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Neuroradiology Case of the Week

Case 99

Xiang Liu, MD, PhD and PL Westesson, MD, PhD, DDS

Clinical Presentation: An 18-year-old male presented with headache and a CT was requested for evaluation of ventricular and cyst shunt malfunction.

Radiological Findings: There is a large cystic lesion in the posterior fossa extending into the fourth ventricle. The fourth ventricle is dilated (Fig. 1), with accompanying vermian and cerebellar hypoplasia. Cerebellar hemispheres are "winged" outwards (Fig. 2). Cerebelli tentorium is displaced upward (Fig. 3).

Diagnosis: Dandy-Walker syndrome

Etiology and pathogenesis is unclear. Possible mechanisms include insult to the developing fourth ventricle and cerebellum; or delayed/absent opening of the foramen of Magendie. Most common presentation is hydrocephalus (>80%) and callosal hypogenesis (32%). Some older children may have symptoms that mimic posterior fossa neoplasm (ataxia, nystagmus, cranial nerve palsies) [1,2].

Clinical Discussion:  According to the pathology features of Dandy-Walker malformation, imaging findings of classic Dandy-Walker malformation are the following [1-5]:

1. Enlarged posterior fossa.
2. Cystic dilatation of the fourth ventricle.
3. Varying vermian, cerebellar hypoplasia.
4. Vermian remnant is displaced anterosuperiorly over the cyst.
5. Cerebellar hemispheres "winged" outwards
6. Upward displacement of torcular transverse sinuses, tentorium (torcular-lambdoida suture inversion).

    Associated abnormalities of classic Dandy-Walker malformation always include hydrocephalus (in 75% to 90%); corpus callosum dysgenesis (1/3); neuronal migration anomalies in 5% to 10%; and cephaloceles (usually occipital) in up to 16%. Brainstem in some patients may be hypoplastic [3-5].
     It should be noted that the so called "Dandy-Walker variant," always has a normal-size posterior fossa and normal or minimally dilated fourth ventricle. The degree of vermian and cerebellar hypoplasia may vary.
     Differential diagnosis includes mega cisterna magna and midline posterior fossa arachnoid cysts. In mega cisterna magna, the cisterna magna is enlarged and communicates freely with the perimedullary subarachnoid space. The fourth ventricle, vermis, and cerebellar hemispheres are normally formed. The midline posterior fossa arachnoid cysts are retrocerebellar CSF collections that do not communicate with the fourth ventricle and perimedullary subarachnoid spaces. They are not associated with cerebellar dysgenesis [5].
      Because it is often difficult to distinguish between the entities of classic DW malformation, DW variant, and mega cisterna magna, some authors suggest using the term “Dandy walker complex (DWC)” for all cases [6].

References:

1. Osborn A, Tong K. Handbook of Neuroradiology: Brain and Skull. (2nd Ed.) Mosby, 1996:153-158.
2. Barkovich AJ. Pediatric Neuroimaging. (3rd Ed.) Raven Press, 1999:339-341.
3. Patel S, Barkovich AJ. Analysis and classification of cerebellar malformations. AJNR Am J Neuroradiol. 2002 Aug;23(7):1074-87.
4. Soto-Ares G, Delmaire C, Deries B, Vallee L, Pruvo JP. Cerebellar cortical dysplasia: MR findings in a complex entity. AJNR Am J Neuroradiol. 2000 Sep;21(8):1511-9.
5. Tan EC, Takagi T, Karasawa K. Posterior fossa cystic lesions - magnetic resonance imaging manifestations. Brain Dev. 1995 Nov-Dec;17(6):418-24.
6. Kollias SS, Ball WS Jr, Prenger EC. Cystic malformations of the posterior fossa: differential diagnosis clarified through embryologic analysis. Radiographics. 1993 Nov;13(6):1211-31.