Surgical Management of Aneurysms of the Vertebral and Posterior Inferior Cerebellar Artery Complex

Epidemiology

Aneurysms of the VA-PICA complex comprise 0.5% to 3% of all intracranial aneurysms. Approximately two-thirds of these aneurysms are located at the bifurcation of the VA-PICA junction, whereas distal PICA aneurysms account for approximately 0.3% to 1% of all aneurysms. In a recently published series of 24 patients, distal PICA aneurysms accounted for only 0.3% of all intracranial aneurysms and for 3.7% of the vertebrobasilar lesions; 74% were saccular, 7% were fusiform, and 19% were dissecting. Until 1992, only 140 patients harboring an aneurysm of the VA-PICA complex were reported in the literature ; of these, approximately 75% were VA or VA-PICA and 25% were distal PICA aneurysms. Multiple occurrence was occasionally reported. ^, Although the incidence of aneurysms arising in association with arteriovenous malformations may be as high as 46%, the combination of a VA-PICA aneurysm and an arteriovenous malformation is rarely reported in the literature. With 85%, there is a clear predominance of female patients, ^{, , ,} which is even more notable in the case of saccular aneurysms. Patients of virtually all ages may be affected, with an average age of 49.3 years.

Aneurysmal Characteristics

In contrast to intracranial aneurysms at other locations, only some 60% of VA-PICA aneurysms are saccular, whereas 30% are dissecting and 10% fusiform. ^{, ,} In Yamaura’s series, there were 60% saccular, 27% dissecting, and 13% arteriosclerotic fusiform aneurysms; moreover, Yamaura found three giant lesions with diameters exceeding 25 mm and two partially thrombosed saccular aneurysms. Drake, Peerless and colleagues treated the world’s largest series of patients with vertebrobasilar aneurysms, comprising 1767 individual patients, of whom 217 (12.3%) harbored aneurysms of the VA-PICA complex. ^{, ,} Among these, 166 lesions were saccular (76.5%), 25 dissecting (11%), 18 fusiform (8%), 4 atherosclerotic (1.8%), 3 associated with an arteriovenous malformation (1.3%), and 1 was traumatic (0.4%). The majority (70%) were small (<12 mm); 27 aneurysms (12%) were large (13 to 24 mm) and 40 (18%) were giant (>25 mm). A left-sided origin was more common (55%). Forty-three aneurysms (19%) were unruptured. ^,

Fusiform aneurysms appear as spindle-shaped dilatations, the vertebrobasilar trunk being the most frequent location for fusiform aneurysms. Due to a clear difference in natural history and optimal therapy, they must be clearly distinguished from dissecting aneurysms. More than two decades ago, nontraumatic dissecting aneurysms were considered to be extremely rare. However, improved neuroradiologic imaging techniques have demonstrated such lesions with increasing frequency, and during the past years, this type of aneurysm has received much attention in the pertinent literature. ^{, ,} On arteriography, such aneurysms appear as a saccular or spindle-shaped vascular dilatations occasionally combined with proximal stenosis. The classic arteriographic features of dissecting arteries include the double-lumen sign, retention of contrast medium, the pearl-and-string sign, and focal outpouching. ^, They are usually not related to vascular branches of the VA. Such dissecting aneurysms of the VA may occur proximal as well as distal to the origin of the PICA, but occasionally they may involve the origin of the PICA as well. Yasui and colleagues believe that a fusiform VA aneurysm is a predisposing condition for a dissecting lesion. The sudden disruption of the internal elastic lamina is the primary mechanism underlying the development of dissecting aneurysms. The plane of dissection extends through the media, and most aneurysms have one entrance to this pseudolumen. Dissecting aneurysms of the VA often cause SAH by rupture of the adventitia and present a high risk of rebleeding. ^{, ,} These lesions are not confined to the VA but may be observed on the distal PICA as well. ^{, ,} Occasionally a dissecting distal PICA aneurysm can develop as a traumatic lesion.

Extreme dilations of the VA, termed “dolichoectasias,” occur less frequently and are usually difficult to treat. The involved artery is elongated and tortuous. On histologic examination, large defects within the muscular and elastic lamina can be detected, and sometimes also extensive arteriosclerotic changes. Although dissecting aneurysms occur more frequently in male patients of younger age, dolichoectasias occur more frequently in the seventh decade. ^{, , ,}

Historical Background

According to Hudgins and colleagues, the first case description of a saccular VA-PICA aneurysm was given by Cruveilhier in 1829. In 1947, Rizzoli and Hayes were the first to treat such an aneurysm surgically by interrupting the parent artery with two silver clips. Interestingly, the aneurysm was detected by these authors on a ventriculogram that showed a displaced fourth ventricle. Lewis and colleagues mentioned that the first case of an aneurysm arising from the distal segment of the PICA was reported in 1864 by Fernet and that the first surgical treatment of a peripheral PICA aneurysm is accredited to Olivecrona. In the 1950s and 1960s, vertebrobasilar aneurysms were associated with the highest mortality rate. Rizzoli and Hayes treated a peripheral PICA aneurysm with trapping in 1953. In 1955, Uihlein and Hughes described the nonsurgical treatment of 14 patients harboring a posterior fossa aneurysm; 8 of these patients died after the aneurysm ruptured. After the introduction of routine vertebral angiography in patients with SAH, such aneurysms were detected with increasing frequency. In 1958, Desaussure and colleagues reported the successful surgical obliteration of two PICA aneurysms found on vertebral angiograms. Further improvements in neuroradiologic techniques after the introduction of transfemoral catheter and subtraction angiography as well as the routine use of microsurgical techniques in neurosurgery have dramatically improved the outcome of surgical procedures for the treatment of vertebrobasilar aneurysms. ^,

Neuroradiologic Imaging

A meticulous preoperative neuroradiologic assessment is indispensable for successful treatment of VA and VA-PICA aneurysms. Neuroradiologic investigations should clarify the following features: (1) the exact location and origin of the aneurysm with respect to the VA and the various segments of the PICA ( Fig. 52.1 ); (2) the size, shape, extent, and limits of the lesion to differentiate between saccular, fusiform, and dissecting aneurysms; (3) the orientation of the neck and the dome of the aneurysm; (4) the presence or absence of sufficient collateral circulation; (5) the patency of both VAs and the dominance of one of them if present; (6) the presence or absence of multiple intracranial aneurysms or an associated arteriovenous malformation; (7) the precise relationship to the major surrounding anatomic structures as well as the degree of involvement of the brainstem and rootlets of the lower cranial nerves; and (8) the presence of hydrocephalus and/or intracerebellar/intraventricular hemorrhage.

Neuroradiologic studies include high-quality arterial digital angiography as well as various techniques of coronal, sagittal, and axial magnetic resonance imaging (MRI) and computed tomography (CT). Digital subtraction angiography may be complemented by rotational angiography with three-dimensional rendering. Yonekawa and colleagues described three distances that can be measured on preoperative angiograms that are important predictors of the difficulty of operative access to VA-PICA aneurysms: the distance of the aneurysm from the midline, the distance from the most lateral point of the foramen magnum, and the distance from the clivus. According to these authors, optimal results are obtained when these distances are more than 5 to 10 mm, less than 10 to 21 mm, and less than 13 mm, respectively.

In a previous communication, we emphasized that high-resolution CT using a bone tissue algorithm is most useful to demonstrate the configuration of the skull base around the jugular foramen, in particular showing the size and shape of the jugular tubercle, the size of the posterior condylar canal, and the distance between the dural entrance of the vertebral artery and the hypoglossal canal or jugular tubercle. Special sections or three-dimensional reconstructions may further add to the understanding of the individual skull base configuration or presence of bony anomalies. When performed with a bolus of contrast medium, this three-dimensional CT image may demonstrate the relationship between the lesion, brain stem, and skull base, thus adding important information required for the planning of the procedure. Huynh-Le and colleagues have confirmed the utility of three-dimensional CT angiography for the surgical management of VA-PICA aneurysms; this diagnostic technique was most valuable in demonstrating not only the exact site and shape of the aneurysm but also the relationships between parent vessel and malformation on the one side and the bony structures of the skull base on the other. CT is important to demonstrate the SAH, the blood distribution within the basal cisterns, an associated hydrocephalus, and an intraventricular or intracerebellar hemorrhage. Intraventricular hemorrhage is present in as many as 82% of patients and associated hydrocephalus in approximately 75% of patients with ruptured aneurysms. ^{, ,} MRI is particularly valuable in fusiform, dissecting, or partially thrombosed aneurysms. ^, MR angiography can depict the aneurysm in relation to the brain stem, cerebellum, caudal cranial nerves, and skull base.

Clinical Presentation

The most frequent presenting symptom of patients with aneurysms of the VA-PICA complex is SAH. Rupture of these aneurysms occurs similarly to the rupture of aneurysms of the anterior circulation. However, the clinical consequences are far more disastrous. Although only a few patients experience intracerebellar hemorrhage or deficits of the caudocranial nerves, in some instances prolonged coma, hemiparesis, or pulmonary embolism can occur. ^, Aneurysmal rupture occurs more frequently in lesions smaller than 12 mm. Large and giant aneurysms rarely rupture; they become symptomatic more frequently by their compressive effect on the lower brainstem or the caudocranial nerves. Ischemic complications such as Wallenberg syndrome may occur in dissecting aneurysms due to the occlusion of perforating arteries that supply the lateral aspect of the medulla. Patients with a dolichoectatic VA and/or basilar artery may have ischemic stroke, brainstem compression, and occasionally hemifacial spasm or trigeminal neuralgia. In the series of Drake and Peerless of 221 patients, 178 had SAH (80.5%). Sixth nerve palsy, not lower cranial nerve paresis, was the most frequent preoperative cranial nerve dysfunction, as one might have expected. It was nearly always associated with SAH and recovered completely in 75%.

Management

The decision as to whether an aneurysm of the VA-PICA complex should be treated, as well as the timing and choice of treatment modality in case treatment appears indicated, depends on criteria such as the patient’s age, actual clinical condition, and neurologic status; progression or resolution of initial symptoms and signs; presence or absence of SAH; the interval between SAH and time of decision-making; aneurysmal characteristics; medical history; and the presence or absence of hydrocephalus and intraventricular or intracerebellar hemorrhage. Surgery, for instance, is preferable when a significant hematoma must be evacuated. For more than a decade, the concomitant availability of endovascular and microsurgical procedures has made possible a multimodal treatment of aneurysms of the VA-PICA complex. In particular, the technologic achievements in neurovascular instrumentation (i.e., coil technology, intracranial stent technique) of recent years have led to a continuing challenge for interventional neuroradiologists, enabling them to treat even previously “untreatable” aneurysms with a high rate of success. However, complications of treatment, such as brainstem infarction and hemorrhages, are also reported for the interventional therapy of VA-PICA aneurysms. ^{, ,} For some rare complex cases, a combined interventional and microsurgical therapy may constitute a reasonable solution.

In some aneurysms, such as proximal PICA lesions, even in cases where endovascular coil occlusion of the aneurysms seems possible, direct microsurgical inspection of the affected segment of the PICA and of perforating brain stem–supplying arteries may offer significant advantages compared with endovascular therapy. Considering the complexity and heterogeneity of VA and VA-PICA aneurysms, many or most lesions may require individual case-by-case decisions.