[Intracranial arachnoid cyst and porencephaly of the middle fossa--its clinical and neuroradiological study].

We reported four cases with well demarkated low density area in the middle cranial fossa, which was not enhanced with contrast medium and had the same absorption coefficient as the CSF. The operations and histological examinations revealed that two cases were arachnoid cysts and the others were porencephalic cysts. The clinicoradiological differential clues are listed below. 1) The porencephaly has intimate relation with focal neurological signs. 2) The thinning and bulging of the temporal bone are not a specific finding of an arachnoid cyst. Hemicranial atrophy, elevation of minor sphenoid wing and dilatation of paranasal sinuses in addition to focal bulging and thinning are indicative of porencephalic cyst. 3) Hemiatrophy of the brain and dilatation of the lateral ventricle on CT as well as the absorption coefficient of the brain surface facing the cyst are important finding of the porencephaly in differentiation. 4) The investigation of CSF dynamic is by no means helpful in differentiation between arachnoid cyst and porencephaly except intraarachnoid cyst. 5) The horizontal portion of the middle cerebral artery is not elevated in both two porencephalic cases. One of them showed a slight shift of anterior cerebral artery toward the affected side. An obstruction of the vessel is an important finding in porencephaly. 6) The electroencephalography can clearly demonstrate a porencephalic cyst as a localized slow focus, whereas it is not so remarkable in an arachnoid cyst.