Synovial Myxoma in the Vertebral Column of a Dog: MRI Description and Surgical Removal

Introduction

Myxomas are benign tumors arising from pluripotent, mucin-producing mesenchymal cells. In dogs, the tumor is most common in the skin, but it has also been reported in such places as the tricuspid valve, the periodontal region, and the appendicular synovial joint.^1–7 Synovial tumors are uncommon in the dog and very rare in the cat. In a retrospective report of 35 synovial tumors, 17% were myxomas, and Doberman pinschers and mixed-breed Doberman pinschers were overrepresented.⁸ Synovial myxomas have been described in various anatomic sites in the dog, with the digit and stifle the most common sites.⁹

To the authors’ knowledge, there are only three reports of a synovial myxoma arising from vertebral articular facet.^9–11 Craig et al. (2010) detail findings at necropsy and, therefore, do not document either imaging findings or clinical disease course, and Teague and Berg (1978) provide a surgical case report, but do not use MRI as its imaging modality.^9,10 The final report, however, describes the MRI characteristic of a synovial myxoma in the lumbar spine, which are similar to what are appreciated in the present case.¹¹

The purpose of this report is to document the MRI findings of a cervical vertebral synovial myxoma. The successful surgical removal of the myxoma is also described.

Case Report

A 12 yr old castrated male mixed-breed dog weighing 26.4 kg was presented with a 2 wk history of neck pain, progressive weakness, and ataxia of all four limbs. Five days prior to presentation, the dog was no longer able to ambulate without assistance and treatment with prednisone^a (0.75 mg/kg per os q 24 hr) was initiated. The patient failed to improve and was referred for a neurologic consultation.

On presentation for the consultation, the dog was bright, alert, responsive, and had normal vital signs. The dog was laterally recumbent with pronounced ventroflexion of the neck and was reluctant to reposition to sternal recumbency without assistance. When assisted to stand, neurologic examination revealed a short-strided choppy gait in the thoracic limbs and a long-strided ataxic gait in the pelvic limbs. Postural reactions were normal in the thoracic limbs but delayed in the pelvic limbs. Reflex evaluation showed a decreased withdrawal reflex bilaterally in the thoracic limbs with very mild crossed-extension. Hyperesthesia was noted in the caudal lumbar region; however, the patient was not overtly painful on palpation of the neck. Atrophy of the infraspinatus and supraspinatus muscle groups was appreciated bilaterally. Given the dog’s history and neurologic exam findings, a lesion in the caudal cervical/cranial thoracic spinal cord (i.e., the sixth cervical [C] vertebra to the second thoracic [T] vertebra) was suspected. Leading differential diagnoses for this patient’s myelopathy included intervertebral disk disease, cervical spondylomyelopathy (Wobbler syndrome), intra- or extramedullary neoplasia, and an infectious/inflammatory spinal cord condition.

Routine preanesthetic blood work was performed, consisting of a complete blood count and a serum biochemical analysis. Results of the complete blood count were within normal limits, and the biochemical analysis revealed elevations in alkaline phosphatase (17.10 μkat/L; reference range, 0.33–2.50 μkat/L) and alanine aminotransferase (4.40 μkat/L; reference range, 0.17–1.93 μkat/L), which were attributed to the recent corticosteroid therapy.

MRI^b of the cervical spine revealed a mass at the level of C6 and C7 on the left side. The mass was hyperintense on T2-weighted images, hypointense on T1-weighted images, and showed strong homogenous enhancement following IV contrast agent^c administration. The mass was centered around the synovial joint of the articular facet of C6–C7 on the left and entered the spinal canal, causing severe dorsolateral compression of the spinal cord on the left side (Figures 1A, B). There was distention of the left C6–C7 articular facet, and the mass extended to the dorsal aspects of the C6 and C7 vertebral bodies. The primary differential diagnosis was a neoplastic process of the facet. Other differential diagnoses considered included a synovial cyst, granuloma, or a focal abscess. A MRI of the lumbar spine was performed under the same anesthetic period and was unremarkable. The owners elected decompressive surgery and excisional biopsy of the cervical lesion.

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A dorsal midline approach to the cervical vertebrae was made through a skin incision that spanned from the cranial margin of C3 caudally to the midbody of the T2 vertebra. Subcutaneous tissue was incised and multiple planes of epaxial musculature were retracted to expose the dorsal spinous processes of C6, C7, and T1. The nuchal ligament was excised, and the spinous processes of C6 and C7 were removed. A 3 cm × 1 cm × 1 cm, semifirm, pale, friable mass was noted in association with the articular facet of C6–C7 on the left side. The dorsal laminae of C6 and C7 and the left articular facet were removed to allow visualization of the spinal cord and associated mass. The mass extended over the spinal cord on the left side where it compressed the left C7 nerve root, extended through the C6–C7 intervertebral foramen, and coursed down to the ventral aspect of the C6 and C7 vertebral bodies. The mass was dorsolaterally adhered to the dura mater on the left side. The mass was sharply excised from the dura with a scalpel blade and removed in its entirety. The C7 nerve root was visualized and was free from compression. An absorbable gelatin sponge^d was placed over the laminectomy site, and the area was copiously lavaged. Closure was routine. Postoperative MRI of the cervical spine revealed complete surgical removal and alleviation of the spinal cord compression (Figures 2A, B). The patient recovered from anesthesia uneventfully.

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The tissue evaluated consisted of the neoplastic proliferation as well as fragments of dense collagen, adipose, and articular cartilage. The neoplasm was arranged in sparsely cellular lobules separated by a fine fibrous stroma (Figure 3). The neoplastic cells were stellate to spindle-shaped and embedded within a light blue-gray myxomatous matrix. The cells had a moderate amount of wispy eosinophilic cytoplasm and fusiform to triangular nuclei with dispersed chromatin and 1–2 nucleoli. There was mild to moderate anisocytosis and anisokaryosis, and 0–1 mitoses/single high-power field (original magnification ×400). The histologic lesions were compatible with a myxoma arising from the synovial joint of the articular facet.

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Examinations at 2 mo, 6 mo, and 12 mo postoperatively showed that the dog had a normal neurologic examination and was pain-free. Follow-up MRI of the cervical spine was performed at 6 mo and 12 mo and showed no evidence of local recurrence.

Discussion

The cell of origin in synovial myxomas is not well understood; however, it is theorized that they may arise from primitive type C synoviocytes.⁹ In addition, exact identification of synovial myxomas with the use of immunohistochemic staining has proven difficult as markers expressed in those tumors are shared among several common articular and periarticular neoplasms.⁷ Thus, histopathologic characteristics, not biochemical markers, of those neoplasms should be used to formulate an accurate diagnosis.

Synovial myxomas are characterized as benign tumors based solely on their histologic appearance and low rate of metastasis. However, those tumors can be very locally aggressive and recurrence is not uncommon. Bony invasion and lysis has been previously described in eight dogs with histologically confirmed synovial myxomas; however, that was not appreciated in the present case.⁹ Treatment of synovial myxomas is weighted heavily on the degree of invasion into the bone. Synovectomy is typically recognized as a curative therapy; however, amputation is frequently needed to ensure complete tumor removal.^1,6 In the present case, a synovectomy was achieved through the C6–C7 facetectomy, and it did not necessitate any further removal of bone in addition to what was removed with the laminectomy.

A case report exists describing a vertebral synovial myxoma at the first and second lumbar vertebra in an 8 yr old rottweiler.¹⁰ Standard myelography was used and showed extradural compression. A dorsal laminectomy and durotomy were performed, and the tumor was removed in its entirety. The dog fully recovered and was walking normally at 8 wk and 1 yr postoperatively. Similarly, Blair et al. (2011) describe extradural spinal cord compression as seen with MRI and tumor removal achieved with a hemilaminectomy.¹¹ That patient, a 14 yr old border collie, showed neurologic improvement with an improved gait and resolution of fecal incontinence at a 2 mo follow-up examination.

The MRI findings in the current case are similar to what have been previously described in the literature in synovial myxomas existing both within and outside of the vertebral column. Hyperintensity on T2-weighted images, hypointensity on T1-weighted images, and strong uniform contrast enhancement was described in the stifle of a 10 yr old Labrador retriever with a confirmed synovial myxoma.⁶ The degree of contrast enhancement was less in the case report described by Blair et al. (2011); however, the precontrast images were similar in description.¹¹ The MRI findings in the aforementioned reports are similar to those reported in dogs with vertebral synovial myxosarcomas.^12,13 Although reports exist of vertebral synovial myxomas in the lumbar spine in dogs, this is the first MRI description in the cervical vertebral column.^9,10

In light of the MRI findings, a reasonable differential diagnosis for the spinal mass was an extradural articular cyst. More specifically, either a synovial cyst or ganglion cyst. Both synovial and ganglion cysts arise from periarticular tissues as a sequela of a degenerative joint process.¹⁴ Synovial cysts are fluid-filled structures lined with a synovial-like epithelium, whereas ganglion cysts are encapsulated in a degenerative layer of connective tissue.¹⁵ Canine synovial cysts have been described in the vertebral column in recent literature. A case report of three dogs with histopathologically confirmed synovial cysts showed occurrence in the thoracic and lumbar spine with complete resolution of clinical signs following decompressive surgery.¹⁶ Dickinson et al. (2001) report on nine dogs with extradural synovial cysts where they further classify the disease into two subtypes.¹⁷ One subtype was young, giant-breed dogs with multiple cysts in the cervical vertebral column and the other were older, large-breed dogs with solitary thoracolumbar cysts. Ganglion cysts are rare in the canine vertebral column, with only one report existing in the literature of cysts occurring either at or near the lumbosacral junction in a middle-aged German shepherd dog.¹⁴

Conclusion

The imaging findings in the current case are compatible with several etiologies, both benign and malignant. MRI findings of a mass arising from and causing distention of the articular facet should increase the suspicion for this neoplasm. Moreover, a diagnosis through surgical biopsy and histopathology should be attained before an accurate prognosis can be established. Clinical improvement described in the current patient is similar to what was described in prior reports, supporting the authors’ beliefs that this neoplasm has a favorable prognosis with prompt surgical treatment.^10,11