Synovial cysts are most commonly found in the lumbar spine, mostly at the L4-L5 level. They usually develop in patients with degenerative disc disease, facet arthropathy, and degenerative spinal stenosis. Quite frequently degenerative spondylolisthesis or facet joint instability is also found at the level of cyst formation. The latter findings, it is thought, support the notion that increased segmental motion plays a role in the pathogenesis of these cysts.
Typically the cysts occupy the posterolateral aspect of the spinal canal, are adjacent to the facet joints, and are attached to the facet joint capsule. They contain serous or gelatinous fluid and measure up to two centimeters in diameter.
The patients, usually females in the sixth decade, present with back pain and radicular pain. The pain is frequently intermittent and is often mechanical in nature. It may be worse standing but may persist in other postures as well. The pain does not respond to conservative management and usually persists. On rare occasions, large-diameter cysts may lead to cauda equina syndrome. Thoracic or cervical cysts compress the cord and result in a slowly progressive myelopathy.
The X-ray findings are those commensurate with age and spinal degeneration: degenerative disc disease, facet arthropathy, degenerative spondylolisthesis, and spur formation. Spinal CT may detect
v_t the cyst when its wall has calcified or when its cavity contains gas or blood due to a hemorrhage. In the latter case the cyst space will appear hyperdense in comparison to the spinal canal. Occasionally large cysts may erode the adjacent bone and become detectable on bone windows. Contrast CT may show some enhancement in the cyst's wall. MRI, the procedure of choice, will show an extradural lesion with smooth surfaces adjacent to the facet joint. T1-weighted images may show the cyst as hypointense, isointense, or hyperintense when it contains blood. T2-weighted images show a hyperintense lesion that, at times, may communicate with the facet joint. In contrast studies the cyst's walls may enhance and show its impact on the adjacent nerve roots (Figures 7-1A, 7-1B, and 7-1C).
The MRI examination helps rule out an extruded disc or a tumor as their T1 and T2 signal characteristics and response to contrast administration are different from those of a cyst. Unlike cysts, extruded discs are not hyperintense on T2-weighted images. Following contrast administration, however, the disc's periphery may enhance due to local scarring or inflammation. Tumors, typically, will become hyperintense to a greater degree after contrast administration.
Following the diagnosis of a facet cyst it is worth trying conservative measures as the symptoms may spontaneously regress. Analgesic
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