SPINAL ARACHNOID CYST

Sagittal and Axial T2 wt images of lumbar spine showing a well defined oval fluid intensity extradural lesion with few internal septations in left lateral aspect of thecal sac from D12 to L2 level with extension into neural foraminae at D12-L1 and L1-2 levels.
Mild smooth displacement of thecal sac to right side.

According to the classification described by Nabors et al for spinal meningeal cysts, extramedullary cysts of the spinal canal can be divided into three main groups.

Group I : Meningeal cysts.

                  Classified into three types:

1.     Extradural meningeal cysts that contain no neural tissue (arachnoid cysts-Ia and sacral meningoceles-Ib)

2.     Extradural meningeal cysts that contain neural tissue (tarlovs cyst, cord herniation into cyst)

3.     Intradural meningeal cysts.

Group II : Nonmeningeal epidural cysts

(includes nonneoplastic lesions such as juxtaarticular synovial cysts, pigmented villonodular synovitis, and, rarely, herniated disks, as well as neoplastic lesions such as dermoids, cystic nerve sheath lesions, and metastases. In addition, trauma, hemorrhage, and inflammation can produce cystlike changes that are visible on cross-sectional imaging)

Group III : Neurenteric cysts.

Spinal Arachnoid cyst:

·        Probably arise from the herniation of the arachnoid through congenital or acquired dural defects.

·        The cysts begin as diverticula and subsequently enlarge, possibly because of a valvelike mechanism.

·        Postural changes and Valsalva's maneuver may produce symptoms.

·        These cysts usually arise in the mid-to-lower thoracic spine, often at the junction of the meninges and the dorsal nerve root sleeves proximal to the dorsal root ganglia.

·        Type 1A lesions most commonly project dorsally and may partially protrude into the adjacent neural foramen.

·        “Arachnoid cyst” is a histologic diagnosis, and the cyst has either no epithelial lining or an attenuated lining of arachnoidlike cells.

·        In the imaging literature, the terms “arachnoid cyst” and “meningeal cyst” are used interchangeably.

Reference : CT Myelography and MR Imaging of Extramedullary Cysts of the Spinal Canal in Adult and Pediatric Patients, AJR January 2002 vol. 178 no. 1 201-207