastically within a 24-hour period, with markedly increased severity of signs of neck pain and progression to nonambulatory tetraparesis. The owners elected to euthanize the dog and gave consent for a postmortem examination. On gross postmortem examination, a fragile, multilobulated, cystic structure containing clear to pale pink gelatinous material was observed within the left and right dorsolateral aspects of the atlanto-occipital joint; it multifocally extended over the right side and dorsal aspect of the C2 and cranial third of the C3 spinal cord segments. This structure was within the vertebral canal and outside the spinal cord and dura mater (ie, extramedullary and extradural). Subsequent removal of the cervical portion of the spinal cord revealed a similar cystic structure that was located within the ventrolateral aspect of the right side of the vertebral canal at the level of C2 through C3 and associated with moderate compression of the spinal cord at the level of C2. Histologic examination of sections of the cystic structure that was seen grossly identified a myxoid mesenchymal tumor within the fascia that infiltrated the adjacent skeletal muscle bundles and adipose tissue on the ventral aspect of the left side of the first cervical vertebra. The sections had low cellularity, mild nuclear pleomorphism, lack of mitotic figures, and lack of atypical cells with vacuolated cytoplasm. The lesion was considered most likely a myxoma, rather than a myxosarcoma or a myxoid liposarcoma, and was thought to have originated from a synovial myxoma in a synovial facet joint within the vertebral column.
Comments
For the dog of the present report, the presumed likely localization of the lesion prior to diagnostic imaging was the C6-T2 region; however, MRI and subsequent postmortem examination findings confirmed a C1-C3 lesion. This inconsistency could be explained by the fact that the dog's signs of pain were poorly controlled at the referral evaluation, causing the dog to assume an antalgic posture and altering its response to stimuli (ie, decreased spinal reflexes), or by possible so-called false localization, which has been noted to occur in humans with an upper cervical lesion.1 The pathophysiology of false localization is still uncertain, and arterial, venous, and mechanical principles have been proposed.1
In dogs, intraspinal cystic structures most commonly originate from periarticular joint tissue, and can be synovial or ganglion cysts.2 The structures have characteristic MRI, CT, and histologic features and their location is generally reflected in the clinical findings of neurologic examinations of affected dogs.3,4 The most common synovial neoplastic tumors are histiocytic sarcomas, synovial myxomata, and synovial cell sarcomas5,6; however, a vertebral location of synovial myxoma, as identified in the case described in the present report, is considered rare. Synovial myxomata are usually solitary and tend to affect the limbs in dogs and humans; their invasive nature, including bone invasion, has been described.6,7 Middle-aged and large-breed dogs are usually affected.5 The MRI findings for synovial myxomata in dogs have been described and typically include a hypointense appearance on T1-weighted images and a hyperintense appearance on T2-weighted images.8–10 These MRI findings are similar to those for intramuscular myxomata.7,11 Descriptions of contrast enhancement of synovial myxomata on MRI images have been variable, ranging from mild to strong enhancement8,10; in the case described in the present report, there was no contrast enhancement of the lesion. Prognosis for dogs with synovial myxoma has been reported to be good following surgical excision with wide margins, although in most cases this procedure has involved amputation because the lesion was in a limb joint and local recurrence may have otherwise occurred.5,6 Surgical removal of a cervical vertebral synovial myxoma through a dorsal laminectomy and facetectomy has been described for 1 dog; in that case, no recurrence was observed clinically or with MRI at 12 months after surgery.10
Footnotes
Philips Intera 1.5-T Pulsar System, Philips Medical Systems, Guildford, England.
Omniscan 0.5 mmol/ml, GE Healthcare, Chalfont St Giles, England.
References
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