Pediatric Arachnoid Cyst

  • Etiology:
    — Cerebrospinal fluid containing cyst covered by arachnoid membrane
    — Can be primary or secondary
  • Imaging:
    — Circumscribed cerebrospinal fluid collection with a large range of sizes
    — Anterior middle cranial fossa most common but can be anywhere
    — Most do not change substantially over time especially in children greater than 4 years old
  • Imaging MRI:
    — Follows cerebrospinal fluid signal on all sequences
    — DWI: Does not restrict
  • DDX:
  • Complications:
    — Can rupture after mild trauma causing subdural hygroma and cysts larger than 5 centimeters are more likely to rupture
    — Intracystic hemorrhage after trauma rare
    — Although cyst rupture is rare contact sports may be restricted with large cysts
    — Spontaneous rupture of cyst causing subdural hygroma
  • Treatment:
    — Mass effect alone not indication for surgery
    — Indicated if causing clear and specific neurologic symptoms
  • Clinical:
    — Very common – in 2% of scans
    — Usually asymptomatic and incidental

Radiology Cases of Arachnoid Cyst

MRI of hemispheric arachnoid cyst
Axial T2 (left) and coronal T1 (right) MRI without contrast of the brain show a large extra-axial cystic structure in the right hemisphere causing significant mass effect to the left.
CT of arachnoid cyst of the posterior fossa
Axial CT without contrast of the brain shows a cystic structure on the left side of the posterior fossa that is shifting the cerebellum, which is structurally normal, to the right. The fourth ventricle is compressed leading to a moderate amount of hydrocephalus.

Radiology Cases of Sacral Arachnoid Cyst

MRI of arachnoid cyst of the sacrum
Sagittal color doppler US of the left (above left) and right (below left) testicles shows normal appearing color doppler flow to each testicle which was confirmed on spectral dopper US. Sagittal T2 MRI without contrast of the lumbar spine (right) shows a large cylindrical structure filling the sacral spinal canal from S1-S5 whose signal intensity followed cerebrospinal fluid on all sequences.