Under this term are included a group of conditions that share in common one sonographic findings: the impression that the fourth ventricle communicates with the cisterna magna. These conditions include: Dandy-Walker malformation, Blake’s pouch cyst, vermian hypoplasia/agenesis. They have a similar sonographic appearance, particularly in early gestation, and differentiation requires a multiplanar approach.

Abstract: Under this term are included a group of conditions that share in common one sonographic findings: the impression that the fourth ventricle communicates with the cisterna magna. These conditions include: Dandy-Walker malformation, Blake’s pouch cyst, vermian hypoplasia/agenesis. They have a similar sonographic appearance, particularly in early gestation, and differentiation requires a multiplanar apparoach. Magnetic resonance mey also be useful. The prognosis is variable, and depends mostly upon the frequent occurrence of associated anomalies. Blake’s pouch cyst, the most common entity, when isolated is probably a normal variant.

Key words: Blake’s pouch cys, Dandy-Walker malformation, Joubert syndrome , vermian agenesis, vermian hypoplasia, COACH syndrome, CORS syndrome, oral-facial-digital syndrome type VI, Senior–Løken syndrome

Authors: Gianluigi Pilu1, Gustavo Malinger2, Selim Buyukkurt3 

  1. Department of Obstetrics and Gynecology of the University of Bologna, Italy
  2. Fetal Neurology Clinic, Department of Obstetrics and Gynecology, Wolfson Medical Center, Holon, Israel
  3. Departmet of Obstetrics and Gynecology, University of Cukurova, Adana, Turkey


Dandy Walker malformation, vermian hypoplasia, vermian agenesis, Blake’s pouch cyst, Joubert syndrome and related entities (vermian agenesis/hypoplasia with ‘molar tooth’ sign)


Megacisterna magna, cerebellar hypoplasia


Dandy-Walker malformation defines the combination of an enlarged cisterna with superior displacement (rotation) of the cerebellar vermis.1-5 The term Dandy-Walker complex (or continuum) indicates a spectrum of anomalies with anatomic similarities to Dandy-Walker malformation. There is no general consensus in the categorization of posterior fossa anomalies and the interested reader is referred to specific works on this subjects.1-5 In this section we include a group of anomalies that share in common one sonographic finding: the impression that in a axial plane of the head the fourth ventricle is open posteriorly and communicates with the cisterna magna. This finding will be referred to in the following as the ‘open fourth ventricle’. The most frequent anomalies that demonstrate this sign include: Blake’s pouch cyst, vermian agenesis/hypoplasia, Dandy-Walker malformation; Joubert syndrome and related entities (vermian agenesis/hypoplasia with ‘molar tooth’ sign) The rational for grouping them together is that they are often difficult to differentiate and they tend to overlap clinically.


Dandy-Walker malformation is found in 4-12% of all cases of infantile hydrocephalus and has an estimated prevalence of about 1:30,000 births.6-8 The incidence of the other varieties of the Dandy-Walker complex is unknown.

Embryology and Pathogenesis

In the late first trimester, the fourth ventricle is large and a relatively small cerebellum is found on top of it. In the following weeks the cerebellum grows to enfold completely the fourth ventricle. However, a small finger-like appendage of the fourth ventricle, the Blake’s pouch, is frequently seen protruding into the cisterna magna, caudally to the cerebellum. It has been suggested that there is a continuum of anatomic anomalies involving the fourth ventricle-Blake’s pouch complex.9 The mildest of these anomalies is the Blake’s pouch cyst, a persistence of the Blake’s pouch that results in an isolated superior displacement of the cerebellar vermis.10, 11 At the other end of the spectrum stands the Dandy-Walker malformation, where the upward displacement of a normal to hypoplastic vermis is associated with an enlargement of the cisterna magna.1-4 In vermian hypoplasia (previously referred to as Dandy-Walker variant) the cisterna magna is of normal size and the vermis is small and frequently (although not always) rotated upward.1-4, 12, 13 Many cases of the mildest anomaly in this group, the Blake’s pouch cyst, resolve spontaneously throughout gestation and therefore probably represent a delay in normal anatomic development.10 In the other entities of the Dandy-Walker complex, the expansion of the fourth ventricle is concomitant with aberrant development of the cerebellum and of the vermis in particular, and these cases are more likely to be associated with neurologic compromise. The Dandy-Walker malformation was originally described in individuals with obstructive hydrocephalus and was postulated to arise from a primary atresia of the foramina of Luschka and Magendie.6, 8 A similar explanation was proposed for the Blake’s pouch cyst.14 It is now clear that most patients with the Dandy Walker complex do not develop hydrocephalus suggesting that the pathogenesis is more complex and is not merely the consequence of obstruction and overdistension of the fourth ventricle. Several hypotheses have been formulated but there is no clear evidence supporting any of them.


Enlargement of the Blake’s pouch (or of the entire fourth ventricle in the most severe forms) is responsible for the superior displacement of the vermis that is encountered in the Dandy-Walker complex. The enlarged Blake’s pouch/fourth ventricle balloons into the cisterna magna and in the most severe cases obliterates it and distend the posterior fossa. With most techniques of diagnostic imaging, whether prenatal or postnatal, the thin walls of the Blake’s pouch/fourth ventricle are difficult to visualize and the impression is that of a communication between the fourth ventricle and the cisterna magna.9, 15

The term Blake’s pouch cyst was originally introduced in infantile neuroradiology to indicate a type of obstructive hydrocephalussecondary to failure of formation of the foramen of Magendie and Luschka, resulting in a compressive cyst of the posterior fossa displacing superiorly the cerebellar vermis.14, 16 More recently, the term has become popular in fetal imaging studies to indicate cases with a posterior fossa cyst displacing superiorly an intact cerebellar vermis, typically in association with a normal ventricular system and a normal size of the posterior fossa.3, 4, 10, 17 This finding has been interpreted as failed or delayed regression of the Blake’s pouch. The entity described in the original neonatal studies and the one later described in fetal studies are likely to be different, as the latter has typically a normal outcome and appears to be rarely associated with ventriculomegaly.

Vermian agenesis/hypoplasia is characterized by a small vermis with a normal cisterna magna; it has been suggested that the term hypoplasia should be limited to cases in which the vermis is small but has a normal morphology (all the lobules are present); conversely, the term partial agenesis should be used in those cases in which a part, typically the most inferior portion is absent. In these patients the cisterna magna has a normal size.4, 5, 10, 18, 19 This condition was originally defined as Dandy-Walker variant20, 21 a term no longer in use.

Dandy-Walker malformation is characterized by the enlargement of the cisterna magna. The cerebellar vermis is significantly rotated and may be intact or incompletely formed.1, 3, 4, 10 Hydrocephalus was classically considered an essential diagnostic element of this condition, but more recent evidence suggests that it is not present at birth in most patients, although it may develops in later life.6, 8

Joubert syndrome and related disorders are a group of conditions that share in common hypoplasia of the cerebellar vermis in association with the characteristic neuroradiologic 'molar tooth sign. These features are associated with extraneural anomalies to constitute different syndromes: classical Joubert syndrome, Coloboma, Oligophrenia/developmental delay, Ataxia, Cerebellar vermis hypoplasia, Hepatic fibrosis (COACH), cerebello-oculo-renal syndrome (CORS), oral-facial-digital syndrome type VI (OFD-VI), Senior–Løken syndrome. The gene products that are affected in these group of conditions are however known to take part into the development of the primary cilium and/or basal body and centrosome apparatus. Essentially, Joubert and related disorders are now considered to belong to the general group of ciliopathies.22-24


Genetic factors have a major role in the etiology of the Dandy-Walker complex. Dandy-Walker malformation and vermian hypoplasia/agenesis may occur as a part of Mendelian disorders and chromosomal aberrations.25 In rare cases, the disease is inherited as an autosomal recessive or dominant trait. Environmental factors, including viral infections, alcohol and diabetes, have also been suggested to play a role in the genesis of Dandy-Walker malformation, but the evidence is uncertain.25 Joubert syndrome and related entities have a genetic etiology with autosomal recessive or X-linled transmission. The genetics is however complex with at least 8 genes involved.

Recurrence risk

In the absence of a recognizable syndrome, a recurrence risk of 1-5% is suggested for Dandy-Walker malformation and vermian hypoplasia/agenesis.25 Autosomal transmission, both dominant and recessive has been documented. There are no available information on Blake’s pouch cyst, and vermian agenesis/hypoplasia include probably a number of heterogeneous entities. Joubert and related cerebellar disorders are transmitted as an autosomal or X-linked recessive trait and have therefore a high recurrence risk. 22-24

Associated anomalies

The Dandy-Walker complex is frequently associated with chromosomal aberrations, syndromes and other cerebral malformations (mostly ventriculomegaly, agenesis of the corpus callosum, holoprosencephaly, cephaloceles), polycystic kidneys, cardiovascular defects and facial clefting.10, 11, 21 A detailed list of condition found in association with Dandy-Walker malformation is reported in Table 1.


The landmark of the Dandy-Walker complex is the open fourth ventricle. This is demonstrated by sweeping the transducer in the posterior fossa along the axial plane and visualizing a fluid-filled tract seemingly connecting the cavity of the fourth ventricle to the cisterna magna. In early gestation this is a normal finding, as the developing cerebellar vermis has not yet completely enfolded the fourth ventricle.26-28 It is therefore imprudent to make a diagnosis of the Dandy-Walker complex at this gestational age, with the possible exception of those (rare) cases in which there is an obvious cystic enlargement of the cisterna magna or other abnormal findings. After 20 weeks the vermis has normally ‘closed’ the fourth ventricle and therefore the demonstation an opening is indicative of the Dandy Walker complex.

The varieties of the Dandy-Walker complex have much different prognostic implications. Differentiation may be difficult at times, and requires a systematic approach using multiplanar examinations possibly aided by magnetic resonance. Assessement includes evaluation of the vermis (absent, hypoplastic, intact), and of the cisterna magna (normal, enlarged).3, 4, 10 The degree of vermian rotation is also relevant.29 The evaluation of these findings is difficult and involves an elements of subjectivity, but an expert examiner using either sonography or magnetic resonance (or both) is able to make a precise diagnosis in the majority of cases,4, 10, 13, 19 probably with the remarkable exception of subtle vermian hypoplasia/partial agenesis.10, 13, 19

To assess the presence of the vermis the best approach is to sweep the transducer in the posterior fossa along the axial plane. The vermis appears as an ovalar echogenic structure interposed between the fourth ventricle and the cisterna magna. When this is not seen and the area of the fourth ventricle is seen to communicate with the cisterna magna at any level, vermian agenesis can be inferred.30, 31

To assess the integrity of the vermis and the position of the torcular a median view is required. This can be obtained directly by multiplanar imaging, scanning preferably through the posterior fontanel as this allows better visualization of posterior fossa and brain stem. Three dimensional ultrasound can also been utilized.32, 33 Indeed, one of the major shortcomings in the median view is the difficulty to obtain with absolute precision the exact plane of section and to confuse the cerebellar hemispheres with the vermis. The advantage of three dimensional ultrasound is to control the sections using as reference the orthogonal planes. The use of volume contrast imaging may also facilitate visualization of subtle anatomic details.32, 33 Once the vermis has been identified in the median plane, both a qualitative and a quantitative evaluation should be performed. It has been suggested that if the posterior apex (fastigium) of the fourth ventricle and the two main fissures of the vermis can be identified, the vermis is presumably intact.10, 19, 32-35 The secondary fissure is at times difficult to define in early gestation and a semiquantitative approach can be used alternatively (normally, twice as much vermis is found below than above the primary fissure.34Measurements of the fetal cerebellar vermis have been reported.36, 37 Most cases of defective vermis involve agenesis of the caudal portion and therefore the vertical diameter is probably the most relevant one. A small vermis with an abnormal configuration (absence of fastigium and/or fissure) indicates partial agenesis. Conversely, a small vermis with a normal configuration indicates hypoplasia.5, 10, 19, 35 In practice, the distinction between these two entities in the fetus is difficult.

A word of caution is needed regarding the possibility to obtain` a truly, apparently normal, median plane by 2D or 3D ultrasound in a fetus with complete vermian agenesis due to the fact that the cerebellar hemispheres may be displaced into the midline and in contact with each other.30, 31 In these patients the coronal plane may be useful to demonstrate that the vermis is absent and the folia abnormally oriented.30, 31, 38

The degree of rotation of the cerebellar vermis is also relevant. It has been recently demonstrated that the Blake’s pouch cyst has usually a rotation of less than 30°, while a value in excess of 45° favors the diagnosis of a Dandy-Walker malformation.29

Postnatally, the expansion of the cisterna magna is evaluated by observing the torcular Herophilii. Normally the torcular Herophilii is implanted in the inner skull at the same level of neck muscles. When the cisterna magna is expanded the torcular is displaced upward. This evaluation is difficult with antenatal ultrasound, and magnetic resonance is usually more effective. 4, 10

The Blake’s pouch cyst, which is by far the most frequent variety encountered prenatally is featured by a slight (< 30°) rotation of an intact cerebellar vermis with a normal cisterna magna.17, 29 Vermian hypoplasia/partial agenesis is featured by an upward displacement of a small vermis, while the cisterna magna and torcula are normal (Figure 7).5, 13 The landmark of the Dandy-Walker malformation is the large cisterna magna and the consequent superior displacement of the torcular. The vermis is always significantly rotated superiorly (>45°)29 and may be either normal or defective.35, 39 It has been suggested that the shape of the opening of the fourth ventricle in the axial plane is relevant for the differential diagnosis of the different varieties of the Dandy-Walker complex.40 Also in our experience, an hourglass opening (buttock sign) is typical of the Blake’s pouch. A triangular or square shaped opening is indicative of either vermian hypoplasia or Dandy-Walker malformation).

Postnatally, the predominant finding of Joubert syndrome and related entities the typical appearance of the mesencephalon in the axial plane commonly referred to as 'molar tooth sign,' that is the consequence of a deep posterior interpeduncular fossa with thick and elongated superior cerebellar peduncles, associated with variable degree of hypoplasia or agenesis of the cerebellar vermis. In pregnancies at risk, the demonstration of an open fourth ventricle in the axial plane has been described as the most important finding. The authors of the original report have not found sagittal planes to be of value.30 Indeed the absence of the vermis results in apposition of the two cerebellar hemispheres in the midline that mimicks the presence of a normal vermis. In low risk pregnancies, this condition can be easily missed. A handful of cases has been recently reported in which the ‘molar tooth’ sign could be identified prenatally with both ultrasound and magnetic resonance,a and it is lickely that the increased resolution of current ultrasound as well as increased experience allow now more confident diagnosis.38

Differential diagnosis

The Dandy-Walker complex should be differentiated by other cystic anomalies of the posterior fossa. In case of megacisterna magna, the cisterna magna is large but the cerebellum is intact and the fourth ventricle is triangular and closed.10, 31, 34 Cerebellar hypoplasia results in ex-vacuo large cisterna magna41 but the main feature of the condition is a global reduction in the size of an otherwise normally shaped cerebellum.4, 10, 31 Arachnoid cysts growing in the posterior fossa have a mass effect and cause asymmetrical distortion of the cerebellum.10, 31 The greatest difficulty is probably encountered by posterior fossa hemorrhage that results at times in destruction of the cerebellar vermis and enlargement of the cisterna magna mimicking very closely a Dandy-Walker malformation or vermian hypoplasia.10, 42 Thrombosis of the torcular Herophilii may result in massive dilatation of the superior sagittal sinus mimicking a posterior fossa fluid collection. A specific diagnosis is however relatively easy, considering that the blood contained in the enlarged dural sinuses has on sonography a corpuscolated appearance, usually with a large echogenci blood clot at the level of the torcular, and that the fluid accumulation is found cranial to the posterior fossa.43, 44

Implications for targeted examination

The Dandy-Walker complex has genetic implications, and sonography is usually offered to couples at risk. Although the most severe forms of this condition can be recognized as early as 14 weeks,45 caution is necessary because of the ambiguous images caused by the incomplete development of the cerebellum at this gestational age.26, 27 We recommend to limit the diagnosis of Dandy-Walker complex to those cases in which either a very large cisterna magna or other abnormal findings are encountered. In the midtrimester, it is stressed that the transcerebellar view only demonstrate the superior part of the vermis and therefore is not sufficient to rule out the Dandy-Walker complex. A lower scan is required to demonstrate the ‘closure’ of the inferior vermis. Couples at risk should be informed that false negatives and false positives cases have been reported, and that an antenatal diagnosis is not possible in all cases, particularly with minor anatomic alterations.

In pregnancies at incraesed risk of Joubert syndrome and related entities because of a previous affected child neurosonography should be performed to assess the presence and integrity of the vermis. An open fourth ventricle after 20 weelks’ gestation and/or failure to visualize the main landmarks of the cerebellar vermis are strongly suggestive of a recurrence. On the basis of the available evidence, MR should also be performed in that it allows a better demonstration of the abnormalities of the mesencephalon (molar tooth sign). In the absence of a familial history the diagnosis of Joubert and related syndromes is a major challenge even for an expert using either ultrasound or MR.

Implications for sonographic screening

In the midtrimester, a cisterna magna measuring more than 10 mm 46 and/or an ‘open’ fourth ventricle} alert to the possibility of Dandy-Walker complex.4, 10, 17 The transcerebellar view, that is recommended as a part of the standard sonographic examination of the fetal brain47will not detect all cases of the Dandy-Walker complex. The available experience suggest that a standard exam may not reveal even severe forms of the Dandy Walker complex.


Neurosurgical series that report the combination of Dandy-Walker malformation with hydrocephalus describe abnormal neurologic development in 40-70% of survivors.6-8 Dandy-Walker malformation is also a part of many genetic syndromes, with a wide spectrum of outcomes. The clinical significance of Dandy-Wallker malformation without ventriculomegaly is debated.48 It has been suggested that the neurologic outcome is mainly related to the appearance of the cerebellar vermis. When this is of normal size and morphology a normal development has been reported in 85% of cases. Conversely, when the vermis is abnormally lobulated and/or there are associated cerebral anomalies, the prognosis is poor.35, 39 In reality this evaluation is difficult prior to birth.4, 10 Most of the antenatal series thus far reported demonstrated a poor outcome in a large proportion of cases. In a recent antenatal stuidiy about 50% of isolated cases had a normal outcome.10 The association with other complex anomalies and/or syndrome was the main risk factor for an abnormal outcome.10

Vermian hypoplasia/agenesis is also frequently a part of muiltiple anomalies and genetic syndromes. When isolated may be asymptomatic but precise risk figures are not available.10, 12, 13, 48, 49

About one third of Blake’s pouch cysts detected antenatally are associated with other anomalies, including chromosomal aberrations (in most cases trisomy 21).10, 11When the Blake’s pouch cyst is an isolated finding and the karyotype is normal, however, the outcome is usually good. Intrauterine remission occurs in about 50% of cases, and a normal developmental outcome is registered in 96% of cases.10, 11 Despite small case series in the pediatric literature in which Blake’s pouch cyst is associated with obstructive hydrocephalus this occurred only in 1/51 cases diagnosed antenatally.10, 11 Blake’s pouch cyst does appear to represent a risk factor for other anomalies, but when isolated is probably a normal variant.10, 11

Most infants with Joubert syndrome and related disorders have severe intellectual impairment and early death is common. Neurologic dysfunction typically includes ataxia, abnormal breathing patterns (hyperpnea intermixed with central apnea in the neonatal period) and abnormal behaviour (temperament, hyperactivity, aggressiveness, and dependency)22-24

Obstetric management

The diagnostic workup must include a detailed search for associated anomalies including karyotyping. Genetic consultation is reccomended. When Joubert syndrome is suspected, a search for the specific mutations may be considered. In counselling couples, it is stressed that with the exception of Joubert sydrome, the Dandy-Walker complex may have a good outcome when there are no associated anomalies. Blake’s pouch cyst, the most common abnormal finding of the posterior fossa, is most frequently a normal variant. Serial scans are suggested because of the potential for cerebral maldevelopment including ventriculomegaly. With the exception of those cases associated with hydrocephalus and macrocrania, that may require cesarean delivery, no modification of the standard obstetric management is indicated.


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