Broadly speaking there are malformations which demonstrate arteriovenous shunting and those which do not. Arteriovenous shunting occurs when blood flows directly into the venous system from the arteries without passing through a capillary bed and/or supplying tissue with oxygen and nutrients. We can also describe them in terms of where they are located in the nervous system.
Vascular malformations may remain entirely asymptomatic through life. As high quality imaging of the brain becomes more accessible they are discovered by accident with increasing frequency. As with most things in nature there are rare entities which demonstrate characteristics of both types- mixed malformations. It is somewhat contentious whether or not Cavernomas constitute a variety of vascular malformation. Cavernomas are dealt with here in more detail.
Arteriovenous Shunts- blood flows directly into the venous system from the arteries without passing through a capillary bed and/or supplying tissue with oxygen and nutrients.:
No Arteriovenous Shunt
Arteriovenous malformations (AVM for short) are complicated tangles of blood vessels which can occur in almost any living tissue. They represent one of the types of vascular malformations found in the nervous system. When found within the central nervous system they may be associated with the development of seizures or brain haemorrhage. It is likely that many remain silent and undiscovered throughout life. Understanding what provokes an AVM to cause symptoms continues to be a focus of much research. In recent years we have come to understand that some vascular malformations may develop under genetic influence during life but there is a great deal yet to be learned.
In some ways each AVM is completely unique and this goes some way to explaining the difficulty experienced in predicting wheny they will cause illness. Normally blood, laden with oxygen, passes from arteries to tiny blood vessels called capillaries where the oxygen is delivered to the organ that needs it. The blood then leaves the organ through veins on its way back to the lungs to collect more oxygen. Capillaries are missing in an AVM and the blood flows directly from arteries to veins without supplying any tissue. The tangle of vessels through which the blood flows under high pressure is called the nidus from the latin word meaning "nest."
The additional work that contributing blood vessels do in delivering high blood flow to the nidus may result in additional stresses on the vessel wall. Signs of wear-and-tear develop such as aneurysms (blister like dilatations) on arteries or within the nidus and these can be weak points from where the AVM may bleed.
By using the buttons at the top of this screen you can learn more about the various different types of vascular malformation and their treatment.