Introduction

Arteriovenous malformations (AVMs) are the most commonly recognized of the vascular malformations of the brain because of their clinical and therapeutic implications. Other forms of vascular malformations (e.g. cavernous malformations, telangiectasias, and venous malformations) are not easily visualized on an angiogram or by ultrasonographic means and have only recently gained more attention because the lesions are so readily seen on magnetic resonance angiography.

Morphology

Because of the complex and variable anatomic changes, some understanding of the morphological and physiological aspects of AVMs that may influence the sonographic findings is worth notation. In addition, knowledge of the clinical presentation of AVMs can facilitate the interpretation of the Doppler findings and their changes over time.

AVMs are usually considered to be congenital, but some are fistulas which have the same angiographic appearance and arise from trauma or from venous occlusion. They are composed of a coiled mass of arteries and veins partially separated by thin islands of sclerotic tissue, lying in a bed formed by displacement, rather than by invasion, of normal brain tissue (McCormick & Rosenfield, 1973) (Fig. 20.1). A characteristic histological feature is the absence of capillaries in those assumed to be congenital, a feature not easily estimated by angiographic study alone; (McCormick, 1996; Stein & Wolpert, 1980a, b). Despite a congenital origin, AVMs usually take many years before they become clinically apparent. Some are more biologically active than others, drawing to themselves huge collaterals, while others seem almost dormant.