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. Diagnosis
Dandy–Walker malformation is diagnosed based on the characteristic neuroimaging findings.
If this occurs, a suboccipital decompression with duraplasty may be attempted.In DWM, it is not agreed whether a shunt should be placed in the fourth ventricle (a cystoperitoneal shunt, or CP shunt), the lateral ventricles (a ventriculoperitoneal shunt, or VP shunt) or both, due to conflicting studies on whether the cerebral aqueduct is affected by the malformation. However, a CP shunt almost always drains both the fourth and lateral ventricles in DWM, and according to strict definitions of the malformation, the aqueduct should be assumed open, though imaging is important to confirm this. Many authors therefore recommend the CP shunt as the logical option. However, it is associated with a high rate of complications, including shifting and overdrainage. Overdrainage can lead to subdural haematomas, a tethered spinal cord, due to scarring, or downward herniation of the cerebral hemispheres. Spennato et al. therefore recommend a flow-regulating or anti-syphon valve. On the other hand, VP shunts have a lower rate of complications than CP shunts and are recommended initially by some. However, they are less effective in DWM, and the elevated position of the tentorium should be considered before installing a VP shunt.In patients older than 1 year, endoscopic third ventriculostomy (ETV) may be considered as the first-line treatment. This less invasive procedure creates an artificial hole in the third ventricle to allow CSF to bypass any obstruction. It cannot be used on those with brain abnormalities such as agenesis of the corpus callosum, due to the risk of CSF escaping to other brain areas.