Congestive brain oedema associated with a pial arteriovenous malformation with impaired venous drainage.

We describe two patients with an unruptured pial AVM accompanied by significant brain oedema at initial presentation. In both cases, the primary drainer was a cortical vein showing varicose dilatation. in which venous congestion was indicated by magnetic resonance imaging (MRI). The restriction of venous drainage presumably caused venous hypertension in the surrounding brain, leading to the brain oedema and neurological symptoms. Brain oedema can develop in patients with an unruptured AVM by venous congestion following spontaneous thrombosis of venous components. Varicosity in a major cortical draining vein and a small nidus are the possible lesions predisposing this fairly rare condition for unruptured AVMs.