Vascular Surgery for Arteriovenous Malformations 

Vascular anomalies are among the most common congenital abnormalities in infants and children. Historically, the treatment of these lesions has been impeded by confusing terminology and lack of a precise classification system. Lesions were named with descriptive terms such as strawberry hemangioma or port-wine stain or histopathologic terms such as capillary hemangioma, cavernous hemangioma, or lymphangioma. Although some of the vascular anomalies may appear similar, their biological behavior differs markedly; therefore, treatment should be based on a proper classification system.^[1]

The current classification system is based on the landmark investigation by Mulliken and Glowacki published in 1982, which proposed a simplified classification of vascular anomalies based on biological activity.^[2] As a result, we now recognize 2 main types of vascular anomalies: vascular tumors and vascular malformations. Differentiating between these 2 types is essential because their treatment is quite different. The management of vascular anomalies is a dynamic and rapidly developing subspecialty, which requires interdisciplinary care and collaboration.

The image below depicts arteriovenous malformations.

The International Society for the Study of Vascular Anomalies (ISSVA) began in 1992 after a series of biennial international workshops begun in 1976 by Drs. John Mulliken and Anthony Young.^[3] The primary goal of the ISSVA is to improve the understanding and management of vascular anomalies by promoting interdisciplinary and international collaboration. The classification system adopted by the ISSVA during its 1996 workshop provides a common language and guides treatment. It is founded on Mulliken and Glowacki’s biological study but also further distinguishes vascular malformations based on hemodynamics and predominant anomalous channels (see Table 1 below).^{[4, 5, 6, 3]}

The goal of this article is to review one type of fast-flow vascular malformation in particular: arteriovenous malformations.

Table 1: ISSVA 1996 Classification of Vascular Anomalies (Open Table in a new window)

The pathogenesis of AVMs is not well understood^[7] but is thought to result from abnormal vasculogenesis. Multiple biological studies since 1982 have demonstrated clear differences between vascular tumors (hemangiomas) and vascular malformations. Some have hypothesized that infantile hemangiomas result from excess angiogenesis, while vascular malformations are due to errors in vessel remodeling.^[8] Although some vascular malformations thicken, expand, or multiply with time, whether true angiogenesis occurs is unclear.

Marler et al suggest that vascular malformations may be angiogenesis-dependent disorders. They found that urinary high-molecular-weight matrix metalloproteinases (hMW MMPs) and bFGF levels are elevated in vascular tumors and some vascular malformations, such as lymphatic malformation (LM), lymphaticovenous malformation (LVM), and AVMs, and that the urinary increase in these proteins parallels the tissue remodeling seen in diffuse and expanding vascular malformations.^[9] They suggest that drugs targeting basic fibroblast growth factor (bFGF) or matrix metalloproteinases (MMPs) may be an adequate therapeutic strategy for patients suffering from these vascular anomalies.^[9]

Inherited forms of vascular malformations are rare^[4] but may allow insight into the molecular mechanisms and signaling pathways involved in the pathogenesis of these lesions. This may in turn identify potential novel therapeutic targets, although whether the more common sporadic vascular malformations share similar biological mechanisms with the infrequent inherited vascular malformations is unclear at this time.

An arteriovenous malformation (AVM) is a hemodynamically active, fast-flow vascular malformation. Arterial feeders and enlarged draining veins directly connect through micro- and macro-arteriovenous fistulas that create the nidus or epicenter of the AVM. AVMs may occur both superficially and viscerally. They are usually present at birth and rarely regress.^[4] They have a normal endothelial cell cycle and grow commensurately with the child.^{[2, 1]} The natural history of AVMs is organized into a clinical staging system proposed by Schobinger at the 1990 ISSVA meeting in Amsterdam (see Table 2 below).^[10]

Table 2: Schobinger Staging for AVMs (Open Table in a new window)

Frequency

United States

Although vascular anomalies are among the most common pediatric abnormalities occurring in approximately 1% of children,^[11] AVMs are rare.^[4]

Mortality/Morbidity

AVMs never regress and usually follow the stages outlined by Schobinger. Morbidity and mortality is dependent on several factors: location and size of the AVM, whether it is surgically accessible or amenable to palliative embolization/sclerotherapy, and the presence of congestive heart failure.

Sex

The female to male ratio for vascular malformations is 1:1.^{[2, 12]}

Age

Most AVMs are evident at birth (40% in a study of 200 AVMs by Enjolras et al),^[12] although they may not be clinically relevant. Mulliken and Glowacki noted 90% of vascular malformations were present at birth in a series of 23 patients; however, these lesions were predominately venous in type and may not be representative of AVMs per se.^[2]