The certain pathogenesis of AVMs is not clear. They are considered to be congenital malformations. The embryological development of cerebral vessels occurs in two phases: vasculogenesis and angiogenesis. In the vasculogenesis, angioblasts differentiate into endothelial cells to form the primary vascular plexus. Later, angiogenesis follows, in which the primary plexus undergoes remodeling and organization, leading to the formation of the final cerebral vessels (Streeter 1918; Risau and Flamme 1995; Risau 1997). The causes of an aberrant vasculo-angiogenesis leading to AVMs are unknown. Many factors are probably involved; among them, some endothelial growth factors (VEGFR1-VEGFR2) and their binding receptors (FLt-1; FLk-1) have been identified as important for the normal development of cerebral vessels. Absence, mutation, or highest levels of these factors could lead to aberrant development and formation of AVMs (Shalaby et al. 1995; Fong et al. 1995; Sonstein et al. 1996; Uranishi et al. 2001; Hashimoto et al. 2001).

When considering the embryological development of the cerebral arteries and veins, some authors (Mullan et al. 1996a, b) have suggested that AVMs could already be present before the third month of gestation. In some cases, an AVM may be relatively small at birth and grow later. There are, however, reports describing the appearance of cerebral AVMs later in life among patients in whom previously performed magnetic resonance imaging (MRI) showed no malformations. In some of these patients, cerebral AVMs occurred in the pathologically altered brain as a result of different causes, such as vascular pathology (Schmit et al. 1996; Song et al. 2007), heterotopia (Stevens et al. 2009), and changes after radiosurgery (Rodriguez-Arias et al. 2000); in others, the brain parenchyma was completely normal (Gonzalez et al. 2005; Bulsara et al. 2002). These observations raise doubts about the congenital nature of cerebral AVMs, which—at least in some cases—seem to be acquired lesions caused by different nonspecific insults on the brain.

The main angioarchitectural characteristic of an AVM is an area called the nidus, in which a direct shunting between arteries and veins occurs without interposed capillaries. The elevated intravascular flow leads to changes of the vessels. Histology shows the nidus to be composed basically of dilated arteries and veins. In some vessels, the wall structure is still recognizable, characterized by the presence of a media with smooth muscle cells and an elastic lamina in the arteries and an absence of muscle cells in the veins. In other arteries, prominent changes, characterized by areas of wall thickening caused by proliferation of fibroblasts, muscle cells, and an increase in connective tissue, are present. Segments with a thinning of the wall also occur, which potentially can lead to aneurysm formation. Severe changes take place in the venous sector, forming so-called arterialized veins, characterized by wall thickening, which is particularly due to fibroblast proliferation, not smooth muscle cells. The interposed parenchyma shows gliosis, hemosiderin pigmentation, and calcifications, resulting from ischemia or previous hemorrhages. The surrounding parenchyma may appear normal or show similar changes (Challa et al. 1995; Kalimo et al. 1997; Brocheriou and Capron 2004).

The incidence of AVMs is not completely known. In general autopsy, they are discovered with a frequency of 0.15–0.8 % (McCormick 1984; Jellinger 1986). Multifocal lesions can occur with a frequency of 1–10 % (Perret and Nischioka 1966; Rodesch et al. 1988; Willinsky et al. 1990); the latter are more frequent in pediatric patients, where they are reported to be twice as common as in adults (Rodesch et al. 1988; Lasjaunias 1997).

Of AVMs, 5–10 % remain asymptomatic and are diagnosed incidentally by CT or MR investigations performed for other reasons. Some 40–50 % present with intracranial hemorrhage, 30 % with seizures, 10–15 % with headaches, and 5–10 % with neurological deficits (Perini et al. 1995; Stapf et al. 2002; Hofmeister et al. 2000; Valavanis et al. 2004); the incidence of symptomatic cerebral malformation in the adult population is reported to be one-tenth the frequency of intracranial aneurysm (Berenstein and Lasjaunias 1992; Valavanis et al. 2004). The most important risk in AVM is hemorrhage, which is calculated to be 2–4 % per year, with an annual rate of mortality of 1 % and severe morbidity of 1.7 % (Graf et al. 1983; Crawford et al. 1986; Ondra et al. 1990; Mast et al. 1997). The risk of a repeated hemorrhage after an initial episode is reported to increase in the first year, later decreasing until it reaches the level of the initial risk (Graf et al. 1983; Mast et al. 1997). It is the most frequent initial symptom in children (Berenstein and Lasjaunias 1992; Rodesch et al. 1995; Lasjaunias 1997).

Cases of spontaneous thrombosis of AVMs (Sukoff et al. 1972; Levine et al. 1973; Mabe and Furuse 1977; Pascual-Castroviejo et al. 1977; Sartor 1978; Nehls and Pittman 1982; Omojola et al. 1982; Wakai et al. 1983; Pasqualin et al. 1985; Barker and Anslow 1990; Ezura and Kagawa 1992; Hamada and Yonekawa 1994; Abdulrauf et al. 1999) as well as its possible recanalization occurring even a few years later (Mizutani et al. 1995) have been reported. A long follow-up of these patients is mandatory.

The majority of AVMs (85 %) are located in the supratentorial area, and only 15 % are infratentorial (Perret and Nischioka 1966; Yaşargyl 1999). Supratentorial AVMs can be further divided (Valavanis et al. 2004): neopallial, including AVMs in the frontal, parietal, temporal, and occipital lobes and corpus callosum, and archi- and paleopallial, including those in the limbic and paralimbic system (amygdala, hippocampal, parahippocampal, septal, gyrus cinguli, and insular AVMs). AVMs can be located in a sulcus (sulcal), gyrus (gyral), or both (sulco-gyral). They can remain superficial or extend deeply toward the ventricle, basal ganglia, and thalamus. AVMs involving primary deep structures or ventricles are rarer. They are more frequent in pediatric patients (Berenstein and Lasjaunias 1992).

Infratentorial AVMs can be divided into those involving the cerebellum (hemisphere, vermis), located on the superior – inferior convexity or on its anterior surface. Deep structures can be primarily involved or be an extension of a superficial lesion. Primary AVMs in the brainstem are very rare, as are those of the fourth ventricle (Garcia Monaco et al. 1990; Liu et al. 2003).

MRI, including functional studies, provides informations about the site and extension of AVMs. Furthermore, it shows which functional changes have occurred in the affected and unaffected hemisphere (Alkadhi et al. 2000). Angiography is essential in defining the angioarchitecture of the malformation. It comprises selective angiography of the internal and external carotid arteries and the vertebral artery, followed, when necessary, by super-selective examinations aimed to characterize the supplying arteries, venous drainage, and aspects of the nidus.