Description

• 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

Epidemiology

• 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).

Prevalence
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.

Genetics
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