This usually presents within the first year of life (85% of the time), most often within the first 3 months. Signs of hydrocephalus in infants include increasing head size, vomiting, excessive sleepiness, irritability, downward deviation of the eyes (known as "sunsetting eyes") and seizures. In contrast to classic DWM, only around 30% of those with Dandy–Walker variant (DWV), in which the posterior fossa is not enlarged, have hydrocephalus. Neurological
Despite the hypoplastic cerebellar vermis, just over half of individuals with DWM (between 27% and 84%) do not appear to have significant intellectual disability or developmental delay. However, many of the genetic conditions associated with DWM can present with developmental delay and other brain anomalies. Agenesis of the corpus callosum has been found in between 5% and 17% of those with DWM. This does not seem to result in intellectual disability on its own, however. Other brain abnormalities known to be sometimes associated with DWM include grey matter heterotopia, pachygyria (fewer ridges in the brain), lissencephaly (shallower ridges), polymicrogyria, holoprosencephaly and schizencephaly. Individuals with these features tend to have developmental delay or seizures. Those without any other central nervous system abnormalities tend to have normal or close-to-normal intellectual development. A 2003 review found that moderate-to-severe intellectual disability and non-DWM brain abnormalities were only present in those with the most severe cerebellar vermis malformations (less than two fissures/three lobules in the vermis), and these comprised 16% of their sample.
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.