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,[6] which can present as increasing head size, vomiting, excessive sleepiness, irritability, downward deviation of the eyes and seizures.

[4] 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.

[5] The mechanism involves impaired cell migration and division affecting the long period of development of the cerebellar vermis.

[6] The mortality rate is roughly 15%, mostly due to complications from hydrocephalus or its treatment, which can include subdural haematomas or infection.

[5] The malformation was first described by English surgeon John Bland-Sutton in 1887,[6][15] though it was named by German psychiatrist Clemens Ernst Benda [de] in 1954[1][6] after American neurosurgeons Walter Dandy and Arthur Earl Walker, who described it in 1914 and 1942, respectively.

[6] 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.

[7] This may be due to herniation of the bottom of the cyst through the foramen magnum (a similar mechanism to Chiari malformation).

[6] In DWM, the fourth ventricle opens up into and is continuous with almost the entire posterior fossa subarachnoid space.

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.

Theories of abnormal development or inflammation of the arachnoid mater in the posterior fossa have been put forward.

[6] The arachnoid mater contains granulations necessary to return CSF from the subarachnoid spaces to the dural veins and circulation.

[6] 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.

[7] Additionally, there are several similar conditions which have at various times been grouped with DWM on a continuum by some authors and separated as distinct by others, further complicating diagnosis.

[6][8] In 1976, Harwood-Nash and Fitz proposed the term Dandy–Walker variant (DWV) for a malformation in which the posterior fossa is not enlarged but the cerebellar vermis is hypoplastic.

[7][6] In 1989, Barkovich et al. proposed the term Dandy–Walker complex (DWC) to include classic DWM and DWV (under type A) plus a third malformation (under type B) in which the cerebellar vermis remains large enough to sit between the fourth ventricle and the cisterna magna beneath it, and instead it is mostly the cisterna magna that is enlarged.

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.

[3] Later authors would put these terms and systems under intense scrutiny and state that they added considerable confusion to the diagnosis of DWM.

[12] DWM can be observed prenatally on ultrasound as early as 14 weeks of gestation,[5] though an MRI scan is the most useful method for diagnosis.

Cardiac-gated phase-contrast MRI can observe the flow of cerebrospinal fluid (CSF) during systole and diastole of the heart.

However, those with DWM have a higher rate of shunt-related complications than other patients with hydrocephalus (mainly due to the unconventional anatomy).

[6] One explanation for a failure of a shunt to reduce intracranial pressure in DWM has been that the cyst may herniate into the foramen magnum and form a scarring adhesion at the cervical junction, preventing it from shrinking again.

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,[6] though imaging is important to confirm this.

Overdrainage can lead to subdural haematomas, a tethered spinal cord, due to scarring, or downward herniation of the cerebral hemispheres.

This less invasive procedure creates an artificial hole in the third ventricle to allow CSF to bypass any obstruction.

[34] Previously, craniotomy of the posterior fossa and excision of the cystic membrane was used, which was often unsuccessful in preventing cyst reformation and carried a degree of mortality.

[6][10] Treatments for any other symptoms are generally focussed on the specific condition involved and may include supported education, physical therapy or other services.

[5] 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.

[6][16] In 1942, American physician John K. Taggart and Canadian–American neurosurgeon Arthur Earl Walker detailed the phenomenon extensively, ascribing the potential cause as underdevelopment of the foramina of Luschka and Magendie,[17] now no longer believed to be significant.

[1][6] In 1976, Harwood-Nash and Fitz proposed the term Dandy–Walker variant (DWV) for a malformation in which the posterior fossa is not enlarged but the cerebellar vermis is hypoplastic.