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Median prosencephalic vein

Dr Frank Gaillard et al.

The median prosencephalic vein is a precursor to the vein of Galen, and usually regresses before birth. If an arterio-venous fistula exists, it balloons out to become the 'aneurysmal' vein of Galen malformation. This occurs since the vein is thin walled, lies free within the subarachnoid space, and is unable to accommodate the arterial flow.