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Arteriovenous Malformations, or also known as AVM's, are when an artery and vein connect. There are no known causes for this abnormality, but they are congenital. More often, AVM's occur in the brain, but can happen anywhere. Most of the time people with AVM's have no symptoms, unless the blood vessel leaks causing a hemorrhage. If this happens, patients develop headaches, seizures, possible vision or hearing loss, etc. However, there are other complications that may happen with AVM's such as decreased oxygen level to the brain, weakened blood vessels, or even brain damage! Although AVM's are more common in boys than in girls, pregnancy may cause a patient to start developing symptoms. Patient younger than 50 are also at an increased risk to progress signs of having an AVM.
Even though cerebral arteriography is the preferred imaging modality, MRI and CT imaging may also be used. Contrast may be given to the patient to enhance the vessels in either modality to show where the pathology is. I have uploaded images to show what an AVM looks like. One section is MRI images, the other is CTA images.
Treatment includes surgery, radiation treatment (to actually clot the vessels), or embolization (where a catheter is put into an artery in the leg then to where the AVM is and a substance is injected into the catheter to decrease flow of blood).
References:
http://mayoclinic.com/health/brain-avm
http://stanfordhospital.org/ImageGallery/images