Cauda equina syndrome is a well described state of neurologic compromise due to 
lumbosacral root compression. In most cases, it is due to a herniated disc, 
tumor, infection, or hematoma. We report a case of rapid lumbar synovial cyst 
expansion leading to acute cauda equina syndrome and compare it to similar cases 
in the literature. The patient is a 49-year-old woman with a history of chronic 
low back pain who developed cauda equina syndrome. Serial lumbar magnetic 
resonance imaging studies demonstrated a significant increase in the size of a 
lumbar synovial cyst over a 2 week interval. After an unsuccessful attempt to 
relieve her acute symptoms with computed tomography-guided cyst aspiration, an 
L4-5 posterior spinal decompression with excision of the synovial cyst was 
performed. Postoperatively the patient's perineal numbness, bladder 
incontinence, and associated pain complaints resolved. The only residual symptom 
at one month follow-up was continued numbness in the right lower limb in an L5 
distribution. This report adds to 6 other well described similar cases found in 
the literature by illustrating several important points. First, a lumbar 
synovial cyst is a rare but possible cause of acute cauda equina syndrome. 
Second, magnetic resonance imaging is the test of choice to diagnose and 
characterize lumbar synovial cysts; serial imaging can detect fluctuations in 
cyst size. Third, percutaneous treatment of lumbar synovial cysts is variable in 
efficacy and proved to be unsuccessful in our patient. Finally, surgical 
management has shown high success rates for symptomatic cysts. Specifically, in 
the setting of acute cauda equina syndrome secondary to a lumbar synovial cyst, 
urgent surgical decompression has led to resolution of neurologic symptoms in 
most reported cases. A lumbar synovial cyst is an uncommon cause of acute cauda 
equina syndrome. Prompt diagnosis and treatment may lead to reduced morbidity 
associated with this condition.