Occult intrasacral meningocele, or more simply sacral meningocele, is a rare congenital lesion characterized by an extradural arachnoid cyst within the sacral canal, classified as a Nabor type Ib meningeal cyst.
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Terminology
Other terms for this entity in the literature include intrasacral meningocele, intrasacral cyst, intrasacral extradural arachnoid cyst, intraspinal meningocele, and giant sacral meningeal diverticula 7.
Epidemiology
It usually presents in childhood 7.
Associations
It is associated with spinal dysraphism, tethered cord syndrome and Tarlov cysts.
Clinical presentation
Cases are often incidentally found but some have symptoms of bowel or bladder dysfunction or back pain related to nerve root compression 7.
Pathology
Intrasacral meningoceles result from arachnoid diverticulum that has herniated out of a congenital defect in the dura but that remains confined within the sacral canal 7.
Radiographic features
CT
Typically seen as a hypoattenuating lesion (isodense to CSF) enlarging the sacral thecal sac, which may displace the nerve roots.
MRI
MRI is the best modality to evaluate an occult intrasacral meningocele. The location is central to the sacral canal but extradural (adjacent and external to the distal thecal sac). The signal characteristics are typical of an arachnoid cyst:
- T1: hypointense (follows CSF signal)
- T2: hyperintense (follows CSF signal)
- T1C+ (Gd): no enhancement
- DWI: hypointense, no restricted diffusion
Provided the wall is thin and smooth, contrast agent administration is not indicated for evaluation 7.
High resolution T2-weighted sequences can be helpful to confirm the absence of neural elements within the cyst and identify the pedicle of the cyst where it takes off from the thecal sac near midline 7.
Differential diagnosis
General imaging differential considerations include
-
Tarlov cyst
- arises more laterally along a nerve root, although the distinction may be difficult if the lesion is large, and can remodel the neural foramen
- dorsal meningocele
- dural dysplasia
- caudal regression syndrome: only when the cyst is very large, but easily distinguishable on MRI
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