Dandy-Walker malformation, a congenital brain malformation that typically presents in infancy and early childhood, is characterised by the triad of agenesis or hypoplasia of the cerebellar vermis, cystic dilation of the fourth ventricle, and an enlarged posterior fossa. The hallmark radiological feature is a ‘keyhole’ appearance of the fourth ventricle on axial imaging.
Description
Dandy-Walker malformation is a rare congenital abnormality that falls within the spectrum of posterior fossa anomalies. It involves the underdevelopment or absence (agenesis) of the cerebellar vermis, the part of the brain that connects the two hemispheres of the cerebellum. Alongside this, dilation of the fourth ventricle into a large cyst (Dandy-Walker cyst) and an enlargement of the posterior fossa occur. These structural changes often lead to hydrocephalus and present radiologically as a ‘keyhole’ appearance of the fourth ventricle on axial imaging due to the cystic dilation. The tentorium and torcula are frequently displaced upwards.
Pathogenesis
The pathogenesis of Dandy-Walker malformation is not fully understood, but it is believed to occur due to a disruption in the development of the brain during the embryonic stage, specifically between the 7th and 9th weeks of gestation. The malformation appears to result from a failure in the formation of the foramina of Luschka and Magendie, which are crucial for the egress of cerebrospinal fluid from the fourth ventricle. This failure leads to cerebrospinal fluid accumulation, resulting in ventricular dilation and an enlarged posterior fossa.
Subtypes
Three recognised variants in the Dandy-Walker complex are:
- Dandy-Walker malformation: The classic variant involving complete or near-complete agenesis of the cerebellar vermis.
- Dandy-Walker variant: A less severe form featuring partial agenesis of the cerebellar vermis.
- Mega cisterna magna: Characterised by a normal cerebellar vermis but an enlarged cisterna magna (>10 mm).
Epidemiology, Risk Factors & Associations
- Dandy-Walker malformation affects approximately 1 in 30,000 live births, making it a rare condition.
- It has been associated with prenatal exposure to certain risk factors, such as infections, alcohol, and antiepileptic drugs.
- The malformation is frequently associated with other congenital anomalies and chromosomal abnormalities, including trisomies. There are also associations with genetic syndromes such as Meckel-Gruber syndrome and Joubert syndrome.
Clinical Features
The clinical features of Dandy-Walker malformation are diverse, ranging from severe neurological impairment to asymptomatic cases. Symptoms often relate to increased intracranial pressure and may include irritability, vomiting, and macrocephaly. Other common neurological features include developmental delay, problems with motor coordination (ataxia), and rapid, uncontrolled eye movements (nystagmus).
Complications
The main complications of Dandy-Walker malformation stem from increased intracranial pressure due to the buildup of cerebrospinal fluid (hydrocephalus). If not adequately managed, this can lead to further neurological impairment. Hydrocephalus is a common complication that may necessitate surgical intervention.
Pathological Features
Histopathology
- Macroscopic: Enlarged posterior fossa, dilated fourth ventricle, and agenesis or hypoplasia of the cerebellar vermis are typically observed.
- Microscopic: The cerebellar vermis, when present, shows underdevelopment.
Serology
There are no specific serological markers for Dandy-Walker malformation.
Biochemistry
No specific biochemical markers have been identified for this condition.
Radiological Features
General Features
- Dandy-Walker malformation characteristically shows an enlarged posterior fossa with cystic dilation of the fourth ventricle (‘keyhole’ appearance) and agenesis or hypoplasia of the cerebellar vermis.
CT
- Non-contrast: Clear visualisation of an enlarged posterior fossa and a cystic dilatation that communicates with the fourth ventricle. There is typically an absence or hypoplasia of the cerebellar vermis, and an upward displacement of the tentorium and torcula.
MRI
- T1: The fourth ventricle cyst appears hypointense, and there is a noticeable absence or underdevelopment of the cerebellar vermis.
- T2: The cystic dilation of the fourth ventricle and surrounding cerebrospinal fluid spaces appear hyperintense, and the hypoplastic cerebellar vermis is more clearly visualised.
- T1 Gad+: No specific enhancement patterns are associated with Dandy-Walker malformation.
US
- B-mode: Ultrasonography can identify an enlarged cisterna magna and the cystic dilation of the fourth ventricle, particularly in prenatal diagnosis.
Grading and Staging
Dandy-Walker malformation does not have an established grading or staging system due to its congenital and non-progressive nature.
Diagnosis
The diagnosis is usually confirmed through imaging, with prenatal ultrasound being a common diagnostic tool. Postnatally, MRI is considered the gold standard for diagnosis, providing detailed evaluation of the cerebellar vermis, fourth ventricle, and posterior fossa.
Differential Diagnosis
- Mega cisterna magna: This condition also presents with an enlarged cisterna magna but has a normal cerebellar vermis.
- Arachnoid cyst: This is an extra-axial cyst that occurs without associated cerebellar malformations.
- Blake’s pouch cyst: This is characterised by a normal cerebellum with a retrocerebellar cyst and a non-elevated fourth ventricle.
Management
The management of Dandy-Walker malformation is typically symptomatic, addressing complications as they arise. Hydrocephalus is a common complication that often requires surgical intervention such as the placement of a ventriculoperitoneal shunt. A multidisciplinary team, including a neurosurgeon, geneticist, and relevant subspecialties, is often involved in the care of these patients. Genetic counselling may be needed due to the frequent association with chromosomal abnormalities. Early intervention services and therapy for developmental delays are crucial in the overall management of the condition.