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Estimated Time for Completion: approximately 1 hour
Date of Release and Review: February 3, 2014, January 15, 2017
Expiration Date: January 31, 2020
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The Dandy-Walker complex consists of the classic Dandy-Walker malformation (DWM), Dandy-Walker variants, and a mega cisterna magna.
The characteristic findings with a Dandy-Walker malformation (Fig. 1) consists of an enlarged cisterna magna, an absent cerebellar vermis, and superior displacement of the tentorium (Fig. 2)1. Although ventriculomegaly is present in the majority of cases of DWM, it is not required for the diagnosis2. Ventriculomegaly often develops postnatally3.
The Dandy-Walker variant has a variable degree of cerebellar vermian hypoplasia with or without enlargement of the posterior fossa. Inferior vermian defects are more common than classic DWM, occurring in 57% - 75% of cases4,5.MRI studies have shown that a clear distinction between a DWM and a Dandy-Walker variant is not consistently possible2.
A mega cisterna magna (Fig. 3) has been defined as a posterior fossa ? 10 mm with a normal cerebellar vermis and 4th ventricle.
The continuum from classic DWM to mega cisterna magna relates to the timing and severity of the insult on the developing brain. With a DWM, the roof of the 4th ventricle and the cerebellar vermis are affected (Fig. 4). A Dandy-Walker variant may be due to a more localized cerebellar insult. If only the roof of the 4th ventricle is affected a mega cisterna magna with a normal cerebellum will result2,6,7.
The prevalence of Dandy-Walker malformation has been estimated at 1/30,000 births8. The recurrence risk for isolated DWM is 1% to 5%9.
Associated malformations occur in 50 to 70% of cases with DWM (Table I)8. As the number of associated anomalies increases, so does the neonatal mortality rate and the likelihood of a karyotypic abnormality3. Neonates with two or more other anomalies are six times more likely to die than neonates with an isolated DWM10. Agenesis of the corpus callosum (Fig. 5) is a frequently associated central nervous system malformation. Cardiac anomalies are also commonly associated with DWM.
Karyotypic abnormalities are present in 40% of fetuses with DWM4. A DWM is, therefore, a significant sonographic marker for a chromosomal abnormality. The most common karyotypic abnormalities associated with DWM are trisomy 13, 18 and 213,4.In a fetus with DWM, a transverse cerebellar diameter < 5th centile suggests cerebellar hypoplasia and a possible karyotypic abnormality11.
There are a number of syndromes that may have a DWM as one component (Table II).
The Aicardi syndrome characteristically consists of callosal agenesis, ocular abnormalities and infantile spasms12. Additional features include vertebral anomalies, mental delay and Dandy Walker malformation13. Early embryologic over distension of the neural tube is the cause for both the congenital absence of the corpus callosum, as well as the Dandy Walker malformation14.
This disorder is lethal in males and is, therefore, only detected in karyotypically normal females.
Fryns syndrome is an autosomal recessive disorder with multiple congenital anomalies (Table III)15.
Phenotypia expression is highly variable. Fryns syndrome is the most common autosomal recessive syndrome with a congenital diaphragmatic hernia16. However, a diaphragmatic hernia is not necessarily a feature of Fryns syndrome17. Survival with Fryns syndrome is improved without a diaphragmatic hernia18. The incidence of FS is 1/14,00016.
A Dandy Walker malformation occurs from in 10%16 to 50%19 of cases.
Approximately 10% of patients with congenital diaphragmatic hernia have Fryns syndrome20.
Meckel-Gruber syndrome (MGS) is a lethal autosomal recessive condition. It has been mapped to six different loci on different chromosomes21.
The triad of common findings with MGS include: cystic renal dysplasia; occipital encephalocele; and post-axial polydactyly. These specific stamata are found in 100% , 90%, and 83.3% of cases, respectively22. However, there are wide phenotypic variations (Table IV).
A Dandy Walker malformation is one of the central nervous system malformations associated with this syndrome23.
Pulmonary hypoplasia, due to the associated dysplastic kidneys and oligohydramnios, is the leading cause of death.
Prenatal diagnosis in affected families has occurred in the 1st trimester24.
The incidence of MGS is between 1/13,250 and 1/140,000. The incidence in Finland is 1/9,00023.
Dandy Walker malformation and occipital encephaloceles are due to disturbances in the development of the rhombencephalon25.
MGS is the most common syndromic etiology for neural tube defects; accounting for 5% of neural tube defects.
Approximately half of families with more than one child affected by MGS may have different manifestations of the syndrome26. In one series only 18% of subsequently affected siblings had all three of the classic defects27.
SLOS is an autosomal recessive disorder with multiple congenital anomalies (Table V), intra-uterine growth restriction and developmental delay.
The biochemical defect results in a significant reduction in the final enzyme (7-dehydrocholesterol reductase) in the cholesterol pathway. While there have been multiple mutations identified, seven mutations account for almost 70% of the cases28.
SLOS has a wide phenotypic expression that extends from isolated syndactyly to holoprosencephaly. The severity of anomalies in a particular case may relate to the degree that cholesterol production is inhibited by the specific mutation resulting in SLOS29.
WWS is a severe form of congenital muscular dystrophy. The characteristic features of WWS are outlined in Table VI. Its estimated incidence is 1.7/100,000. Most children die by the age of three30.
*Data obtained from 21 patients.
WWS is an autosomal recessive disorder. 10-20% of cases can be confirmed by DNA analysis of mutations in the protein O-mannosyltransferase 1 (POMT 1) gene31,32. The genetic basis for the majority of cases are not known. In families with a known family member, antenatal ultrasound has detected ventriculomegaly, as early as 13 weeks’ gestation33 and an occipital encephalocele was detected in another case at 18 weeks’ gestation34.
First trimester congenital infections, and warfarin or alcohol exposure have been associated with DWM35,36.
The 1996 guidelines for the performance of a standard 2nd trimester ultrasound examination included an assessment of the cerebellum and cisterna magna (Fig 6)37.
The sonographic detection of DWM has been reported as early as 14 weeks' gestation38.
The cerebellar vermis forms beginning superiorly at a gestational age of 9 weeks and progresses inferiorly until the process is complete at around 15 weeks' gestation4. By 18 weeks' gestation the cerebellum and vermis have their customary appearance40. A diagnosis should not, therefore, be made until ? 18 weeks’ gestation39,40.
False positive diagnoses of a DWM or Dandy-Walker variant result when only axial images are obtained or the cerebellum is scanned in an angled semi-coronal plane (Fig. 7)41,42.
An elevation of a normal vermis towards the tentorium (Fig. 8) can suggest the presence of vermian defect when one is not present1,4,41. With two-dimensional sonography the suggestion of a small vermian defect is usually a false positive. The characteristic gap between the cerebellar hemispheres with Dandy-Walker malformation has a trapezoid (Fig. 9), rather than a "key-hole", appearance43. To improve the diagnostic accuracy of antenatal sonography in the detection of Dandy-Walker malformation, at least two scanning planes should be used44. A three-dimensional coronal view provides the best evaluation of the vermis (Figs. 10 a,b).
Because of the subtleties involved in the antenatal sonographic detection of either cerebellar vermian hypoplasia or cerebellar dysplasia, antenatal sonography permits the diagnosis of only a classic DWM with any degree of certainty. Neuropathologic examination fails to corroborate a diagnosis of DWM in 59% of cases. A similar, or higher, false positive rate occurs with Dandy-Walker variant, i.e. agenesis of the inferior cerebellar vermis43.
Developmental delay has been reported in approximately 50% of cases with isolated DWM3. It has been suggested that developmental delay is due to associated central nervous system anomalies rather than to vermian absence35. The overall prognosis, i.e. normal development to severe disability, appears to be similar with a complete absence of the vermis and inferior vermian agenesis4,11. The liveborn mortality rate among fetuses with DWM is approximately 40%4.
The outcome of fetuses with DWM is worse than for neonates with a similar condition. This discrepancy is due to severe associated anomalies or karyotypic abnormalities found in the fetuses diagnosed with DWM35.