Complications from overdrainage such as subdural haematomas are also possible and can lead to mortality. Shunts in the fourth ventricle (cystoperitoneal shunts, or CP shunts) have a generally high rate of successful cyst and ventricle size reduction, especially in the cyst (at least 80%). With a shunt in the lateral ventricles (ventriculoperitoneal shunt, or VP shunt), studies have generally found a roughly 50% successful cyst size reduction rate, with successful ventricle size reduction roughly two thirds of the time.Other systemic or genetic conditions are often present with DWM, and each have their own significant effect on prognosis. Epidemiology
The prevalence of DWM is estimated at between 1 in 25,000 to 1 in 50,000. DWM is the cause of around 4.3% of cases of congenital hydrocephalus and 2.5% of all cases of hydrocephalus.A 2017 review found that most patients (65%) were diagnosed with either "Dandy–Walker malformation" or "Dandy–Walker syndrome", while 20% were diagnosed with "Dandy–Walker variant" and 1.1% with "mega cisterna magna". History
The malformation was first described in 1887 by English surgeon John Bland-Sutton as hypoplasia of the cerebellar vermis, an enlarged posterior fossa and hydrocephalus. In 1914, American neurosurgeon Walter Dandy and American paediatrician Kenneth Blackfan described the malformation as partial or complete absence of the cerebellar vermis, an enlarged fourth ventricle and hydrocephalus.
Some scientists speculate that some forms of essential myoclonus may be a type of epilepsy with no known cause. Juvenile myoclonic epilepsy (JME) usually consists of jerking and muscle twitches of the upper extremities. This may include the arms, shoulders, elbows, and very rarely, the legs. JME is among the most common types of epilepsy and can affect one of every 14 people with the disease. These seizures typically occur shortly after waking up. Onset for JME can be seen around puberty for most patients. Administration of medications that also treat multiple seizure types is usually the most effective form of treatment. Lennox-Gastaut syndrome  (LGS), or childhood epileptic encephalopathy, is a rare epileptic disorder accounting for 1–4% of childhood epilepsies. The syndrome has much more severe symptoms ranging from multiple seizures daily, learning disabilities, abnormal findings in electroencephalogram (EEG). Earlier age of seizure onset is correlated with higher risk of cognitive impairment. Progressive myoclonus epilepsy (PME) is a group of diseases characterized by myoclonus, epileptic seizures, tonic-clonic seizures, and other serious symptoms such as trouble walking or speaking. These rare disorders often get worse over time and can be fatal. Studies have identified at least three forms of PME. Lafora disease is inherited as an autosomal recessive disorder, meaning that the disease occurs only when a child inherits two copies of a defective gene, one from each parent. Lafora disease is characterized by myoclonus, epileptic seizures, and dementia (progressive loss of memory and other intellectual functions).