<unk>  is diagnosed based on the characteristic neuroimaging findings. It can be diagnosed prenatally on ultrasound as early as 14 weeks of gestation, although it is usually diagnosed postnatally by MRI. It is diagnosed within the first year of life 41% of the time, normally due to increasing signs of hydrocephalus, but 28% of the time it is discovered in adolescence or adulthood due to mental health problems, such as psychosis or mood disorder. Criteria and classification The precise diagnostic criteria and classification systems of DWM are not agreed upon, and significant dispute exists as to which terms or criteria should be used. The core criteria of DWM are hypoplasia of the cerebellar vermis and an enlarged fourth ventricle and posterior fossa (the space behind the cerebellum), though the specific degree of hypoplasia or cystic enlargement for diagnosis of DWM is not agreed upon. 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. In 1976, Harwood-Nash and Fitz proposed the term<unk><unk><unk>  variant (DWV) for a<unk>  in which the posterior fossa is not enlarged but the cerebellar vermis is hypoplastic. In 1989, Barkovich et al. proposed the term<unk><unk><unk>  complex (DWC) to include classic DWM and DWV (under type A) plus a third<unk>  (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. 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<unk><unk><unk>  continuum when referring to proposals that a condition known as Blake's pouch cyst falls under the umbrella of the<unk><unk><unk>  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. * The confluence of sinuses, part of the drainage system located at the far rear of the occipital lobe, is elevated due to the enlarged posterior fossa. (The adjacent cerebellar tentorium is also elevated.) Due to the inconsistency of the presence of hydrocephalus in DWM, Spennato and Klein suggested that it should not be considered a criterion for DWM. Klein's criteria differed from Spennato's mainly in that it required no apparent cerebellar hemisphere hypoplasia, but it may also have required the vermis to touch the tentorium or an absence of brainstem abnormalities. Methods DWM can be observed prenatally on ultrasound as early as 14 weeks of gestation, though an MRI scan is the most useful method for diagnosis. MRI can delineate the shape and extent of the<unk>  as well as assessing additional areas for<unk> s such as the cerebellar hemispheres, cerebral aqueduct or corpus callosum. Cardiac-gated phase-contrast MRI can observe the flow of cerebrospinal fluid (CSF) during systole and diastole of the heart. In true DWM, this will find a flow from the cerebral aqueduct to the posterior fossa and no flow between the cisterna magna and the space behind the cervical spinal cord. CT may also be used if MRI is unavailable, but it provides less detail. Klein et al. (2003) suggested that a suspected diagnosis based on CT or ultrasound should not be confirmed until an MRI is performed, due to the large number of conditions that can present highly similarly and confound diagnosis. Differential diagnosis DWM has a large number of conditions that can present highly similarly on imaging and confound diagnosis. Blake's pouch cyst Blake's pouch cyst (BPC), or persistent Blake's pouch, is a condition that arises when Blake's pouch, an invagination in the fourth ventricle that ruptures at around 4 months of gestation to form the foramen of Magendie (medial aperture), fails to rupture. This can lead to a dilated fourth ventricle and subsequent hydrocephalus of all four ventricles. In a Blake's pouch cyst, unlike in DWM: * The cerebellum is not hypoplastic, though it may be compressed by the enlarged posterior fossa (mass effect). * The posterior fossa is not enlarged. * The cerebellar tentorium/confluence of sinuses is not raised. * Hydrocephalus, when it occurs, involves all four ventricles. Some authors, however, consider Blake's pouch cyst part of a continuum with DWM (the<unk><unk><unk>  continuum"). Mega cisterna magna Mega cisterna magna is a condition in which the cisterna magna, the subarachnoid cistern below the fourth ventricle, is enlarged. It has been proposed to be due to a delayed rupture of Blake's pouch rather than a failed rupture. In mega cisterna magna, unlike in DWM: * The cerebellum is not usually hypoplastic. * The fourth ventricle is of relatively normal shape. * Hydrocephalus is uncommon. There is debate as to whether this<unk>  is distinct from DWM or forms part of the<unk><unk><unk>  continuum". Posterior fossa arachnoid cyst An arachnoid cyst is a collection of cerebrospinal fluid (CSF) in the arachnoid mater. 10% of these occur in the posterior fossa. In a posterior fossa arachnoid cyst, unlike in DWM: * The cyst is clearly localised in a specific location separate from the fourth ventricle outlets. * The cerebellum is not hypoplastic, though it may be compressed by the cyst (mass effect). * The CSF flow in the cyst is not continuous with that of the fourth ventricle. * Hydrocephalus, if it occurs, is due to the cyst pressing on the cerebellum and compressing the cerebral aqueduct or fourth ventricle outlets.
Dandy–Walker malformation
Glial tumor
Subdural hygroma
Schizencephaly
Chiari malformation
Hemimegalencephaly
Subependymoma
Hydranencephaly