RADIONUCLIDE EVALUATION OF THE DANDYWALKER MALFORMATION AND CONGENITAL ARACHNOID CYST OF THE POSTERIOR FOSSA

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Abstract
The "cyclops sign" or the "isogonal sign" noted in the posterior view of a brain scintigram, along with a high midline position of the torcular, have been found to be associated with congenital cystic malformations in the posterior fossa. Differentiation between the Dandy-Walker malformation and congenital arachnoid cyst of the posterior fossa cannot be achieved on the brain scintigram. Filling of the cyst from a radionuclide ventricular injection without visualization of the subarachnoid spaces supports the diagnosis of Dandy-Walker malformation. Filling of the cyst from a radionuclide subarachnoid injection would more likely suggest an arachnoid cyst than the Dandy-Walker malformation. The angle produced by the transverse sinuses of the torcular (torcular angle) provides a ready index for the detection of congenital or acquired malformations of the brain. A standard of normalcy in children is established with a mean angle of 162° and a standard deviation of 8°, whereas the cystic malformations had a mean angle of 110° with a standard deviation of 10°. An abnormal torcular angle requires further definitive studies such as angiography or pneumoencephalography.

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