Spinal Extradural Arachnoid Cysts: Clinical, Radiological, and Surgical Feature

James K. Liu, M.D.; Chad D. Cole, M.D.; Peter Kan, M.D.; Meic H. Schmidt, M.D.

Disclosures

Neurosurg Focus. 2007;22(2) 

In This Article

Neuroimaging Features

Magnetic resonance imaging appears to be effective as an initial modality for diagnosing arachnoid cysts and does not require the intrathecal injection of contrast medium.[22] It can define the anatomical relationship to surrounding structures. The imaging characteristics of arachnoid cysts are similar to those of CSF signal intensity (Fig. 3). Epidural fat capping of the lesion at its superior and inferior poles can be seen on sagittal T1-weighted MR images, which further suggests its extradural location. The presence of vertebral body scalloping and expansion of the neural foramina bilaterally from osseous remodeling suggests longstanding mass effect from the lesion.

Sagittal T1-weighted (A) and T2-weighted (B) and axial T1-weighted (C) and T2-weighted (D) MR images obtained in a 20-year-old woman who had presented with 2 months of lower back pain and neurogenic claudication, demonstrating a large spinal extradural arachnoid cyst (white arrows) extending from T-11 to L-2. Note the severe dorsal compression and flattening of the thecal sac. Epidural fat capping can be seen at the superior and inferior poles of the cyst (A). The scalloping and remodeling of the inner osseous margins of the spinal canal with bilateral expansion of the neural foramina is suggestive of longstanding mass effect. There are no obvious communications between the cyst and the intrathecal subarachnoid space, although one is suspected.

Note, however, that MR imaging may not demonstrate a communication between the cyst and subarachnoid space. Historically, routine water-soluble myelography has been used as the initial study and usually has demonstrated a filling defect at the dural diverticula.[16] Filling of the cyst through the communicating pedicle may not always be visualized, however, particularly if the patient is placed prone.[24] The diagnostic study of choice for demonstrating the communication with the subarachnoid space is CT myelography (Fig. 4). Some authors have recommended delayed CT scanning—performed approximately 8 hours after metrizamide myelography—to visualize the communication.[8] In our practice, we obtain one CT scan immediately after myelography, one 3 hours later, and one the following morning. We have found that delayed CT scanning allows more contrast filling of the cyst for better visualization. Demonstration of this communication allows for an accurate diagnosis of a Type IA spinal extradural arachnoid cyst. Magnetic resonance imaging is valuable in diagnosing a cyst, whereas CT myelography is useful in detecting the communication of the cyst with the subarachnoid space.[8,12,15] Longstanding hydro static pressure within the cyst can cause osseous changes in the spine, such as widening of the interpedicular distance, erosion of the pedicle and posterior elements, scalloping of the posterior aspects of the vertebral bodies, and kyphoscoliosis.[8,10]

Sagittal (A) and axial (B and C) delayed CT myelograms obtained in the patient whose images appear in Fig. 3, showing CSF blockage at the middle L-3 level due to a smoothly marginated extradural cyst extending from T-11 to L-2. There is filling of the cyst on these delayed images, which was not seen on the immediate post-CT myelography, suggesting a communication between the thecal sac and the cyst. The conus medullaris and cauda equina are compressed anteriorly. Extensive osseous remodeling and posterior body scalloping are also apparent. The cyst also extends through the neural foramina bilaterally.

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