43 Posterior Inferior Cerebellar Artery Aneurysms
Abstract
Posterior inferior cerebellar artery (PICA) aneurysms constitute a heterogeneous group, and management varies greatly depending on the artery segment that is involved. They are often fusiform or dissecting. As with other aneurysms, the rupture risk of a PICA aneurysm goes up with increasing size, family history, smoking, and hypertension, yet there is evidence to suggest that PICA aneurysms have a higher risk of rupture than other types of aneurysms, even at smaller sizes. PICA aneurysm rupture often involves hemorrhage into the fourth ventricle, which may lead to a presentation of obstructive hydrocephalus. Both endovascular and open surgical treatments for PICA aneurysms are utilized, and the decision is based on morphological and systemic considerations. Proximal PICA aneurysms are appropriate for primary coiling if the aneurysm has a narrow neck and the sac is accessible. However, if the parent artery is involved with the neck of the aneurysm, coil packing may be insufficient, and there is a higher risk of parent artery encroachment. For more distal aneurysms, the small caliber of the PICA generally precludes placement of stents or flow diverters therein. Due to these challenges, microsurgery has remained prominent in the management of these lesions despite an overall shift to endovascular treatments for other aneurysms. The close association of PICA aneurysms with the lower cranial nerves (CNs) presents a challenge for open surgical treatment, but the far-lateral approach, including C1 laminectomy and transcondylar extension, permits a more anterolateral operative corridor that minimizes inadvertent damage when operating between the lower CNs.
Introduction
Posterior inferior cerebellar artery (PICA) aneurysms account for 0.49 to 3% of all intracranial aneurysms and 18% of aneurysms of the posterior circulation. They are the second most common aneurysm of the posterior circulation, after basilar artery (BA) aneurysms. Due to the left-sided vertebral artery (VA) dominance in 45% of people, PICA aneurysms tend to occur more frequently on the left.
PICA aneurysms constitute a heterogeneous group, and management varies greatly depending on the artery segment that is involved. The most common location is at the PICA-VA junction (40%), followed by proximal PICA (35%), and distal PICA (24%). Compared to other intracranial aneurysms, aneurysms of the PICA are more often fusiform or dissecting. As a result of this variability, both in morphology and anatomy, decision making for PICA aneurysms can be complex.
Major controversies in decision making addressed in this chapter include:
Whether or not treatment is indicated.
Open versus endovascular treatment for ruptured and unruptured PICA aneurysms.
Influence of anatomical factors such as proximal versus distal location.
Use of advanced surgical techniques such as trapping and bypass.
Whether to Treat
As with other intracerebral aneurysms, the rupture risk of a PICA aneurysm goes up with increasing size and a history of subarachnoid hemorrhage (SAH) from a previous aneurysm in the same patient. Family history, smoking, and hypertension are also taken into account. The results of the International Study of Unruptured Intracranial Aneurysms (ISUIA) and other natural history studies show that PICA aneurysms have a higher risk of rupture than other types of aneurysms, even at smaller sizes. In ruptured aneurysms, prompt treatment is paramount, as rebleeding rates can be as high as 78% ( 1 , 3, 4 in algorithm ). Case series of ruptured PICA aneurysms have shown good outcomes even in patients presenting with poor clinical grades. Ruptured PICA aneurysms frequently present with obstructive hydrocephalus, which can worsen the Hunt/Hess grade, due to the artery′s proximity to the ventricular system. Swiftly addressing hydrocephalus can reverse clinical deterioration. In our practice, most patients with unruptured PICA aneurysms are offered treatment, unless there are extenuating circumstances such as a limited life expectancy or prohibitive comorbidities. Ruptured aneurysms almost always warrant timely intervention.
Conservative Management
The role for conservative management of PICA aneurysms is limited. It may play a role in instances of severe comorbidities precluding even endovascular intervention. In patients with ruptured aneurysms and moribund appearance after ventriculostomy, conservative treatment may be pursued.
Anatomical Considerations
The VA enters the intradural space between C1 and the foramen magnum and crosses anterior to the dentate ligament (▶ Fig. 43.1 ). The PICA is the largest intradural branch of the VA, with the origin approximately 10 mm above the foramen magnum. The PICA is divided into five segments: anterior medullary (p1), lateral medullary (p2), tonsillomedullary (p3), telovelotonsillar (p4), and cortical (p5). The choroidal point, which is part of the tonsillomedullary segment, is perhaps the most important anatomical consideration because proximal to this point are perforating arteries, which supply the ventrolateral medulla that should be preserved in any intervention. Among the major intracranial arteries, the PICA has a variable course; it irrigates the ventrolateral brainstem, inferior cerebellar peduncle, cerebellar vermis, and the suboccipital surface of the cerebellar hemispheres.
The PICA is closely associated with cranial nerves (CNs) IX, X, XI, and XII. At its origin, it is most closely associated with CN XII, whose fibers emerge anteriorly and posteriorly to the PICA. The PICA has a variable amount of leptomeningeal anastomoses with the contralateral PICA and the ipsilateral anterior inferior cerebellar artery (AICA). Occasionally, PICA and AICA territories are irrigated by a shared combined artery emerging usually from the proximal basilar trunk. This variable anatomy can affect considerations for treatment, including endovascular treatment such as vessel sacrifice or flow diversion, and microsurgical procedures such as bypass.
The close association of many PICA aneurysms with the lower CNs presents a challenge in the surgical management of PICA aneurysms. The majority of PICA aneurysms are accessed through the vagoaccessory triangle. This space is bordered by the medulla medially, CN XI laterally, and CN X superiorly. It is further divided by the hypoglossal nerve into the suprahypoglossal and infrahypoglossal triangles.
PICA aneurysms most frequently project posteroinferiorly and medially. More challenging lesions may incorporate the proximal PICA. The exact projection of each particular aneurysm should be identified during preoperative imaging, as it may significantly alter management.
Pathophysiology
Due to the proximity of the PICA to the foramina of Lushka and Magendie, rupture of a PICA aneurysm frequently causes intraventricular hemorrhage (IVH). Hemorrhage into the fourth ventricle frequently results in a presentation of obstructive hydrocephalus. In their series of ruptured PICA aneurysms, Mericle et al found that 81% of patients had Fisher grade IV hemorrhage; they placed a ventriculostomy in 84% of patients.
Workup
Clinical Evaluation
The most common clinical presentation of a PICA aneurysm is SAH with IVH. Large aneurysms can present with mass effect on the brainstem, or brainstem infarction. As with other cerebrovascular lesions, many PICA aneurysms are found incidentally during evaluation of unrelated symptoms.
Imaging
Typically, a diagnostic cerebral angiography (DSA) that includes a three-dimensional reconstruction of the aneurysm is performed (▶ Figs. 43.2 and ▶ 43.3 ). For large aneurysms, computed tomography angiography (CTA) is also recommended to evaluate for calcifications, possible partial thrombosis of the aneurysm, and craniotomy planning. Cone beam CTA, a commonly performed aspect of modern cerebral angiography, provides more specific and high-resolution information. Magnetic resonance imaging (MRI), especially with construction interference steady state (CISS) sequencing, can be used to visualize the vascular anatomy with respect to the CNs, in addition to any mass effect on the brainstem, or edema therein. Attention should always be paid to ruling out an extradural PICA origin, which is present in 18% of patients.
Differential Diagnosis
PICA aneurysm rupture should be considered in the presence of cisterna magna SAH, fourth ventricle IVH, or posterior fossa ICH. Other lesions mimicking aneurysm rupture include vertebrobasilar dissection or dural arteriovenous fistulas.
Treatment
Initial management of patients with SAH should include stabilizing the patient systemically and placing a ventriculostomy, especially for PICA aneurysms. Even in cases without overt hydrocephalus, ventriculostomy can be used to effect brain relaxation. After placement of the ventriculostomy, the clinical grade and management choices should be reevaluated/reassessed ( 1 in algorithm ).
Microsurgical treatment has traditionally been the mainstay of PICA aneurysm management. Due to challenges in applying endovascular techniques, as detailed later in this chapter, microsurgery has remained prominent in the management of these lesions, despite the shift to endovascular treatments for other posterior fossa and supratentorial aneurysms.
PICA aneurysms are heterogeneous, and aneurysm location along the artery is a major consideration for treatment. Proximal PICA aneurysms should be considered for primary coiling if the aneurysm has a narrow neck and the sac is accessible ( 9 in algorithm ). However, if the parent artery is involved with the neck of the aneurysm, coil packing may be insufficient, and there is a higher risk of parent artery encroachment. If a patient cannot tolerate surgery (due to poor neurological grade, advanced age, or significant comorbidities), then endovascular therapy, including endovascular vessel sacrifice, is considered ( 9, 10 in algorithm ).
For patients whose aneurysms are not readily amenable to endovascular coiling using high packing density due to dome morphology or a difficult location, we recommend a microsurgical approach, which usually means direct clip ligation ( 11 in algorithm ). Advanced techniques are considered for the following types of PICA aneurysms: large, fusi-form, dissecting, or one that is not sufficiently distinct from the parent vessel. These advanced techniques can include trapping, bypass, aneurysmorrhaphy, excision with reanastomosis, or reimplantation ( 12, 13 in algorithm ). These options are further discussed under sections Cerebrovascular Management—Operative Nuances and Endovascular Management—Operative Nuances.
Despite the occasional complexity of PICA aneurysms, a comprehensive cerebrovascular neurosurgeon with sufficient experience should be able to tackle most lesions with microsurgery. As a group, they are considered some of the easiest posterior circulation aneurysms to cure surgically. In cases with mass effect from ICH or IVH of the fourth ventricle, microsurgical treatment can include clot evacuation and decompressive craniectomy ( 1 in algorithm ).
Cerebrovascular Management—Operative Nuances
Craniotomy Planning
When planning for a craniotomy, either a midline suboccipitial or far-lateral approach can be considered, depending on what segment of the PICA is affected (▶ Fig. 43.1 ). Incisions can be a “hockey-stick,” “lazy S,” or “large C” style; each has its proponents. Positioning, especially optimizing the space between the shoulder and the head/ear, is critical to surgical ergonomics.
In their respective series of surgically treated PICA aneurysms, several experienced surgeons (Lawton, Connolly, Samson, others) described utilizing far lateral craniotomy with C1 laminotomy to allow improved exposure and limit CN manipulation. Preoperative preparation for alternative strategies should be considered, including prepping of the radial artery, preservation of the occipital artery (OA), or a craniotomy exposing the contralateral PICA for potential bypass options. This is a particularly important consideration, as approximately 25% of PICA aneurysms are not amenable to direct clip ligation. Lawton′s experience noted the use of trapping or bypass in up to 17% of the cases. The contralateral far-lateral approach has also been described for these lesions, depending on the medial-lateral displacement of the VA and PICA. The distance of the aneurysm from foramen magnum and the midline are important factors in craniotomy planning. Review of axial imaging from MRI, “Dyna” CT, or CTA is useful, revealing vascular relationships with the cranial base.
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