In comparison to low-flow vascular malformations, extracranial high-flow vascular malformations are rare, with a slightly higher prevalence in females. As is true for most vascular malformations, AVMs generally grow in proportion to the child, but they have been shown to grow rapidly during puberty, pregnancy, infection, and trauma [20]. Some extracranial AVMs have been associated with mutations of the RASA1 gene [21]. RASA1 encodes for a p120 Ras GTPase-activating protein which itself is involved in angiogenesis and cell adhesion, and children harboring mutations present with a combination of skin capillary stains and a propensity to develop AVMs throughout the body (CM-AVM). The PTEN gene on chromosome 10q has also been associated with AVMs. This gene encodes a tumor suppressor protein involved in regulating angiogenesis, cell growth, and cell proliferation, and mutations are seen in multiple eponymous syndromes, such as Bannayan-Riley-Ruvalcaba and Cowden syndrome (Table 2). The preferred designation for syndromes resulting from such PTEN mutations is PTEN hamartoma tumor syndrome (PHTS). Interestingly, greater than half of these patients have high-flow vascular anomalies of the head and neck. However, these are unlike typical AVMs, in that they often have ectopic fat within or adjacent to the malformation, disproportionate dilation of draining veins, and multiple non-contiguous sites of involvement [22]. PTEN-associated arteriovenous lesions tend to respond poorly, if at all, to embolization.

In the head and neck, AVMs most frequently manifest with symptoms of hemorrhage, pain, or deformity. Particularly because total cure is so elusive and can only be entertained for the smallest of lesions, where complete resection might be possible, therapeutic goals are typically focused on symptomatic control, on preservation of critical functions such as vision, and on cosmesis. There is no universal treatment algorithm, with different teams not infrequently opting for different embolic agents, frequency of treatment, modality of treatment, etc. In those few cases where complete cure is the goal of therapy, the best results seem to occur with preoperative embolization followed by surgical resection.

The choice of embolic agent is dependent on the overall context of therapy for a given patient. Specifically, preoperative embolization can be effectively performed with short-term embolic agents such as PVA particles, Gelfoam, or Embospheres. While these provide excellent devascularization for the purposes of reducing intraoperative blood loss, these are not durable embolic agents, and thus, if the goal of treatment is permanent (i.e., anything other than preoperative embolization), liquid embolic agents, such as ethylene vinyl alcohol copolymer (Onyx), ethanol, and n-butyl cyanoacrylate (nBCA) glue, are preferred. nBCA is a clear, liquid, monomeric substance which is frequently mixed with tantalum powder and Ethiodol for radiopacity. Exposure to anions in blood causes the glue to polymerize and form a cast that is adherent to the endothelium, with the rate of polymerization of the glue controllable by the glue-to-Ethiodol ratio. In addition to the acute inflammatory fibrosis caused by the glue, there is a delayed foreign-body giant cell granulomatous reaction [23]. Onyx is a liquid embolic agent that precipitates at its surface in contact with blood, while maintaining a liquid core. Thus embolization technique consists of the formation of a proximal plug around the microcatheter tip, with further infusion allowing for controlled delivery of the liquid embolic, with longer and more extensive embolization than is possible with nBCA. Onyx additionally causes less endothelial damage and incites less inflammatory response than nBCA, resulting in improved handling of embolized malformations during surgical resection. Some advocate ethanol, which results in endothelial denudation, as the embolic treatment of choice [24], though with potentially the greatest morbidity risk.

The sonographic appearance of arteriovenous malformations is characterized by multiple hypoechoic vascular channels, without a well-defined soft tissue mass. Doppler analysis reveals arterialized draining venous waveforms [3]. CT angiographic imaging demonstrates increased caliber of feeding arteries and enlarged draining veins. Similar findings are also noted on MRI, where the rapid flow may also manifest as abnormal flow voids on spin-echo sequences. On all imaging modalities, the absence of an enhancing soft tissue mass is critical in distinguishing these lesions from others. DSA is virtually never needed to establish the diagnosis. Dedicated diagnostic catheter angiography is the routine standard of care for intracranial lesions, where multiphasic evaluation of flow dynamics to clearly identify arteriovenous shunting, precise delineation of arterial inflow, nidal vessels, and venous drainage and superselective angiography for identification of normal parenchymal branches arising from feeding pedicles is critical [25]. In contradistinction, for extracranial AVM, diagnostic angiography can typically be performed as part of the first session of embolization.

In terms of location within the head and neck, midface lesions, supplied by the internal maxillary artery, are disproportionately represented. The cheek and ear are relatively common sites, with the latter generally confined to the outer ear and possible extra-auricular spread; middle and inner ear involvement virtually never occurs (Fig. 5) [26]. Because arterial supply to ear AVMs is almost always from end arteries, complete endovascular embolization may unavoidably induce tissue necrosis and lead to amputation. As such, ear AVMs are generally observed until symptoms necessitate intervention. Although amputation and plastic surgical reconstruction may be the likely end point, staged palliative embolizations can delay amputation for several years.

Mandibular and maxillary arteriovenous malformations pose unique risks for life-threatening hemorrhage associated with dental eruptions or dental extractions. Arterial supply to maxillary malformations is usually from internal maxillary branches, though ascending pharyngeal branches, labial branches of the facial artery, and posterior ethmoidal artery branches may also contribute (Fig. 6). Given the intraosseous nature of these malformations, surgical resection is challenging and necessitates bony excision and reconstruction with graft. While some authors advocate for a combined embolization and surgical approach from the outset for these lesions, others have reported that most can be controlled with embolization alone [27]. High-volume vascular anomaly centers often use embolization as first-line therapy for newly discovered maxillary or mandibular AVM and to approach any loose tooth or required dental extraction with a further preparatory embolization, with extraction in the angiography suite.