Unlike AVFs, which are typically acquired lesions and usually characterized by one or a few large AV shunts, AVMs are complex developmental anomalies composed of a myriad of small abnormal arteriovenous connections that bypass the normally controlled, high-resistance vascular capillary bed that is replaced by a nidus of abnormal intervening capillaries.⁹,¹⁰ This capillary nidus may be primary or may reflect secondary capillary hypoplasia. But for unknown reasons, some pure eyelid AVMs do not show this capillary nidus.¹⁰

The origin of AVMs is not well established, but they are generally believed to be heritable. Several somatic mutations in the RASA1 gene, which regulates the RAS/MAPK pathway, have been implicated in AVMs as well as in the autosomal-dominant capillary malformation-AVM syndrome.³,⁶,¹⁰,¹¹ However, this genetic origin is contested by some authors who believe these lesions are acquired. This point of view reflects the frequent clinical observation of adult-onset AVMs, as well as the severity of these defects when they manifest clinically. According to this hypothesis, if AVMs were transmitted genetically, they should have resulted in early embryonic lethality.¹,²,⁶,⁷,¹²,¹³,¹⁴ It should be noted that the concept of an acquired vascular malformation developing much later in life is not unheard of and is well documented not just in AVMs but to a lesser extent in acquired port-wine stains (the so-called Fegeler syndrome) as well.⁶ An interesting conciliatory concept was proposed recently to explain the development of spontaneous acquired AVMs later in life.⁶ According to this theory, spontaneous AVMs are true developmental lesions that are initially quiescent. Their adult vascular remodeling may require interaction between genetic susceptibility involving downstream alterations in the RAS-MAPK pathway and an environmental second hit (hypoxia, hormonal influences, trauma, radiation, etc). This second hit acts as a “revealing trigger” and is required to initiate the development of an AVM through the loss of angiogenic suppressor mechanisms and upregulation and activation of angiogenesis, which results in abnormal vessel formation and sprouting later in life.⁶

AVMs occur principally in the head, neck, and brain.¹²,¹³,¹⁴,¹⁵,¹⁶,¹⁷,¹⁸ Head and neck AVMs are evident at birth in 59% of patients and are noted in childhood or puberty in another 20% of individuals.¹⁶ AVMs may remain asymptomatic until puberty or adulthood when they grow and start to manifest clinically.¹²,¹³,¹⁴,¹⁵,¹⁶,¹⁷ Pure preseptal eyelid AVMs are rare, and orbital cutaneous lesions are far more common (Figure 50.1). Fewer than 20 cases of pure palpebral AVMs have been described in the literature.¹⁰,¹³,¹⁴,¹⁵,¹⁶ Eyelid AVMs more commonly involve the upper eyelids; however, the significance of this observation is unknown.¹⁰

AVMs are exceedingly uncommon heterogeneous lesions that have been described in a multitude of subclassification systems in the literature, despite the absence of any clear differences in treatment options.⁶ The Schöbinger severity scoring system is concerned with the prognostication or natural progression of AVMs and is divided into four stages or grades that include stage I, the quiescent stage with pink or blue dot lesions, a red flag sign that may occasionally be encountered in the periorbital region (Figure 50.2A); stage II, the expansive stage characterized by pulsatile, expansive lesions, with engorged draining veins; stage III, similar to stage II but includes necrosis, infection, or hemorrhage (Figure 50.2C and D); and stage IV, the decompensated stage of stage III with the addition of cardiac failure.⁶,¹⁴ From an oculoplastic perspective, Warier and associates classified orbital AVMs based on location into (1) purely orbital AVM; (2) orbital and periorbital AVM; and (3) orbital with retinal or cerebral AVM (Wyburn-Mason syndrome).⁷ Other rheologic subclassification systems that are primarily concerned with brain AVMs were proposed to effectively treat those complex lesions. The most favored ones are the Houbert, the Cho-Do, and the Yakes AVM classification systems, but these may be difficult to adapt to the periorbital region.