Dandy–Walker malformation (DWM), also known as Dandy–Walker syndrome (DWS), is a rare congenital brain malformation in which the part joining the two hemispheres of the cerebellum (the cerebellar vermis) does not fully form, and the fourth ventricle and space behind the cerebellum (the posterior fossa) are enlarged with cerebrospinal fluid. Most of those affected develop hydrocephalus within the first year of life, which can present as increasing head size, vomiting, excessive sleepiness, irritability, downward deviation of the eyes and seizures. Other, less common symptoms are generally associated with comorbid genetic conditions and can include congenital heart defects, eye abnormalities, intellectual disability, congenital tumours, other brain defects such as agenesis of the corpus callosum, skeletal abnormalities, an occipital encephalocele or underdeveloped genitalia or kidneys. It is sometimes discovered in adolescents or adults due to mental health problems.DWM is usually caused by a ciliopathic or chromosomal genetic condition, though the causative condition is only identified in around half of those diagnosed before birth and a third of those diagnosed after birth. The mechanism involves impaired cell migration and division affecting the long period of development of the cerebellar vermis. The mechanism by which hydrocephalus occurs in DWM is not yet fully understood. The condition is diagnosed by MRI or, less commonly, prenatal ultrasound. There are other malformations that can strongly resemble DWM, and disagreement exists around the criteria and classifications used for the malformation.Treatment for most involves the implantation of a cerebral shunt in infancy.
In this type, the hypoplasia of the cerebellar vermis does not reach past the horizontal midline of the fourth ventricle, and the posterior fossa is also not as large. The authors noted that this form would previously have been classified as simply mega–cisterna magna. In 1999, Calabró et al. first used the phrase Dandy–Walker continuum when referring to proposals that a condition known as Blakes pouch cyst falls under the umbrella of the Dandy–Walker complex proposed by Barkovich. Later authors would put these terms and systems under intense scrutiny and state that they added considerable confusion to the diagnosis of DWM. However, they remain commonly used.In 2011, Spennato et al. came up with a set of criteria based on Klein et al. (2003) that they considered necessary for diagnosis of DWM:
The lower portion of the cerebellar vermis is absent to varying degrees (three quarters, one half or one quarter missing). The posterior fossa (the space behind the cerebellum) is enlarged, and its cerebrospinal fluid flow is continuous with that of the fourth ventricle. The rest of the cerebellar vermis is hypoplastic and is pushed upwards and rotated forwards due to the enlarged posterior fossa. The cerebellar hemispheres are pushed forwards and to the side by the enlarged posterior fossa. The angle at the centre of the cerebellar vermis (representing the location of the fastigial nucleus) is large, giving a flattened appearance to the bottom of the vermis, or the fastigial nucleus is absent entirely.