Pathophysiology
Cranial cysts
Distribution of Intracranial Arachnoid Cysts
| Localization | Prenatal (n=54) | Pediatric (n=67) | Elderly (n=23) |
| Supratentorial | 34 | 31 | 20 |
| Middle Fossa | 1 | 13 | 11 |
| Interhemispheric | 16 | 9 | |
| Hemispheric | 1 | 5 | 9 |
| Suprasellar | 5 | 4 | |
| Skull base | 7 | ||
| Intraventricular | 5 | ||
| Infratentorial | 12 | 31 | 3 |
| CPA | 1 | 1 | |
| Hemispheric | 1 | 22 | |
| Other | 10 | 8 | |
| Tentorial incisure | 8 | 5 |
Modified from Westermaier et al. (25).
- No unifying theory for origin: Many theories exist about the origin of arachnoid cysts, including infection, trauma, hydrocephalus, and metabolic derangements, but no unique explanation has been accepted (25).
- Splitting of arachnoid membrane: Arachnoid cysts occur between layers of the arachnoid membranes. This formation is possibly the result of duplication and splitting of the arachnoid endomeninx, a germinal tissue that normally develops into the pia and arachnoid layers as well as the subarachnoid space.
- Growth possible over time: Arachnoid cysts may enlarge over time. The reasons for growth are poorly understood, with many theories having been proposed including CSF pressure gradient / form fruste of hydrocephalus, unidirectional ball-valve mechanism, and oncotic gradient from increased proteins in cyst or secretory cyst membranes(25).They rarely grow in children after the age 4 years (1).
- Hydrocephalus may be present: The arachnoid cyst may be related to hydrocephalus or anomalous CSF circulation.
- Rupture possible with related hemorrhage: Arachnoid cysts may rupture from even minor trauma. Rupture may result in hemorrhage from torn bridging veins.
Spinal cysts
- Most thoracic or cervical: Spinal arachnoid cysts are usually found in the thoracic and cervical regions, posterior to the spinal cord. They may be intradural/extramedullary or extradural, with extradural cysts predominating.
- Intradural cysts due to splitting arachnoid: These cysts may be developmental, due to splitting of the arachnoid, or they may be due to loculations of CSF that have arisen due to scarring and inflammation occurring after hemorrhage or infection.
- Extradural cysts associated with dural defect: These cysts may be due to congenital or traumatic defects in the dura, with herniation of the arachnoid into the extradural space. They can rupture from trauma.
Genetic Pathology
- Genetic studies positive: Several studies examining cranial cysts in siblings and twins and one study examining spinal cysts in a family have suggested that there could be a genetic pathology for the cysts; however, no exact genetic cause has been isolated (11, 22, 23).
Histopathology
- Normal arachnoid: The walls of the cysts consist of normal arachnoid.
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