Seizures, headaches, and transient ischemic attacks are far more frequent in CPA than in classical AVMs, while hemorrhages are exceedingly rare. Patients may present with progressive neurological deficits or with TIA-like symptoms indicating that hypoperfusion may be an important underlying pathological mechanism (Lasjaunias et al. 2008).

The pathophysiology of CPA is unknown, but it is assumed to be an acquired condition induced by tissue hypoxemia related to local hypoperfusion. As such, transdural supply is a well-known feature of CPA, likely to be provoked by local hypoxemia. Ducreux et al. applied perfusion-weighted MRI in seven CPA patients to demonstrate hypoperfusion throughout the affected hemisphere (Ducreux et al. 2004). Therefore, stroke-like symptoms, e.g., TIAs and nonhemorrhagic neurological deficits, are likely to occur during the natural disease course. Hemorrhage occurs much less frequently as a presenting symptom than in a BAVM, but once a hemorrhage has occurred, there is a high risk for further hemorrhages (Lasjaunias et al. 2008).

Since the typical gliotic plane of a BAVM nidus with the surrounding normal white matter is nonexistent in CPA, the surgical resection or endovascular embolization of CPA is impossible without damaging healthy brain parenchyma. As such, obliterative treatment possibilities of CPA carry a high risk of neurological deficits. In patients presenting with hemorrhages, partial targeted embolization may be employed if a target (i.e., an intranidal aneurysm) can be identified. Since the pathological mechanism is related to local hypoxemia, treatment should be targeted at increasing the blood supply to the affected hemisphere. Both in the series of Lasjaunias and in a recent case report by Ellis et al., the treatment of these lesions by pial synangiosis (burr hole treatment) leads to an increase in blood supply to the affected hemisphere and an amelioration of symptoms (including seizures, headaches, and TIA-like symptoms) (Ducreux et al. 2002a; Ellis et al. 2011).

With CT and MRI the presence of a vascular malformation in the brain is well identifiable, but further discrimination between BAVM and CPA may be problematic. On CT and MRI, CPA presents as a diffuse network of densely enhancing vascular spaces intermingled with normal brain parenchyma. T1- and T2-WI show small, widely distributed flow voids that may involve multiple lobes or the entire hemisphere. In most cases, the primary lesion extends from the surface into the basal ganglia and thalamus and involves more than one vascular territory. Compared to the size of the nidus, relatively few draining vessels are seen on CT or MRI, and these are only moderately enlarged. Perfusion-weighted MRI (p-MRI) demonstrates perfusion abnormalities far beyond the boundaries of the morphological malformation seen on conventional MR sequences, so the disease actually affects the entire hemisphere. The nidus shows increased cerebral blood volume, slightly decreased TTP (time to peak), and prolonged mean transit time (MTT). Remote from the nidus, in normal-appearing cortical and subcortical areas, the TTP is increased and the blood volume is decreased, indicating remote, widespread hypoperfusion. Lasjaunias et al. reported that in the majority (67 %) of CPA cases, its localization was in between the different vascular territories (watershed zone). In addition he noticed that in comparison with the size of the CPA nidus, there was a relative paucity of draining veins that were often only moderately enlarged (Lasjaunias et al. 2008). Another finding indicative of CPA is that both on CT and MRI, most vessels densely enhance with intravenous contrast because of relatively slow flow, while in BAVM, most vessels do not enhance due to the high flow and turbulence (Vargas and Castillo 2011).

A BAVM usually has a more or less compact nidus (Fig. 23.3). T2-WI shows punctate to linear flow voids in the subarachnoid space over the brain reflecting the dilated surface vessels. Within the brain, the punctate flow voids often lie within hyperintense, gliotic parenchyma. This gliosis may extend to the surrounding brain tissue. T1-WI shows highly variable signal within the AVM, due to turbulent flow, blood degradation products, and the variable flow rate in the veins. Different from CPA, the perfusion abnormalities do not extend far beyond the boundaries of the nidus (Ducreux et al. 2002b, 2004; Vargas and Castillo 2011) (Lasjaunias et al. 2008). Recent studies by the Toronto group with BOLD-MRI have demonstrated a severely impaired cerebrovascular reserve in patients with CPA, which was also demonstrated in BAVMs in combination with seizures, although functional imaging with the BOLD technique in the past by Ducreux et al. failed to point out the functional aspect of the brain tissue in between the CPA nidus (Ducreux et al. 2002a; Fierstra et al. 2010, 2011).

Lasjaunias et al. described the characteristics of cerebral catheter angiography in CPA in his launching paper in 2008. In BAVM pathognomonic dominant arterial feeders are present, while CPA is fed by multiple arteries that are not enlarged or are only moderately enlarged. Unlike classical AVMs, all arteries of the affected region contribute equally to the malformation. Stenosis of the proximal arteries is present in 40 % and affects the internal carotid artery (ICA) and the proximal horizontal segments of the middle cerebral artery (M1) and anterior cerebral artery (A1). Flow-related proximal aneurysms can be present in BAVM but are not seen in CPA. The nidus has a classical appearance with scattered areas of “puddling” of contrast within what looks like capillary ectasias (Fig. 23.4). These persist into the late arterial and early venous phase. The nidus is usually fuzzy, poorly circumscribed, and >6 cm in diameter. Intranidal vessels show a capillary ectasia. Perinidal angiogenesis is often present and difficult to distinguish from the nidus proper. There is no high-flow fistulous component to the arteriovenous shunt, so early opacification of draining veins is uncommon. The size of the draining veins, the “shunt volume,” and the time until the veins are visualized never “correspond” to the size of the nidus. CPA has a higher tendency for transdural supply from the external carotid circulation than BAVM (Lasjaunias et al. 2008).