= subset of meningeal cysts

Cause: posttraumatic, postinfectious, congenital diverticulum, idiopathic (arachnoid herniation through dural defect /abnormal distribution of arachnoid trabeculations)

Histo: lined with fibrous tissue + scattered meningothelial cells

Age: 15–45 years (in 77%); range of several months to 80 years; M=F

Relationship to CSF:

1. encapsulated completely separate from CSF
2. communication with subarachnoid space via a neck ± one-way valve

• pain (waxing + waning), sensory changes, urinary dysfunction, weakness
• progressive spastic / flaccid paresis improving in supine position + worsening during Valsalva maneuver
• symptomatic with compression of spinal cord / nerve root ← arachnoid cyst expansion (osmotic gradient, ball-valve mechanism, active secretions from cyst lining)
• oval sharply demarcated extramedullary mass
• filling with intrathecal contrast material depending upon size of opening between cyst + subarachnoid space:
  initially in 50%, nearly 100% with delayed imaging
  DDx: epidermoid cyst (asymmetric filling defect)
• local displacement + compression of spinal cord
• higher SI than CSF (from relative lack of CSF pulsations)
• NO enhancement

MR:

• iso- to hyperintense to CSF on T1WI + T2WI
• variability in signal intensity ← pulsatility of CSF / higher protein content
• isointense to CSF on FLAIR MR images
• NO restricted diffusion at DWI

Types:

• I extradural arachnoid cyst without nerve root fibers
• Ia extradural arachnoid cyst
• Ib sacral meningocele
• II extradural meningeal cyst containing neural tissue (= Tarlov cyst)
• III intradural arachnoid cyst

Pitfalls of CT myelography / MRI:

1. Nonopacification of noncommunicating arachnoid cyst → misidentification as a different type of lesion
2. Absence of mass effect on cord → failure to detect additional cysts / small CSF-isointense arachnoid cyst

DDx: traumatic / lateral thoracic meningoceles

Cx: spinal cord myelopathy, nerve root radiculopathy