Arachnoid cysts
AKA leptomeningeal cysts, distinct from posttraumatic leptomeningeal cysts (AKA growing skull fractures), and unrelated to infection. Arachnoid cysts (AC) are congenital lesions that arise during development from splitting of arachnoid membrane (thus they are technically intra-arachnoid cysts) and contain fluid that is usually identical to CSF. They do not communicate with the ventricles or subarachnoid space. May be unloculated or may have septations. Typically lined with meningothelial cells positive for epithelial membrane antigen (EMA) and negative for carcinoembryonic antigen (CEA). AC may also occur in the spinal canal. “Temporal lobe agenesis syndrome” is a label that had been used to describe the findings with middle cranial fossa ACs. This term is now obsolete since brain volumes on each side are actually the same, bone expansion and shift of brain matter account for the parenchyma that appears to be replaced by the AC. Two types of histological findings:
- “simple arachnoid cysts”: arachnoid lining with cells that appear to be capable of active CSF secretion. Middle fossa cysts seem to be exclusively of this type
- cysts with more complex lining which may also contain neuroglia, ependyma, and other tissue types