Their presence in utero and their high prevalence in children who have no history of trauma support the assumption that they are congenital 1) 2) 3) , although there is some question as to how early in development the cysts are formed.
These cysts progressively enlarge from an abnormality in the membrane of Liliequist or in the interpeduncular cistern 4) 5).
Typically, expand from the prepontine space, displacing the floor of the third ventricle upwards, the pituitary stalk and optic chiasm upwards and forwards, and the mammillary bodies upwards and backwards. As the cyst increases in size it fills and occludes the third ventricle, and distorts and blocks the aqueduct, which finally results in hydrocephalus.
A slit-valve mechanism seems to be at least one factor of the genesis of suprasellar prepontine arachnoid cysts 6).
In postraumatic cases one hypothesis is that the inflammatory process due to subdural hematoma may locally result in arachnoiditis, and thus to the creation of a neomembrane, and eventually to cyst formation 7).