Arachnoid cyst

20 year old gentleman came to the ER following a motor vehicle accident. Routine CT brain was asked for to rule out any internal injury.



  • no intra axial or extra axial hemorrhage.
  • no calvarial fracture.
  • large extra axial CSF attenuating lesion is seen occupying the left middle cranial fossa splaying the sylvian fissure and displacing the frontal, parietal and temporal lobes.
  • secondary mass effect is seen in the form of effacement of left cerebral hemispheric sulci, mild midline shift to right,  effaced ipsilateral lateral ventricle.
  • no transtentorial herniation.
  • the incidental nature suggested a benign nature of the lesion and an MRI was asked for confirmation


Coronal T2


  • non enhancing nature of lesion.
  • CSF attenuation.
  • extra axial location
    • displaced cortical mantle and cortical vessels between lesion and calvarium
  • no underlying white matter changes in left cerebral hemisphere.

Key points

Most common site of origin of the arachnoid cyst is in the middle cranial fossa, splaying the Sylvian fissure. Depending on the size and extent, it is classified by the Galassi classification. This a type III variant.

They tend to be asymptomatic and pounced upon incidentally. The larger ones may cause symptoms secondary to mass effect.