Synovial myxomas are rare tumors across species and are even more rarely reported in association with vertebral articulations. Here we report a case of L4/L5 vertebral synovial myxoma in a 12 year old, male neutered, Basset Hound.
This dog presented for chronic gait abnormalities involving the rear limbs which bilateral TPLO surgery had failed to improve. An acute worsening occurred over the course of 5 – 7 days prior to neurologic assessment. On presentation the patient was nonambulatory, paraparetic with neuroanatomic localization to the L4 – S2 spinal cord segments. A more cranial lesion in the T3-L3 spinal cord segments could not be completely ruled out due to the difficulty in assessing reflexes in the rear limbs from previous knee surgeries. Differential diagnoses based on these findings included intervertebral disc disease (IVDD), primary or secondary neoplasia, discospondylitis, meningitis, or polyneuropathy.
Plain radiographs showed multiple narrowed intervertebral disc spaces and mild vertebral endplate sclerosis along the lumbar spine. The primary MRI finding was of an irregular but well-defined, multilobulated mass along the right dorsal aspect of the vertebral canal at the level of L4-5 (Fig.1). The mass was centered at the right dorsal articular processes of L4 and 5, with slight widening and distortion of the synovial space. Extensions of the mass protruded into the epaxial muscles and caused significant extradural compression of the spinal cord (Fig. 2).
Differential diagnoses for these MRI findings included both malignant and benign processes and a synovial cell sarcoma or other benign synovial process was considered most likely. Based on the non-destructive nature of the lesion, a surgical decompression of the spinal cord in this area was considered to have a good prognosis. Surgery was performed revealing a lobulated, whitish grey, gelatinous mass extending from the L4-5 vertebral articulation. This was easily separated from normal spinal cord structures and submitted for analysis. Histopathologic analysis was of a synovial myxoma.
Significant improvement in gait occurred within 24 hours of surgery and steady improvement continued over the next 6 months of follow up.
Myxomas are characterized by the production of mucinous ground substances (Hayes, Teague). The tissue of origin of synovial myxomas is still unclear (Craig). A juxtaarticular tumor occurring in humans is very similar to synovial myxomas in dogs and these tumors grow slowly, can be infiltrative over time, do not usually metastasize, and can reoccur (Teague, Craig, Blair).
Synovial myxomas have been described in dogs. Clinical signs may be nonspecific and evidence of subtle osteolysis or distortion on survey radiographs may lead to a mistaken assumption of malignancy. In three of the previous case reports of synovial myxomas (2 vertebral and 1 stifle) the clinical signs were attributed to other diseases including synovitis, IVDD, and bilateral cruciate ligament deficiency. (Craig) MRI is significantly helpful in narrowing the list of differential diagnoses. All synovial myxomas that had MRI available for review, including the present case, had similar imaging characteristics and MRI appearance of changes to the involved joint. Gross pathologic findings corresponded to the MRI findings in all cases. This consistency in MRI changes may help in directing suspicion toward a myxoma versus a more malignant process.
Treatment for suspected synovial myxomas should include aggressive surgical removal. There is no benefit to follow-up chemotherapy or radiation because of the lack of demonstrated metastatic potential in these tumors (Craig, Meis). In all case reports of synovial myxoma, complete surgical excision was pursued as treatment and had both initial and long term success. Follow up times ranged from 2months to 2.5 years.
Fig 1. Sagittal T2-weighted image (A), showing a hyperintense mass (white arrow) at L4/L5 articulation completely obscuring the spinal cord at this site. A transverse T2 image (B) shows the multilobulated hyperintense mass (orange arrows) extending through the L4/L5 intervertebral foramen on the right. This mass is causing significant right dorsolateral spinal cord compression.
Fig. 2 (A) Transverse T1- pre-contrast image at the level of the L4/L5 articulation, showing an iso- to hypointense multilobulated mass on the right side, extending through the articulation (white arrows). (B) A post-contrast T1 transverse image at the same level with little to no contrast enhancement.
Fig. 3 The bulk of the tumor is seen in place (yellow arrow) between at the L4-L5 articulation.
Fig. 4 Portion of tumor exterior to articulation is seen here with the thin lobulated stalk extending dorsally between L4 and L5 dorsal spinous processes.
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