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.