Arachnoid Cyst



  • Histologically benign, cerebrospinal fluid (CSF)-filled, space-occupying lesion of the central nervous system
  • May be intracranial (most are supratentorial), spinal, or perineuronal
  • Classification (1)
    • Primary (congenital)
      • Intra- or extradural, found around the brain or spine
      • Often no communication with the subarachnoid space
      • Contains clear CSF-like fluid
      • Cyst walls are meningothelial cells often capable of secreting CSF.
    • Secondary (acquired)
      • CSF sequestration due to inflammatory or traumatic processes
      • Surrounded by arachnoid scarring
      • Contains CSF and possibly hemosiderin-stained or inflammatory cells
      • Usually communicates with the subarachnoid space
  • System(s) affected: central nervous; peripheral nervous
  • Synonym(s): leptomeningeal cysts


  • Diagnosed most frequently in the first 2 decades of life (1)
  • 2:1 male predominance
  • Middle cranial fossa cysts are most common, with an increased incidence on the left side (2,3).

0.2–1.7%, although as imaging becomes more common, a prevalence up to 2.9% is suggested, with peaks at age 1 and 5 years (3).

Etiology and Pathophysiology

  • Several theories of arachnoid cyst formation (4)
    • Agenesis of part of the brain
    • Defect in the arachnoid development, leading to membrane splitting or duplication creating diverticula
    • Failure of temporal embryonic meninges to merge as the sylvian fissure forms
    • Defect in condensation of the mesenchyme from abnormal CSF flow
    • Arachnoid adhesions due to trauma, meningitis, mastoiditis, subarachnoid hemorrhage, or iatrogenic causes
  • Cyst growth also has many suggested causes (4):
    • One-way ball-valve allowing CSF to enter but not exit the cyst (CSF obstruction and vent mechanism) (5)[A]
    • Distribution of arachnoid trabeculae leading to trapping of fluid in the cyst
    • Fluid production by cells lining the walls of the cyst
    • Osmotic gradients due to higher protein content of cyst fluid (6)
  • Symptoms are due to compression and mass effect on adjacent brain or spinal cord structures, interruption of CSF flow, or intracyst/subdural hemorrhage.
  • Primary arachnoid cysts are congenital.
  • Secondary arachnoid cysts are associated with inflammation and trauma.

No specific genetic association with arachnoid cysts, although bilateral cysts may occur in Hurler syndrome

Risk Factors

  • Primary arachnoid cysts are congenital.
  • Risks for secondary arachnoid cysts
    • Inflammation from infection, hemorrhage, or iatrogenic causes
    • Trauma from lumbar puncture, anesthetic procedures, or intradural surgery

Commonly Associated Conditions

Rarely, associated with the following:

  • Hydrocephalus
  • Chronic subdural hematoma
  • Dysgenesis of the corpus callosum
  • Malformation of the cerebral venous system
  • Glutaric aciduria type I
  • Trisomy chromosome 12
  • Neurofibromatosis
  • Autosomal dominant polycystic kidney disease

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