The Dandy–Walker malformation is a nonspecific congenital brain malformation that results from a number of diverse causes. There are two principal features of the Dandy–Walker malformation: aplasia or hypoplasia of the cerebellar vermis and posterior fossa cysts that represent cystic dilatation of the fourth ventricle (Nyberg et al., 1988).

The first case of Dandy–Walker malformation was reported in 1887 (Murray et al., 1985). In 1914, Blackfan and Dandy described a hindbrain abnormality in a patient with cystic dilatation of the fourth ventricle, hypoplasia of the cerebellar vermis, separation of the cerebellar hemispheres, and absence of the lateral and median apertures of the fourth ventricle (cited in Chang et al., 1994). The term Dandy–Walker malformation was first used in 1954, combining case reports of Blackfan and Dandy, and a subsequent report of Taggart and Walker (Chen and Chu, 1994).

The Dandy–Walker malformation originates before the 6th or 7th week of embryonic development (Russ et al., 1989). The malformation may occur in single-gene disorders, in chromosomal abnormalities, in environmentally induced malformation syndromes, or in conjunction with other multifactorial anomalies (Cornford and Twining, 1992).

The full Dandy–Walker malformation consists of complete vermian agenesis, cystic dilatation of the fourth ventricle that communicates with an enlarged cisterna magna, and an enlarged posterior cranial fossa. While obstructive hydrocephalus is nearly always an associated finding postnatally, it is usually not present prenatally (Niesen, 2002). It is unclear if such hydrocephalus is due to failure of the foramina of Lushka and Magendie to open, or to some other embryological disruption.

The full Dandy–Walker malformation is sometimes distinguished from the Dandy–Walker variant and mega cisterna magna (Chen and Chu, 1994). The Dandy–Walker variant consists of variable hypoplasia of the cerebellar vermis, without enlargement of the posterior fossa. There is usually a communication between the fourth ventricle and the cisterna magna. Ventricular dilatation may or may not be present and the cerebellar hemispheres are generally within normal limits (Estroff et al., 1992; Bromley et al., 1994). In mega cisterna magna (greater than 10 mm in anteroposterior dimension), an enlarged cisterna magna is present with a normal cerebellar vermis and fourth ventricle (Estroff et al., 1992). While many authors have tried to distinguish between Dandy–Walker malformation and its variant to better delineate the expected prognosis for the affected fetus, it is unclear how useful this is in clinical practice. In one series of 50 cases of Dandy–Walker malformation and 49 cases of Dandy–Walker variant, the incidence of additional CNS and non-CNS anomalies, and the incidence of karyotypic abnormalities, were similar in both groups (Ecker et al., 2000). Some authors feel that the term Dandy–Walker variant is imprecise and should not be used, and the preferred descriptive term “cerebellar hypoplasia” be used instead (Niesen, 2002). Others consider Dandy–Walker variant to be little more than a variant of normal (Patel and Barkovich, 2002).

A further normal anatomic entity, referred to as Blake’s pouch cyst, is formed from the normal dorsal expansion of the fourth ventricle into the cisterna magna, and is detected prenatally by transverse lines, or septa, within the cisterna magna behind the cerebellar vermis (Robinson and Goldstein, 2006). This finding may be associated with development of symptoms such as headache and recurrent loss of consciousness (Calabro et al., 2000).

Dandy–Walker malformation occurs in at least 1 in 5000 live born infants (Parisi and Dobyns, 2003). In a series of postnatally ascertained Dandy–Walker malformation, it occurred in 12% of cases of congenital hydrocephalus and 2% to 4% of cases of childhood-onset hydrocephalus (Murray et al., 1985; Chen and Chu, 1994).

A major consideration in the diagnosis of Dandy–Walker malformation is gestational age. Although Dandy–Walker malformation has been diagnosed in the first trimester (Achiron and Achiron, 1991; Gembruch et al., 1995; Nizard et al., 2005), a false-positive diagnosis can be made at gestational ages of less than 18 weeks. Bromley et al. (1994) prospectively evaluated 897 fetuses between 13 and 21 weeks of gestation to determine the normal development of the fetal cerebellum. A total of 147 fetuses were shown to have an open vermis at the time of initial scanning, of which 56% were open at 14 weeks of gestation, 23% were open at 15 weeks of gestation, and 6% remained open at 17 weeks of gestation. After 17.5 weeks of gestation, all fetuses were noted to have a closed vermis. These authors concluded that prenatal diagnosis of cerebellar malformations, in particular subtle findings such as the Dandy–Walker variant, should not be made at less than 18 weeks’ gestation because development of the cerebellar vermis may be incomplete. True Dandy–Walker malformations are usually visible earlier in gestation, because the cerebellar hemispheres are hypoplastic and laterally displaced, and in addition, a cyst can be visualized (Figure 11-1).

The sonographic features of the Dandy–Walker malformation include a central cyst communicating with the fourth ventricle, agenesis or hypoplasia of the cerebellar vermis, and splaying of the cerebellar hemispheres with anterolateral displacement against the tentorium, thereby enlarging the posterior fossa (Russ et al., 1989). In a study of 15 prenatally diagnosed cases of Dandy–Walker malformation, the anteroposterior diameter of the posterior fossa cyst was between 7 and 45 mm, with most being >10 mm (Russ et al., 1989). Macrocephaly was seen in 3 of 15 cases (20%), and there were additional brain anomalies in 68% of cases. Agenesis of the corpus callosum was seen in 7% to 17% of cases. In this study, extracranial anomalies were present in 60% of fetuses studied, including involvement of the cardiac, genitourinary, gastrointestinal, and skeletal systems. Of the 12 fetuses who had karyotyping, one-third were abnormal. Similar findings were noted by Nyberg et al. (1988), who reviewed seven proven cases of prenatally diagnosed Dandy–Walker malformation. Of the seven patients, five had hydrocephalus, four had multiple malformations, and two of these had abnormal karyotypes. These authors concluded that the presence of a Dandy–Walker malformation should prompt a careful search for concurrent abnormalities and consideration of a karyotype. In another study of 78 cases of prenatally diagnosed Dandy–Walker malformation, almost 20% had an associated karyotypic abnormality (Has et al., 2004).

Diagnostic criteria for Dandy–Walker variant are quite unclear, with some suggesting that this term should no longer be used as it does not appear to be predictive of outcome (Niesen, 2002; Patel and Barkovich, 2002). The sonographic criteria for the Dandy–Walker variant include partial or complete absence of the cerebellar vermis (Figure 11-2), with near normal-sized cerebellar hemispheres. The sonographic continuity between the fourth ventricle and the cisterna magna gives the appearance of a cleft (Estroff et al., 1992). In one study of 17 cases of Dandy–Walker variant, 4 fetuses also had ventriculomegaly and 3 had agenesis of the corpus callosum (Estroff et al., 1992). Almost half of the affected fetuses had other non-CNS abnormalities, including congenital heart disease, gastrointestinal malformations, renal malformations, and intrahepatic calcifications. Additionally, the presence of the Dandy–Walker variant was associated with a high incidence (29%) of abnormal karyotype (Estroff et al., 1992). In contrast, a recent study of 19 cases of Dandy– Walker variant, diagnosed strictly as isolated inferior vermian hypoplasia by prenatal MRI, demonstrated near normal outcome (Limperopoulos et al., 2006). This would underscore the importance of using strict prenatal diagnostic criteria, and perhaps using the term isolated inferior vermian hypoplasia for the select cases previously referred to as Dandy–Walker variant.

The appearance of the cisterna magna has taken on increasing importance over recent years (Pretorius et al., 1992). The effacement of the cisterna magna gives the “banana sign” of the cerebellum seen in myelomeningocele (see Chapter 19 and Figure 19-2). A small cisterna magna implies an associated neural tube defect and a Chiari II malformation. In contrast, however, an enlarged cisterna magna can be associated with a Dandy–Walker cyst, cerebellar hypoplasia, and communicating hydrocephalus (Pretorius et al., 1992). Nyberg et al. (1991) studied 33 fetuses with the sonographic appearance of an enlarged cisterna magna. Of these 33 fetuses, 18 (55%) had underlying chromosomal abnormalities. Interestingly, the absence of hydrocephalus and milder enlargement correlated more strongly with the presence of an underlying chromosomal abnormality. The negative correlation between ventriculomegaly and chromosomal abnormality was also seen in another study (Chang et al., 1994). These authors tried to distinguish the prognosis based on the sonographic appearance of the fetal vermis. They reviewed sonographic findings in 65 fetuses with Dandy–Walker malformation. Of these, 37 had inferior vermian agenesis, or the milder form of the disorder, and 28 had complete vermian agenesis. Chromosomal abnormalities were seen in 23 of the 51 fetuses who were karyotyped (45%). Chromosomal abnormalities were less prevalent among fetuses with ventriculomegaly. Extracranial abnormalities were seen in 66% of fetuses with inferior vermian agenesis (Chang et al., 1994). In another series, chromosomal abnormalities were seen in 36% and 46% of cases of Dandy–Walker variant and Dandy–Walker malformation, respectively (Ecker et al., 2000).

Major considerations in the differential diagnosis include distinguishing between Dandy–Walker malformation, Dandy– Walker variant, enlarged cisterna magna, and Blake’s pouch cyst, as well as the dorsal cyst seen in holoprosencephaly, and arachnoid cysts (see Chapter 8). The normal anteroposterior depth of the cisterna magna does not exceed 10 mm. The pathognomonic finding in Dandy–Walker malformation is a defect in the vermis through which the cyst communicates with fourth ventricle. The true Dandy–Walker cyst appears as a triangular midline fluid collection with symmetric splaying of the cerebellar hemispheres (see Figure 11-1) (Russ et al., 1989). Neither arachnoid cysts nor enlarged cisterna magna is associated with vermian defects or other cerebellar or cerebral abnormalities. Retrocerebellar arachnoid cysts compress but do not communicate with the fourth ventricle. In general, arachnoid cysts are asymmetrically positioned in the posterior fossa and tend to be rounded rather than triangular. If the Dandy–Walker cyst herniates through the foramen magnum or a defect in the occiput, this may be mistaken for a primary encephalocele (Lee et al., 2005).