Dandy-Walker Malformation Imaging

Dandy-Walker malformation (DWM) is a rare congenital malformation that involves the cerebellum and fourth ventricle. Dandy-Walker malformation and related variants have a prevalence of 1 in 35,000 live births in the United States. ^{[1, 2, 3, 4]} Dandy-Walker malformation is characterized by agenesis or hypoplasia of the cerebellar vermis, cystic dilatation of the fourth ventricle, and enlargement of the posterior fossa. A large number of concomitant problems may be present, but Dandy-Walker malformation is recognized whenever these 3 features are found. Approximately 70-90% of patients have hydrocephalus, which often develops postnatally. Dandy-Walker malformation may be associated with atresia of the foramen of Magendie and, possibly, the foramen of Luschka. ^{[5, 6, 7, 8, 9]}

(See the images of Dandy-Walker malformation below.)

Dandy-Walker complex

Classically, posterior fossa cystic malformations have been divided into Dandy-Walker malformation, Dandy-Walker variant, mega cisterna magna, and posterior fossa arachnoid cyst. Precisely differentiating the malformations may not be possible using imaging studies. Dandy-Walker malformation, variant, and mega cisterna magna are currently believed to represent a continuum of developmental anomalies on a spectrum that has been termed the Dandy-Walker complex. ^{[10, 11, 12, 8]}

Dandy-Walker complex is characterized by an enlarged posterior fossa; a high position of the tentorium, with upward displacement of the lateral sinuses; torcular herophili associated with varying degrees of vermian aplasia or hypoplasia; and a cystic dilatation of the fourth ventricle that nearly fills the entire posterior fossa. Since the vermis is present in posterior fossa arachnoid cyst, this is considered separately from Dandy-Walker malformation.

Dandy-Walker variant

Dandy-Walker variant (see the image below) consists of vermian hypoplasia and cystic dilatation of the fourth ventricle, without enlargement of the posterior fossa.

Mega cisterna magna

Mega cisterna magna (see the image below) consists of an enlarged posterior fossa secondary to an enlarged cisterna magna, with a normal cerebellar vermis and fourth ventricle. Ventriculomegaly occurs rarely (2.3%) before or after birth. ^[13]

Arachnoid cyst

Retrocerebellar arachnoid cysts of developmental origin (see the image below) are uncommon but clinically important. True retrocerebellar arachnoid cysts displace the fourth ventricle and cerebellum anteriorly and show significant mass effect. Because there are different surgical therapy approaches for posterior fossa arachnoid cyst and Dandy-Walker malformation, it is essential to differentiate between the 2 entities. ^[14]

Blake pouch cyst

The Blake pouch cyst (Blake pouch remnant or persistent Blake pouch)  is another posterior fossa embryologic anomaly that must be included in the differential diagnoses of Dandy-Walker malformation. This anomaly can cause obstructive hydrocephalus, which requires early surgical intervention. ^[15]  A retrospective analysis of fetal MRIs with posterior fossa malformations found that Dandy-Walker malformation could be differentiated from Blake pouch cyst by a significantly higher tegmentovermian angle (TVA) at any gestational age. Other biomarkers such as anterior-to-posterior and craniocaudal vermes were not significantly different until 23.1 weeks' gestation or later. ^[16]

Clinical presentation

Most definitions of Dandy-Walker malformation have included dysgenesis of the vermis, cystic dilatation of the fourth ventricle, and enlargement of the posterior fossa with elevation of the tentorium/torcula.

Dandy-Walker malformation consists of a malformation of the roof of the fourth ventricle and of the cerebellum. The cerebellum is poorly developed and is displaced upward and laterally.

The enlarged fourth ventricle balloons out backward. The fourth ventricle is grossly misshapen because it is a membrane-wrapped cyst that extends into the foramen magnum. The membrane-wrapped cyst may lift and displace the posterior portion of the brain, as well as cause an internal obstruction of normal CSF flow, with resultant hydrocephalus.

Dandy-Walker malformation may be associated with atresia of the foramen of Magendie and, possibly, the foramen of Luschka. ^[17]

Common findings of Dandy-Walker malformation include the following:

• Enlarged posterior fossa
• Varying degrees of cerebellar and vermian hypoplasia or complete vermian absence
• Cyst formation in the posterior fossa
• Vermian remnant everted above the posterior fossa cyst
• Hypoplastic cerebellar hemispheres winged anterolaterally (outward) in front of the cyst
• Absence of the foramina of Luschka and Magendie
• Obstructive hydrocephalus secondary to cystic dilatation of the fourth ventricle (70-90%)
• Abnormally high position of the straight sinus, torcular herophili, and tentorium
• Sinus confluence and lateral sinuses elevated above the lambdoid sutures (high tentorial insertion, also called lambdoid-torcular inversion)
• Aqueductal obstruction, which may affect the need for supratentorial decompression (and is, therefore, an important component)
• If callosal agenesis coexists (20-25%), development of dilatation of the occipital horns (colpocephaly)
• Possible brainstem compression and hypoplasia; the degree of pontine hypoplasia is directly related to the degree of cerebellar hypoplasia ^[18]
• Thinning and bulging of the occipital bones

Associated central nervous system (CNS) abnormalities of Dandy-Walker malformation are reported in 70% of children and include the following:

• Dysgenesis of the corpus callosum (20-25%) ^[7]
• Lipoma of the corpus callosum
• Holoprosencephaly (25%)
• Porencephaly
• Dysplasia of the cingulate gyrus (25%)
• Schizencephaly
• Polymicrogyria/gray matter heterotopia (5-10%)
• Cerebellar heterotopia
• Occipital encephalocele (7%) ^[7]
• Microcephaly
• Dermoid cysts
• Malformation of the cerebellar folia (25%)
• Malformation of the inferior olivary nucleus
• Hamartoma of the tuber cinereum
• Syringomyelia ^[6]
• Klippel-Feil deformity
• Spina bifida ^[19]
• Lumbosacral meningoceles
• Spinal lipoma

Non-CNS-associated malformations are reported in 20-33% of children and include the following:

• Orofacial deformities and cleft palate (6%)
• Polydactyly and syndactyly ^[20]
• Cardiac anomalies ^[21]
• Urinary tract abnormalities (polycystic kidneys) ^[22]
• Cataracts, retinal dysgenesis, and choroid coloboma
• Facial hemangioma
• Hypertelorism
• Meckel-Gruber syndrome ^[5]
• Neurocutaneous melanosis ^{[18, 23]}

Imaging modalities

Dandy-Walker malformation is best diagnosed with the help of ultrasonography (US) and magnetic resonance imaging (MRI). US may be the initial examination performed because it can be done portably and without sedation, as well as allowing multiplanar imaging. ^{[24, 25, 26, 27]} US, however, is limited because it is heavily operator-dependent. Abnormalities such as the gyral, dural, tentorial, and skull anomalies that accompany Dandy-Walker malformations are not clearly depicted by US.

Measurement of the cisterna magna forms part of the ultrasound evaluation of the fetal brain. A prominent cisterna magna shown on prenatal ultrasound may raise the concern for congenital posterior fossa abnormalities. ^{[1, 28, 29]}

The introduction of modern imaging techniques, specifically MRI, has radically changed the evaluation of symptoms related to the posterior fossa. MRI is usually performed for detailed evaluation of Dandy-Walker malformation lesions and complications after the diagnosis is suspected using computed tomography (CT) and US. Distinguishing milder forms of vermian hypoplasia from a mega cisterna magna or an arachnoid cyst on US is difficult in the third trimester, where ossification of the skull can limit sonographic assessment of the posterior fossa structures. With its multiplanar capabilities, fetal MR imaging can better evaluate the morphology of the vermis, as well as the anatomic relationship between a retrocerebellar cyst and the fourth ventricle, which can help differentiate a Dandy-Walker variant from other entities, such as a mega cisterna magna. ^[30]

MRI allows surgeons to accurately view the cerebellum and associated structures, determine which form the malformation has taken, and gauge the progress of the malformation. MRI also demonstrates which space should be shunted first. MRI has been frequently used for diagnosing fetal craniospinal anomalies.

Since it can distinguish between hydrocephalus associated with Dandy-Walker and hydrocephalus associated with other etiologies, CT scanning is also useful in Dandy-Walker malformation; however, it exposes the infant to ionizing radiation. Clearly distinguishing the subtypes of Dandy-Walker complex on axial CT images is difficult. In addition, evaluating subtle supratentorial pathologies and associated abnormalities on CT scans may not be easy because its routine use is constrained by the axial plane.

The classic abnormal findings of Dandy-Walker malformation described on cranial CT and MRI can also be demonstrated on cranial sonography. US is routinely used during the antenatal period as a screening method, and it is particularly used for postnatal follow-up studies of hydrocephalus. US evaluation of posterior fossa cystic abnormalities in the newborn is best accomplished via a posterolateral fontanelle approach or through the cisterna magna posteriorly.

According to a retrospective MRI study by Whitehead et al of cerebellar development prenatally and postnatally, the Dandy-Walker malformation phenotype is best defined by inferior predominant vermian hypoplasia, an enlarged tegmentovermian angle, inferolateral displacement of the tela choroidea/choroid plexus, an obtuse fastigial recess, and an unpaired caudal lobule. ^[31]

Measurement of the brainstem-vermian (BV) angle and the brainstem-tentorium (BT) angle helps determine increased size of the cisterna magna. The BV angle increases with severity (ie, angles below 18 degrees are considered normal, angles 18 to 30 degrees suggest Blake pouch malformation, and angles greater than 45 degrees are strongly suggestive of dandy-walker malformation). ^{[1, 29]}

Plain radiography has been primarily used in the evaluation of shunt malfunction, as well as for diagnosing associated anomalies.

Salihu et al studied infant mortality in non-Hispanic black and non-Hispanic white neonates with Dandy-Walker syndrome and found that Dandy-Walker syndrome has one of the largest Black-White disparities in postneonatal survival. Neonatal mortality was similar for non-Hispanic Blacks and non-Hispanic Whites, but non-Hispanic Blacks had an 8-fold increased risk for postneonatal mortality. ^[32]

In another study by Salihu and co-workers, infants with Dandy-Walker syndrome and 2 or more affected organ systems were approximately 6 times as likely to die postneonatally as were patients with isolated Dandy-Walker syndrome. According to the authors, these findings confirm the widely held belief that Dandy-Walker syndrome is a heterogeneous, rather than a homogeneous, disorder. ^[33]