Yin-yang sign on magnetic resonance imaging in a child with left frontal arterio-venous malformation and foot drop

We report an 8-year-old girl who presented with acute right foot drop with preserved reflex following minor trauma. Brain magnetic resonance imaging showed a large pial arterio-venous malformation (AVM) over the left frontal paramedian convexity with a large varix draining into the superior sagittal sinus. On post-contrast T1-weighted imaging, the AVM showed a swirling pattern with sharply marginated outer hypo-intense area and inner hyper-intense area reminiscent of the Chinese yin-yang symbol. A partially thrombosed varix in an AVM nidus was suspected which was confirmed on histopathology following resection. Unilateral foot drop with preserved reflex is one of the rare clinical presentations of a frontal paramedian AVM. Yin-yang sign is a unique magnetic resonance imaging finding suggestive of a partially thrombosed AVM.