Diagram of the cerebellum, fourth ventricle and pons. The white arrow shows the foramen of Magendie (medial aperture) connecting the fourth ventricle to the cisterna magna (3). This usually remains open in DWM. The cerebellum begins forming at the fifth week of embryonic development. It differentiates at the top of the metencephalon, while the pons (in the brainstem) differentiates at the bottom, separated by the fourth ventricle. The cerebellar hemispheres form from the rhombic lips on the forward surface of the fourth ventricle, which expand and roll over to fuse in the midline to form the cerebellar vermis by the 15th week. If this process does not complete, the cerebellar vermis will not form fully. This long period of development of the cerebellar vermis makes it particularly vulnerable to disruptions. In DWM, the fourth ventricle opens up into and is continuous with almost the entire posterior fossa subarachnoid space. Pathophysiology of hydrocephalus The reason why hydrocephalus occurs in DWM is not yet fully understood. The earliest authors had put it down it to blockage or narrowing of the foramina of Magendie and Luschka, the two apertures in the fourth ventricle that allow cerebrospinal fluid (CSF) to escape into the subarachnoid space of the posterior fossa. However, later studies found that these foramina are usually open in DWM. Hydrocephalus is also usually (80% of the time) not present at birth in those with DWM. The impairment to CSF flow may lie beyond the outlets of the fourth ventricle. Theories of abnormal development or inflammation of the arachnoid mater in the posterior fossa have been put forward. The arachnoid mater contains granulations necessary to return CSF from the subarachnoid spaces to the dural veins and circulation. Excisions of the cyst in DWM have not been able to show whether impaired arachnoid absorption is involved, since the subarachnoid space always takes days to weeks to fill up following excision. Aqueductal stenosis (narrowing of the passage between the third and fourth ventricles) does not seem to be a factor in DWM. It is usually open, and shunts placed in the posterior fossa cyst almost always drain all above ventricles. When it is present, it may be the result of compression from a herniated vermis or cyst or an associated developmental abnormality. It is known that once hydrocephalus has started, the compression by the posterior fossa cyst against the venous passages in the arachnoid mater is involved in the worsening pathology.
Dandy–Walker malformation
Glial tumor
Subdural hygroma
Schizencephaly
Chiari malformation
Hemimegalencephaly
Subependymoma
Hydranencephaly