Dandy-Walker Malformation vs Arnold-Chiari on MRI findings

Dandy-Walker malformation is diagnosed based on structural defects as shown below:
1. Dilatation of 4th ventricles, along with presence of a large posterior fossa cyst with open communication between the
fourth ventricle.
2. Enlargement of posterior fossa
2. Complete of partial agenesis of cerebellar vermis
3. Upward location of tentorium cerebelli
4. Hydrocephalus

dandy-walker CT axialdandy-walker CT sagittal

Image above shows the CT(Upper) and MRI(Lower) of a child with Dandy-Walker malformation. Notice the enlarged posterior fossa which was completely occupied by a cyst. The vermis is absent in the upper CT images and hypoplastic in the lower MRI images. The third and lateral ventricles is also markedly dilated. Images are obtained from [ref1].

Although hydrocephalus is usually present in Dandy-Walker malformation, it is not a mandatory finding in making the diagnosis. Dandy-Walker malformation is part of the continuum of developmental anomalies on a spectrum known as Dandy-Walker Complex alongside with the variant, and mega cisterna magna. The “classical” Dandy-Walker malformation is presented with pathology as mentioned above. In Dandy-Walker variant, however, is believed to be a milder form of Dandy-Walker malformation. The findings of Dandy-Walker variant are shown as below:
1. The cistern magna is enlarged in size
2. Vermis and cerebellar hemispheres are hypoplastic to various extents
3. Lateral and the third ventricles are enlarged to various extents
4. Normal-sized posterior fossa with a small cystic cavity that communicates with the fourth ventricle.

As written in the previous article regarding Chiari malformation; although both Dandy-walker and Chiari involved in malformation of posterior fossa structure, they are different. The main differentiating features in general are:
1. Size of posterior fossa:
In all types of Chiari malformation, the posterior fossa is often reduced, however in Dandy-Walker it is often enlarged.
2. Herniation through foramen magnum:
Herniation of posterior fossa component such as tonsil of cerebellum and fourth ventricle is part of the complex. This is not seen in Dandy-walker.
3. The tentorium attachment:
It is low in Chiari malformation but relatively high in Dandy-Walker
4. The cerebellum:
There is reduction of cerebellum size in Chiari malformation (except type 1), however its reduce as a whole in size, however for Dandy-Walker, it involved mostly the vermis( although there is some hypoplastic features of the hemisphere)

References
1. Valery N. Kornienko and Igor N. Pronin, eds. Diagnostic Neuroradiology. First edition. Springer, Leipzig, Germany. 2009.
2. Deanna S. Adams et al. The Dandy–Walker variant: a case series of 24 pediatric patients and evaluation of associated anomalies, incidence of hydrocephalus, and developmental outcomes. J Neurosurg Pediatrics. 2008. 2:194–199
3. Melissa A. Parisia and William B. Dobynsb. Human malformations of the midbrain and hindbrain: review
and proposed classification scheme. Molecular Genetics and Metabolism. 2003; 80:36–53.
4. Andrew J. Larner, Alasdair J. Coles, Neil J. Scolding, Roger A. Barker. A-Z of Neurological Practice A Guide to Clinical Neurology. New York, NY: Springer; 2011:176-178.

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