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Dandy–Walker syndrome (DWS) refers to Dandy-Walker Malformation and related congenital malformations of the cerebellar vermis and the fourth ventricle of the brain. Dandy-Walker Malformation is characterized anatomically by enlargement of the fourth ventricle, incomplete formation of the cerebellar vermis, enlargement of the posterior cranial fossa, and upward displacement of the dural infolding over the cerebellum called the tentorium cerebelli.

The condition is typically diagnosed on prenatal ultrasound or during infancy. Signs and symptoms vary based on the severity, but infants may show signs of hydrocephalus causing increased intracranial pressure such as bulging fontanelle, downward gaze, irritability, and vomiting. Individuals with Dandy-Walker Syndrome may have intellectual and motor developmental delay including impairment of motor skills related to balance and coordinated movement.

Signs and Symptoms
Presentation can be highly variable depending on severity. Dandy-Walker Syndrome is typically diagnosed in utero or in early infancy, but can become apparent at any age. Infants may show delayed motor development and progressive enlargement of the skull. Symptoms of hydrocephalus and increased intracranial pressure such as irritability, vomiting, and convulsions and signs of cerebellar dysfunction such as unsteadiness and lack of muscle coordination or jerky movements of the eyes may occur. Other symptoms include increased head circumference, bulging at the back of the skull, problems with the nerves that control the eyes, face and neck, and abnormal breathing patterns.

Genetics
The Dandy–Walker syndrome is estimated to occur one in every 30,000 live births. It is a genetically sporadic disorder, meaning that it can arise spontaneously instead of being inherited from a parent. There is thought to be a slight female predominance. One study observed that Dandy–Walker syndrome occured 3 times as often in females as in males.

Risk Factors
Exposure to certain substances during development is associated with a greater risk of developing Dandy–Walker syndrome. Surveys suggest that prenatal exposure to teratogens such as rubella or alcohol are correlated with development of Dandy-Walker malformation. There is a relative contraindication of taking Warfarin during pregnancy, as it is associated with an increased risk of Dandy–Walker syndrome if taken during the first trimester.

Pathophysiology
Dandy-Walker syndrome is classified as an obstructive cause of hydrocephalus that develops during embryogenesis. At mouse embryonic day 12 (or at a crown-rump size of 68 cm for a human), a temporary membrane forms in the roof of the fourth ventricle that goes on to form the foramen of Magendie. In Dandy-Walker, this membrane persists. Failure to fully develop the fourth ventricle outflow tracts (the Foramen of Magendie and/or Luschka) results in a dilated fourth ventricle which prevents the two sides of the cerebellum from fusing. An alternate argument is based on the fact that the vermis forms before the foramina, and thus the lack of foramina per se cannot be the direct cause of cerebellar dysplasia. Some researchers argue that Dandy-Walker only involves atresia of the Foramen of Magendie. Formation of additional arachnoid granulations may compensate for reduced CSF circulation and help to increase CSF drainage.

The normal vermis contains a collection of cell bodies called the fastigial nucleus, which controls rapid scanning eye movements (saccade), balance, proprioception, and coordination of muscle groups for standing and walking. When the vermis is hypoplastic or absent, there is difficulty with balance. In the embryo, the cerebellar vermis develops independently from the cerebellar hemispheres.

Diagnosis
Dandy–Walker syndrome is diagnosed prior to the age of one in 80% of patients. Prenatal diagnosis may be possible with ultrasound or fetal MRI. However, due to technical limitations of the imaging modalities, it is not always possible to make an accurate diagnosis. For example, on ultrasound it is difficult to determine if the tentorium cerebeli has upward displacement because the nuchal muscles cannot be identified as a landmark structure. Because the syndrome is associated with an increased risk for fetal karyotype abnormalities, amniocentesis may be offered after prenatal diagnosis.

Classification
Though there is debate about should be classified as a Dandy–Walker syndrome, the most inclusive definition includes DW malformation, DW variant, mega cisterna magna, and Blake Pouch Cyst.


 * 1) DWS Malformation Type The Dandy–Walker syndrome malformation is the most severe presentation of the syndrome. The cerebellar vermis is absent or incompletely formed. The posterior fossa is enlarged and the tentorium is elevated. There is also cystic dilation of the fourth ventricle, which fills the posterior fossa. This often involves hydrocephalus and complications due to associated genetic conditions, such as Spina Bifida.
 * 2) DWS Variant Type Variant form is less severe than DW malformation. This form represents the most wide-ranging set of symptoms and outcomes of Dandy–Walker syndrome. Many patients who do not fit into the two other categories of Dandy–Walker syndrome are often labeled as variant. The fourth ventricle is only mildly enlarged and there is mild enlargement of the posterior fossa. The cerebellar vermis is hypoplastic and has a variably sized cyst space. This is caused by open communication of the posteroinferior fourth ventricle and the cisterna magna through the enlarged vallecula. Patients exhibit hydrocephalus in 25% of cases and supratentorial CNS variances are uncommon, only present in 20% of cases. There is no torcular-lambdoid inversion, as usually seen in patients with the malformation. The third and lateral ventricles as well as the brain stem are normal. 
 * 3) DWS Mega cisterna magna Type This is the most benign form of Dandy–Walker syndrome. It typically goes unnoticed unless incidentally discovered during a CT, ultrasound or MRI done for some other reason.  Infants with this variation are believed to have normal development and intellect.
 * 4) Blake Pouch Cyst  A Blake pouch cyst causes the superior medullary velum to balloon into the cisterna magna, below and posterior to the vermis. It is thought to caused by failure of the foramen of Magendie to canalize.

Management
Treatment for individuals with Dandy–Walker Syndrome generally consists of treating the associated problems as necessary. Neurosurgery may be required to control hydrocephalus. Surgical correction is typically performed at 5 months of age. Treatment significantly reduces the chance of morbidity and mortality. Surgical approach may include either placing a (shunt) to drain the cerebrospinal fluid, or performing an endoscopic third ventriculostomy with possible cauterization of the chorid plexus. Individuals with Dandy–Walker syndrome may also benefit from various therapies such as occupational therapy, physiotherapy, speech therapy or specialized education. Services of a vision teacher may be helpful if the eyes are affected.

Prognosis
Outcome for children with Dandy-Walker syndrome depends greatly on the severity of the individual's disease. In one study, cases who had developed a vermis having at least three lobes had good neurological outcome, while those with fewer developed lobes suffered from mental delay. In one study, 44.7% of infants with Dandy-Walker Malformation died, mostly before the age of one year. Mortality is increased without treatment for hydrocephalus.

Related disorders
Dandy–Walker syndrome is frequently associated with disorders of other areas of the central nervous system including absence of the corpus callosum, the bundle of axons connecting the two cerebral hemispheres, and malformations of the heart, face, limbs, fingers and toes. Dandy–Walker syndrome often occurs in patients with PHACES syndrome. DWM has been reported to be in association with a wide array of chromosomal anomalies, including trisomy 18, trisomy 9, and trisomy 13. Joubert syndrome is a similar disorder of cerebellar vermis hypoplasia.

History
Dandy–Walker syndrome is named for Walter Dandy and Arthur Earl Walker, who first described the syndrome in 1921.

Research
Two zinc finger genes (ZIC1 and ZIC4) have been implicated in development of Dandy–Walker Syndrome. When the genes are knocked out in mice, the mice develop a similar disorder to that seen in humans. Furthermore, there is evidence that the zinc fingers might be involved in ciliary function, which is known to be essential for normal embryonic development. Genetic association studies found that Dandy–Walker syndrome was associated with four out of five ciliopathies studied.