Specific Imaging Findings

Arachnoid cyst (AC) is an extra-axial mass that follows the CSF appearance on all imaging modalities. It is sharply demarcated and displaces brain parenchyma with buckling of the adjacent intact cortex. It also displaces arteries and cranial nerves and frequently leads to thinning and remodeling of the overlying bone. AC is T1 hypointense, hyperintense on T2WI and ADC maps, dark on FLAIR and DWI and does not enhance with contrast. Arachnoid cysts are also CSF-like hypodense on CT. Hemorrhage may rarely occur within the cyst which changes the imaging appearance, depending on the blood products degradation state and proteinaceous contents. AC has a predilection for the middle cranial fossa followed by the suprasellar cisterns, cerebellopontine angle, convexities and quadrigeminal plate region.

Pertinent Clinical Information

AC is typically an incidental imaging finding, but it has been sporadically associated with headache, dizziness, sensorineural hearing loss and hemifacial spasm, depending on its location. There is increased prevalence of subdural hematomas in patients with middle cranial fossa AC. Mass effect from midline and posterior fossa ACs may lead to obstructive hydrocephalus and suprasellar AC may also cause endocrine symptoms, such as hyperprolactinemia and precocious puberty. Treatment is usually not necessary and ACs may recur following surgery. When needed, fenestration, endoscopic resection and cystoperitoneal shunt are possible options.

Differential Diagnosis

Epidermoid Cyst (143)

1. • engulfs vessels and cranial nerves with insinuating growth pattern

2. • usually heterogenous and brighter than the CSF on FLAIR images

3. • characteristically very bright on DWI (and signal similar to brain on ADC maps)