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Arachnoid Cyst

Summary

  • Congenital benign, non-neoplastic, fluid-filled cysts within the arachnoid membrane of the brain or spinal cord
  • Almost always incidental and asymptomatic although symptoms are possible if there is increasing mass effect or CSF obstruction

Pathophysiology

  • Developmental splitting or duplication of the arachnoid membrane
  • Accumulation of CSF within the cyst due to:
    • One-way valve mechanism
    • Active fluid secretion by the cyst wall
    • Osmotic gradient
  • May enlarge over time, causing compression of adjacent structures

Demographics

  • Prevalence: ~1-2% and more common in males
  • Most common locations:
    • Middle cranial fossa (50-60%)
    • Posterior fossa (10-20%)
    • Suprasellar region (10-15%)
    • Convexity (5-10%)

Imaging

  • CT:
    • Well-defined, extra-axial, CSF-density lesion
    • May cause local mass effect or remodelling of adjacent bone
  • MRI:
    • CSF signal on all sequences (T2 hyperintensity with complete signal suppression on FLAIR)
    • No soft tissue or post-gadolinium enhancement
    • The wall is rarely seen although potentially visible on high resolution imaging (e.g., CISS)

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  • An incidental cystic lesion to the right side of the medulla caused bowing of the lower cranial nerves.
  • There is no soft tissue component or diffusion restriction to suggest anything other than an arachnoid cyst.

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  • Incidental finding of a very large CSF-signal cyst centred on the suprasellar cistern.

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  • Incidental expansion of the subarachnoid space at the vertex.
  • Only clearly identified on FLAIR where there is greater CSF suppression within the static CSF within the cyst that the pulsating CSF elsewhere

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  • A 30-year-old patient with worsening headaches had an arachnoid cyst in the left middle cranial fossa.
  • The cyst decreased in size following fenestration.

Treatment

  • Usually no treatment is necessary
  • If responsible for symptoms, they can be fenestrated to allow free communication of CSF between the cyst and the subarachnoid space

Differential diagnosis

Differential Diagnosis Differentiating Feature
Epidermoid cyst Diffusion restriction on MRI; irregular margins
Porencephalic cyst Associated with brain parenchymal injury; communicates with ventricles
Subdural hygroma Recent history of trauma; crescent-shaped appearance
Mega cisterna magna Normal anatomical variant; no mass effect