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Sacral Extradural Meningeal Cyst: Is It an Incidental Finding? 2006

General Spine

Peter _ Yoo, MD, Non ASSR Member
Shane Lee, BA, Non ASSR Member
Nikhil Bhagat, MD, Non ASSR Member
Huey-Jen Lee, MD, Non ASSR Member

Scientific Paper

Purpose

(1) To determine the incidence of extradural meningeal cysts in adult patients who underwent routine magnetic resonance imaging (MRI) of the lumbar spine for lower back pain. (2) To describe the current classification of sacral meningeal cysts and its postulated pathophysiology. (3) Literature review of the clinical significance and management of sacral extradural meningeal cysts.

Methods & Materials

A total of 584 MRI studies of the lumber spine were retrospectively reviewed. The studies were performed for the evaluation of lower back pain and were performed between May 2005 and November 2005. Pediatric and trauma cases were excluded. Ages ranged from 18 years to 80 years. Routine MRI of the lumbar spine was performed on a 1.5 Tesla GE scanner. The study consists of sagittal T1, fast spin echo (FSE) and short TI inversion recovery (STIR) sequences of the lumbar and sacral spine. Axial proton and T2 weighted images of the disc spaces between L1 and S1 are also obtained. Post gadolinium axial and sagittal T1 weighted images are only obtained in post-spinal surgery patients.

Results

A total of 7 sacral extradural meningeal cysts were identified (1.2 %). 4 of the patients were male and 3 were female with a mean age of 49 years (range 32 years to 70 years). 5 were determined to be type I and 2 were indeterminate. All of the cysts occurred in the sacral spine. 4 cases also demonstrated coexistent disc disease of the lumbar spine and 1 case demonstrated a compression fracture of L5. 2 cases demonstrated no concurrent disc disease. Of these two patients, one patient presented with sacral and lower extremity pain with paresthesia while the other patient complained of nonspecific lower back pain.

Conclusion

The prevalence of sacral extradural meningeal cysts in our study is 1.2%. Sacral meningeal cysts have been classified into three categories: extradural cysts not containing nerve root fibers (Type I), extradural cysts containing nerve root fibers (Type II) and intradural cysts (Type III). The exact mechanism of formation of sacral extradural meningeal cysts is unknown. It has been postulated that the arachnoid membrane herniates through a dural defect, which is most likely congenital. Sacral extradural meningeal cysts are generally considered to be incidental findings. However, there are several published case reports that have attributed symptoms to these lesions and these patients have undergone surgical intervention. In our study, 2 out of the 7 cases demonstrated no coexistent disc disease. Therefore, the clinical symptoms may be related to the sacral cysts. In light of these reports and our findings, sacral extradural meningeal cysts should not be easily dismissed as an incidental finding.

References

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