Classification of vascular malformations in general
A classification of pial brain AVMs that is based upon the size, the pattern of venous drainage and the eloquence of the portions of brain adjacent to the AVM [2] is of limited use when considering an endovascular treatment. First, such classification cannot predict the natural history of a specific AVM for an individual patient; second, it does not anticipate the risk of treating brain AVMs by endovascular techniques; third, it does not enhance our understanding of this disease.
Concerning the latter, a classification that is based upon the aetiology of vascular malformations may be a more useful approach to assessing vascular malformative diseases in general, of which arteriovenous malformations are but a subgroup [3]. This aetiological classification takes into account the target, the timing, and the nature of the triggering event [4, 5]. Because arteries and veins are already differentiated early during vasculogenesis, the target of a triggering event may vary depending on its location along the vessel tree: Arteries, the arteriovenous junction at the capillary level, veins, and lymphatic vessels are molecularly distinct vascular channels and therefore different targets [4]. The second determinant will be the timing when the trigger hits its target: an early hit during vasculogenesis (such as a germinal mutation) will affect more cells and will lead to a metamerically arranged defect, whereas a late hit (such as a somatic mutation that occurs late during the fetal life or even postnatally) will have a more focal impact on the vessel [3]. Finally, the nature of the triggering event (intrinsic, i.e., genetic vs. extrinsic, i.e., environmental, traumatic or infectious) will add another level of complexity to this schematic approach of classifying brain vascular malformations [6]. Although there is likely to be a continuous spectrum of vascular diseases rather than clear-cut disease entities, the scheme that is based on the above-mentioned assumptions may help to discern vascular malformations from an aetiological standpoint [7].
The main focus of this article will be on the endovascular treatment of the shunting lesions. While the above-mentioned classification may be helpful to broadly categorise vascular lesions, the most frequently encountered shunting malformation i.e., the pial brain AVMs, deserve further sub-classification as the presented classification is too crude to predict the natural history of a specific AVM for an individual patient and is unable to anticipate the endovascular treatment risk [8].