see also Posterior fossa tumor.
see also Cerebellar hemangioblastoma.
Cystic or cyst-like malformations of the posterior fossa represent a spectrum of disorders, including the Dandy-Walker malformation, cerebellar vermis hypoplasia, mega cisterna magna, and posterior fossa arachnoid cyst. Differentiation of these lesions may be difficult with routine cross-sectional imaging; however, an accurate diagnosis is essential for proper treatment planning and genetic counseling. Dandy-Walker malformation is easily diagnosed on the basis of the classic triad: complete or partial agenesis of the vermis, cystic dilatation of the fourth ventricle, and enlarged posterior fossa. Vermian-cerebellar hypoplasia is a general classification that describes congenital malformations with a normal-sized posterior fossa, varying degrees of vermian and cerebellar hypoplasia, and a prominent retrocerebellar cerebrospinal fluid space that communicates freely with a normal or dilated fourth ventricle. Mega cisterna magna can be asymmetric and can manifest apparent mass effect, simulating the appearance of an arachnoid cyst; therefore, ventriculography or cisternography may be needed to demonstrate communication of the cystic mass with the subarachnoid space. A careful review of the embryologic development is essential in understanding these malformations and in making a more accurate radiologic diagnosis 1).
Posterior fossa intracranial cysts are usually divided into Dandy Walker malformations, posterior fossa arachnoid cysts, and isolated and/or trapped fourth ventricles.
Chiari I malformation can be due to a multitude of etiologies such as craniosynostosis or hydrocephalus. A posterior fossa extra-axial cyst (PFEAC) appears to be an extremely rare cause of this form of hindbrain herniation.
Khan et al. report a case of PFEAC that presented with no Chiari I malformation and then presented months later with a significant Chiari I malformation. Following shunt placement of a PFEAC, striking reversal of the Chiari malformation as well as reconstitution of the cerebellum was noted.
Patients with PFEAC might develop a Chiari I malformation and this might be treated with shunting of the PFEAC alone 2).
In cases with the following neuroimaging findings, surgery appears to be indicated: (1) occipital bossing or petrosal scalloping with distortion or obliteration of cerebrospinal fluid (CSF) cisterns of the posterior fossa; (2) compression and deformity of the brain surrounding the cyst; (3) radioisotope and/or computed tomography cisternographic findings suggestive of disturbance of intracystic CSF circulation; (4) a non-communicating cyst 3).