Spinal Arachnoid Cyst¶

Definition¶

Spinal arachnoid cysts are CSF-filled cysts within or adjacent to the thecal sac. They can be intradural (most common) or extradural, and congenital or acquired (post-traumatic, post-inflammatory). They are most common in the thoracic spine and can cause cord compression or myelopathy if large.

Imaging Findings¶

MRI¶

CSF-signal cyst (follows CSF on all sequences — T1 dark, T2 bright, FLAIR suppressed)
Located within the thecal sac (intradural) or outside the dura (extradural)
Intradural cysts are usually dorsal to the cord
May cause focal cord compression and displacement
No enhancement (unless associated with inflammation or infection)
Cine MRI may show flow dynamics within the cyst

CT Myelography¶

Filling pattern helps characterize the cyst communication with the subarachnoid space
Intradural cysts may fill immediately or with a delay depending on the communication

Distinguishing from Other Cystic Lesions¶

Arachnoid cyst — CSF signal, no enhancement, no solid component
Epidermoid — Bright on DWI (restricted diffusion), does NOT suppress on FLAIR
Neurenteric cyst — May be hyperintense on T1 (proteinaceous content), prevertebral location
Tarlov cyst — Sacral, involves nerve root

Clinical Pearl

A dorsal intradural CSF-signal collection causing cord compression in the thoracic spine is most likely an arachnoid cyst. The key distinction from other intradural cysts: arachnoid cysts follow CSF on all sequences (including FLAIR suppression and no DWI restriction). Epidermoid cysts look similar on T1/T2 but show restricted diffusion on DWI and do not suppress on FLAIR.

Key Points¶

CSF-signal intradural or extradural cyst, most common in thoracic spine
Can cause cord compression and myelopathy
Follows CSF on all sequences including FLAIR (distinguishes from epidermoid)
Intradural cysts are usually dorsal to the cord
Surgical fenestration or excision for symptomatic cysts

References¶

Nabors MW, Pait TG, Byrd EB, Karim NO, Davis DO, Kobrine AI, Rizzoli HV. Updated assessment and current classification of spinal meningeal cysts. J Neurosurg. 1988;68(3):366-77. doi:10.3171/jns.1988.68.3.0366. PMID:3343608.
Liu JK, Cole CD, Kan P, Schmidt MH. Spinal extradural arachnoid cysts: clinical, radiological, and surgical features. Neurosurg Focus. 2007;22(2):E6. doi:10.3171/foc.2007.22.2.6. PMID:17608349.
Ahmed AK, Anisetti B, Huynh T, Agarwal A, Gupta V, Desai A, Singh R, Vibhute P. Clinical and imaging features of spinal extradural arachnoid cysts: a retrospective study of 50 cases. Neuroradiology. 2022;64(12):2409-2416. doi:10.1007/s00234-022-03042-4. PMID:35970946.
Kalsi P, Hejrati N, Charalampidis A, Wu PH, Schneider M, Wilson JRF, Gao AF, Massicotte EM, Fehlings MG. Spinal arachnoid cysts: A case series & systematic review of the literature. Brain Spine. 2022;2:100904. doi:10.1016/j.bas.2022.100904. PMCID:PMC9560677.
Spinal arachnoid cyst. Radiopaedia.org. Available from: https://radiopaedia.org/articles/spinal-arachnoid-cyst
Classification of spinal meningeal cysts. Radiopaedia.org. Available from: https://radiopaedia.org/articles/classification-of-spinal-meningeal-cysts-1