Occult intrasacral meningocoele is a rare congenital lesion characterised by the presence of a cyst within the sacral thecal sac. It is an extradural sacral arachnoid cyst- Nabor's type Ib meningeal cyst. It is not a true meningocoele, since meninges are not involved.
It is associated with spinal dysraphism, tethered cord syndrome and Tarlov cysts. Most of the time, it is a fortuitous finding on MRI in teenagers or the elderly, but very large cysts can be symptomatic. It is rarely diagnosed in children.
Typically seen as a hypoattenuating lesion (isodense to CSF) enlarging the sacral thecal sac, which may displace the nerve roots.
MRI is the best modality to evaluate an occult intrasacral meningocoele. It consists of an extradural sacral arachnoid cyst, where signal characteristics are typically:
- T1: hypointense (follows CSF signal)
- T2: hyperintense (follows CSF signal)
- T1C+ (Gd): no enhancement
- DWI: hypointense, no restricted diffusion
General imaging differential considerations include
- Tarlov cyst
- dorsal meningocele
- dural dysplasia
- caudal regression syndrome: only when the cyst is very large, but easily distinguishable on MRI
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