Diagnosis:
Usually made by MRI imaging
Findings on investigations:
MRI:
- Extraaxial. Supratentorial or infratentorial. Supratentorial; middle cranial fossa & third ventricle, perisellar cistern, along the convexities. Infratentorial; retrocerebellar cistern, cerebellopontine angle cistern, quadrigeminal cistern. Rarely intraventricularly. Mild mass effect.
- isointense with CSF on T1 & T2.
- Nonenhancing
CT: isodense with CSF
Intrathecal contrast: initially nonenhancing, but takes up enhancement later on.
Pathology:
Histology:
- Thin walled. Lined by arachnoid cap cells (meningothelial cells)
Immunohistochemistry: positive for= EMA. Negative for =CEA & GFAP.