Introduction

   Cerebral arteriovenous malformation (AVM) associated with an aneurysm was first mentioned in the literature in 1925, and in parallel with the neuroradiologic technical developments made since many more AVMs associated with aneurysms have been described.¹⁸⁾ When associated with AVMs, aneurysms occur not only at the bifurcation points of the circle of Willis but may also be found in unusual locations such as in the more distal segments of the cerebral vasculature, or within the AVM nidus.⁵⁾ 
   The pathogenesis of intracranial aneurysms associated with AVMs remains a subject of debate, particularly as whether it is related to a congenital defect of the vessel wall or caused by increased blood flow to an AVM.^1)6)10)12) Moreover, it has been suggested that this association may by coincidental.³⁾ Nevertheless, it should be borne in mind, that different pathogeneses influence treatment decisions and acute patient management. In this report, we describe three patients with a rare combination of a PICA aneurysm with an AVM.

Illustrative Cases

1. Case 1
   A 34-year-old man suffered from the acute onset of headache, nausea, and vomiting a day prior to admission. He had a mildly drowsy but oriented mentality, and in a previous neurologic examination he had no focal neurological deficits. Initial computerized tomography (CT) of his brain showed hemorrhage of right cerebellum and intraventricular hemorrhage (IVH) of fourth ventricle (Fig. 1A). Brain CT also showed a subtle subarachnoid hemorrhage in the suprasellar cistern and acute hydrocephalus. Vertebral angiography revealed a small saccular aneurysm of the right distal PICA and a 1 cm-sized AVM which is located distally from the aneurysm (Fig. 1B). The patient underwent a midline suboccipital craniectomy. Although the aneurysm of the distal PICA was detected after retraction of the cerebellar tonsil, there was no hematoma around the aneurysm. Surgical clipping of the aneurysm and subsequent resection of the AVM was performed without complication. Although we did not perform postoperative repeated angiography, postoperative contrast-enhanced CT of brain showed no residual enhancing vascular mass (Fig. 1C). He was discharged without any neurologic deficits, although he complained of short period of mild gait disturbance and dizziness postoperatively.

2. Case 2
   A 25-year-old female presented with a four-day history of severe headache, dizziness, and neck stiffness on physical examination. She showed an alert mentality and no focal neurologic deficits except for her right cerebellar symptoms. Brain CT revealed vermian hemorrhage of lateral ventricle (Fig. 2A). Vertebral angiography showed a right PICA aneurysm at the supratonsillar segment and a cerebellar AVM feeding mainly from the right PICA (Fig. 2B). Surgical clipping of the aneurysm and subsequent resection of the AVM was done without complication. Postoperative repeated angiography showed complete obliteration of the AVM and aneurysm (Fig. 2C). She complained of mild dizziness postoperatively which disappeared during follow-up.

3. Case 3
   A 60-year-old man visited the emergency room having suddenly developed severe headache. He was drowsy, but cooperative. Brain CT showed both subarachnoid hemorrhage and IVH of fourth ventricle. Left vertebral angiography showed a 3 cm-sized cerebellar AVM mainly fed by the left PICA and the superior cerebellar artery, and a saccular aneurysm on the telovelotonsillar segment of PICA. He underwent surgical clipping of the PICA aneurysm only. During follow-up, a ventriculoperitoneal shunt was performed because of symptomatic hydrocephalus. Four months after the surgical clipping of the aneurysm, the AVM was treated by gamma knife radiosurgery in other institute. His postoperative course was uneventful. Follow-up angiography or magnetic resonance imaging was not performed.

Discussion

   Most patients with cerebral AVMs present with symptoms caused either by seizure, focal neurological deficits, or a sudden neurological deficit associated with hemorrhages.¹³⁾ Recent clinical studies on AVM treatment have revealed that angioarchitectonic features such as venous outflow obstruction, extensive secondary vascularization through perforating vessels, deep venous drainage, and aneurysms related to AVMs are closely related to the incidence of hemorrhage.^{5)7)11)14)16)} Moreover, it is well-known that concurrent arterial aneurysms have a higher risk of hemorrhage in patients with AVMs.^1)2)17)
   At present, three theories have been proposed to explain the mechanism of the formation of an AVM-associated aneurysm.¹⁾¹²⁾ 1) Aneurysms are caused by hemodynamic stresses resulting from the presence of an AVM. 2) Both lesions are congenital vascular developmental disorders. And, 3) the association is purely coincidental. Thus, it is apparent that the etiology of cerebral aneurysm and AVM in the same patient has not fully elucidated. These aneurysms can be divided into four types：1) unrelated dysplastic or incidental aneurysms (hemodynamically independent), 2) flow-related aneurysms on proximal feeding vessels, 3) flow-related on distal small feeding vessels and 4) intranidal aneurysms associated with the AVM.^9)14)16)17) It has known that true spontaneous intracra-nial arteriovenous fistulas (AVFs) of posterior circulation rather than traumatic AVFs are very rare.⁴⁾ However, it has known that pedicle aneurysms on feeding vessels occur more frequently in conjunction with infratentorial AVMs.¹⁷⁾ As for the specific situation of AVMs in the infratentorial location, only recently a review focused specifically on PICA aneury-sms associated with cerebellar AVMs.⁸⁾
   The most popularly accepted hemodynamic theory of what was first proposed by Paterson and McKissock in 1964.¹²⁾ In a report of six patients treated with carotid ligation for intracranial aneurysms, two developed new aneurysms on the contralateral carotid arteries, presumably because of altered hemodynamics.¹⁵⁾ Additional support for a hemodynamic approach is provided by case reports of aneurysms that have diminished in size or disappeared after the elimination of the shunt through an AVMs.⁶⁾¹⁵⁾ This phenomenon was explained as being due to restoration of normal blood flow to the feeding artery caused by the reappearance of normal autoregulation.⁸⁾ However, it is not clear whether process is due to a partial or complete thrombosis of the aneurysm or because of a reduction in the size of the aneurysm. Conversely, the growth of aneurysms has been documented when both lesions are untreated, as has the case of an aneurysmal rupture following AVM resection, raises the issue of perfusion overload factors.⁵⁾
   We believe that in view of the high probability of intracranial hemorrhage development in AVM-associated aneurysm patients, an aggressive treatment approach is indicated. The decision of whether and when to treat an unruptured AVM-associated aneurysms must be made on an individual basis.⁵⁾¹⁶⁾ Westphal et al.¹⁷⁾ concluded that AVM-associated aneurysms that are located on major vessels can initially be left untreated if the exposure warranted to remove the AVM does not allow easy access to the aneurysm. These aneurysms should then be observed because they may well be flow related and may regress or, at least, remain stable once the AVM has been removed. If the aneurysm is proximal to the AVM on a main trunk, and does not regress on further observation, treatment should be considered, as for any other asymptomatic aneurysm, and such consideration should include all available endovascular and surgical alternatives. If a large high-flow AVM distally located to an aneurysm is to be removed, and a high-flow proximal aneurysm-bearing bifurcation will be maintained by noninvolved vascular territories, Westphal et al.¹⁷⁾ recommended that the aneurysm be occluded as the first step by available interventional means, because the aneurysm does not necessarily have to be included in the surgical exposure. When aneurysms are located in distal feeding vessels, they recommended that aneurysms should be clipped or eliminated when accessible because they are a source of the hemorrhage, rather than the AVM. Aneurysms on exclusive feeding vessels with no other dependent territory can be left alone, because the carrying vessel will be obliterated. Thompson et al.¹⁶⁾ developed a management protocol designed to treat these aneurysms using surgical or endovascular means before administering definitive AVM treatment. In patients presenting hemorrhage, an important first step is to determine which lesions are responsible, and it the pattern and location of the hemorrhage indicate aneurysmal bleeding, they recommended early surgery or its securement by endovascular coiling. When a hemorrhage is clearly secondary to an AVM, it should be treated expectantly with measures to reduce brain swelling and control blood pressure to prevent subsequent bleeding. Once the patient stabilizes, the associated aneurysms can be treated before administering definitive AVM treatment. In some cases, the source of the hemorrhage cannot be identified with certainty. Under these circumstances, they recommended early surgery or coiling of the aneurysms. Simultaneous treatment of both lesions is recommended when the two lesions are in close proximity and accessible by a single operative approach, and when the size of the AVM permits surgical resection in a single stage. One of the important management considerations in these patients is the surgical treatment of aneurysms located on vessels not directly associated with the AVMs. In these situations, it is reasonable to manage aneurysms on the individual basis, taking into account such factors as the location of the aneurysm relative to the potential pathway traversed by the catheter during embolization. Intraoperative hemorrhage from an AVM during aneurysm surgery is an additional consideration in the management of patients with coexisting intracranial vascular lesions. A number of important variables may contribute to AVM bleeding during aneurysm surgery, including the anesthetic technique, fluid and blood pressure management, and patient positioning. Close follow-up of these patients is mandatory and should include magnetic resonance imaging and angiography during the treatment phase to evaluate the progressive thrombosis of the AVMs and to monitor for the development of new aneurysms that may increase the risk of rupture.

Conclusion

   Recently, we experienced two cases of presenting with cerebellar hemorrhage secondary to AVM rupture and one case of presenting with SAH due to PICA aneurysmal rupture.
   We were able to obtain good surgical results with single stage operations because the aneurysms were located distally on a feeding artery and the lesions were in close proximity and accessible by a single approach. Moreover, as our third case, after surgical clipping of the ruptured aneurysm, and subsequent treatment of the AVM by gamma knife radiosurgery or endovascular treatment instead of surgical resection could be considered as a good treatment modality.

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