Arachnoid cysts are congenital, benign, intra-arachnoidal lesions which typically have thin walls and contain clear, colorless fluid that resembles CSF. Microscopically, flattened arachnoid cells line them. These lesions may be loculated or communicate with adjacent subarachnoid cisterns. Etiology of these lesions is controversial and not well understood.

The majority of arachnoid cysts occur in the middle cranial fossa (50-65%), suprasellar cistern (5-10 %), and quadrigeminal cistern (5-10%). 5-10% occur in the posterior fossa (CPA angle and cisterna magna) and 5% occur around the cerebral convexities. They are space-occupying lesions that vary in size and may result in compression of adjacent structures. Temporal lobe hypogenesis is common with middle cranial fossa cysts.

Arachnoid cysts occur present at any age, but 75% are seen in children. They are more common in males (3:1 male: female ratio). Patients may present with seizures, headaches or other focal neurologic signs. These lesions may enlarge but the mechanism by which this is accomplished is controversial. Some suggest that lesions may have secretory capacity while others suggest entrapment of CSF by a ball-valve mechanism. Potential complications include intracystic or subdural hemorrhage.

Plain CT demonstrates nonenhancing, noncalcified, and smoothly marginated, extra-axial mass with attenuation similar to CSF. On MR, these lesions follow CSF signal intensity on all pulse sequences, have no internal architecture, and are nonenhancing. Hemorrhage and/or abnormal protein may alter the CT and MRI appearance of these lesions. Arachnoid cysts may demonstrate mass effect on adjacent structures.

Differential diagnosis includes epidermoid tumor, cystic tumors, infarcts, open-lip schizencephaly, and loculated chronic subdural hygromas. Of these entities, epidermoid tumor is the most difficult to differentiate from arachnoid cysts. Epidermoids may follow CSF signal intensity on all pulse sequences, but most are slightly hyperintese on proton density images. Epidermoid tumors commonly engulf arteries and cranial nerves whereas an arachnoid cyst displaces adjacent structures. Epidermoids may also show some fine wisps of internal structure.



DDX FOR CPA CISTERN MASSES:

COMMON
-   Vestibular Schwannoma, meningioma, epidermoid, vascular
ectasia/aneurysm, other schwannomas (e.g. trigeminal )

UNCOMMON
-   Metastases, paraganglioma, arachnoid cyst, lipoma,
meningitis