Fusiform, Dolichoectatic, and Dissecting Aneurysms

Fusiform and Dolichoectatic Aneurysms

Fusiform or dolichoectatic aneurysms cause symptoms in three ways: compression, ischemia, or rupture. Compared with saccular aneurysms, dolichoectatic aneurysms are more likely to cause compression or ischemia and may be less likely to rupture.

If a patient presents with symptoms suggestive of hemorrhage, then computed tomography (CT) is diagnostic. If the CT is negative, then a lumbar puncture is warranted. For patients who present with new neurological deficits, CT may show ischemia or edema, but magnetic resonance imaging is usually required. CT and CT angiography can also be used to better define an aneurysm’s relationship to the skull base prior to surgery.

Catheter angiography in multiple projections, preferably with 3D computer reconstruction, remains the gold standard for diagnosis and surgical planning. Preoperative angiographic studies determine: (1) the artery of origin; (2) the aneurysm’s size, shape, and relationship to afferent and efferent arteries; (3) the presence and location of vasospasm or hypoperfusion; (4) the displacement of adjacent vessels suggesting mass effect from hematoma or partial thrombosis of an aneurysmal sac whose dimensions are larger than those seen on angiography; (5) the degree of collateral supply to territory distal to the aneurysm, and (6) the presence of other aneurysms or vascular abnormalities.. We also use magnetic resonance imaging and contrast-enhanced magnetic resonance angiography in the evaluation of giant fusiform and dolichoectatic aneurysms to evaluate their compressive effect on adjacent structures and to visualize organized thrombus within the aneurysm that is not visualized angiographically.

The goals of surgery are to eliminate the risk of hemorrhage, reduce mass effect and compression of adjacent brain and cranial nerves, and preserve normal arterial circulation distal to the aneurysm. ² The indications for surgery have to balance the risks of treatment with the natural history of these aneurysms. The decision to operate considers the patient’s age, medical comorbidities, presenting symptoms, and the size, location, and configuration of the aneurysm. Nonoperative treatment options include antiplatelet therapy for patients with ischemic symptoms, anticonvulsants for patients with neurovascular compression syndromes, or serial imaging observation for asymptomatic or minimally symptomatic lesions. The natural history of fusiform aneurysms is notoriously poor, with 5-year mortality rates > 20% for posterior circulation lesions, and failure of conservative management justifies intervention for these aneurysms.

18.2.2 Dissecting Aneurysms

Anterior circulation and basilar artery dissecting aneurysms classically present with acute focal neurological deficits, whereas intradural vertebral artery dissections often cause subarachnoid hemorrhage (SAH). Blister aneurysms of the dorsal carotid artery are dissecting pseudoaneurysms of the supraclinoid segment, often present with SAH, and have high rates of early rerupture and intraoperative rupture. ³ Angiography is the principal method for detecting these lesions but CT, CT angiography, and magnetic resonance imaging are typically obtained also. The goal of surgery for ruptured lesions is to prevent rebleeding while preserving the circulation distal to the aneurysm. The dissection should generally be trapped. Trapping may be complicated by the presence of eloquent branches along the dissected arterial segment. If necessary, excision with reanastomosis or extracranial–intracranial bypass can be done to preserve circulation.

Pseudoaneurysms due to intracranial dissection have a high rerupture rate, and timely treatment is indicated unless recovery from the initial hemorrhage seems unlikely. Unruptured dissections presenting with ischemia may be treated medically and surgery reserved for symptoms due to mass effect, although management of these has to be individualized.

18.3 Preoperative Preparation

Electroencephalographic and evoked potential monitoring are used in all cases. Barbiturate or propofol burst suppression is used for cerebral protection during temporary clipping. Intravenous adenosine or rapid ventricular pacing can provide a sufficient window of circulatory arrest without the morbidity of hypothermia and cardiopulmonary bypass.

Whenever a parent artery is to be surgically occluded, revascularization of the involved vascular territory must be considered to prevent ischemic complications. Angiographic assessment of collateral blood flow is essential but a balloon test occlusion helps identify patients who are unlikely to tolerate permanent arterial sacrifice. Patients with poor tolerance to occlusion typically require a high-flow bypass such as a saphenous vein or radial artery bypass ( ▶ Fig. 18.1). An ultrasonographic Allen’s test is performed in patients considered for high-flow bypass in whom radial artery harvest may be required. Patients with marginal tolerance may need only a low-flow bypass. In the setting of SAH, the threshold for revascularization is lower, as even a successful test occlusion does not ensure that perfusion will be adequate if vasospasm develops.

Fig. 18.1 A 15-year-old boy presented with double vision. A left cavernous internal carotid artery aneurysm was seen on (a) left internal carotid artery (ICA) angiography (anteroposterior view). A balloon test occlusion of his left ICA with hypotensive challenge and a single photon emission computed tomography cerebral blood flow study demonstrated asymptomatic diminished perfusion to his left hemisphere. The aneurysm was therefore treated with surgical trapping and petrous-to-supraclinoid ICA bypass. (b) A proximal end-to-side anastomosis was performed using a short saphenous vein graft, and a distal end-to-side anastomosis to the supraclinoid ICA was performed. (c) The completed bypass enabled the cavernous aneurysm to be trapped, and (d) the postoperative angiogram shows good reconstitution of blood flow to the left hemisphere.

18.4 Operative Procedure

The size, shape, and location of the aneurysm determine the surgical approach. A pterional craniotomy with extensive drilling of the sphenoid wing provides access to most anterior circulation aneurysms. Occasionally, an orbitozygomatic osteotomy is needed to increase exposure. The superficial temporal artery (STA) must be preserved during the approach. Dissecting aneurysms have a high intraoperative rupture rate, and adequate proximal control is assured by removal of the anterior clinoid process or exposure of the cervical internal carotid artery (ICA). Aneurysms on the second and third segments of the anterior cerebral artery require an interhemispheric approach. Aneurysms of the vertebral artery or vertebrobasilar junction are approached through a far-lateral (transcondylar) craniotomy with laminectomy of the atlas. Midbasilar aneurysms are approached through a transpetrosal route that removes portions of the petrous bone (retrolabyrinthine, translabyrinthine, or transcochlear). With smaller midbasilar aneurysms, an extended retrosigmoid approach may be sufficient and can avoid the complications associated with a transpetrosal approach. When greater exposure is needed, the transpetrosal approaches can be combined with a subtemporal craniotomy and division of the tentorium.

18.4.1 Clip Reconstruction of the Parent Artery

By definition, fusiform and dolichoectatic aneurysms lack a neck, making clip reconstruction more difficult than for simple saccular aneurysms. Typically, multiple clips and fenestrated clips are needed, and the aneurysm has to be temporarily trapped to accomplish clip reconstruction. These aneurysms are also frequently filled with thrombus. To clip the aneurysm, the mass effect must first be reduced by thrombectomy ( ▶ Fig. 18.2). Thrombectomy requires opening the aneurysm under local or global circulatory arrest. Thrombus is removed piecemeal with cup forceps or an ultrasonic aspirator and proceeds until the thrombus has been adequately debulked or until the lumen of the aneurysm is encountered, at which point bleeding is controlled with application of a hemostatic agent (oxidized cellulose) and gentle pressure. It can be difficult to determine if the reconstructed channel through the aneurysm has a sufficient internal caliber to supply the distal branches, so clips are applied to leave a generous lumen, and intraoperative Doppler velocity measurements and angiography are useful.

Fig. 18.2 (a) Dolichoectatic aneurysms involving the intradural vertebral artery typically produce symptoms from compression of the brainstem and cranial nerves. (b) This compressive mass is eliminated by opening the aneurysm and (c) removing thrombus with an ultrasonic aspirator. (d) The thrombectomy proceeds until the lumen of the aneurysm is encountered. (e) Bleeding is controlled with oxidized cellulose or a comparable substance and (f) the walls of the aneurysm are brought together with clips to reconstruct the parent artery. Note that the thrombectomy eliminates mass effect and generates the redundant vessel wall needed to reconstruct the artery.

(Reproduced with permission from the Barrow Neurological Institute, Phoenix, AZ.)

In the case of dissecting blister aneurysms of the dorsal carotid wall, the dissection usually involves only a portion of the artery circumference and direct clip reconstruction is the preferred treatment. The segment is temporarily trapped, which typically requires temporary clip placement proximally on the cervical or clinoidal ICA, and distally at the ICA terminus. In select cases, temporary clipping of the ophthalmic and/or posterior communicating artery may also be needed to reduce turgor within the segment sufficiently to allow permanent clip reconstruction. Clip blades are applied parallel to the parent artery and must oppose the normal intima, often narrowing the parent lumen by 15 to 30% as the diseased section of wall is excluded ( ▶ Fig. 18.3). If the carotid is circumferentially diseased, clip reconstruction may not be possible, and contingency maneuvers such as clip reinforced wrapping, or trapping along with high-flow bypass (typically cervical carotid–middle cerebral artery [MCA]) must be prepared for in advance.

Fig. 18.3 (a) Preoperative three-dimensional angiographic reconstruction (right ICA) demonstrated a dorsal ICA blister aneurysm. (b) The ICA blister aneurysm was clipped with two stacked right-angled clips with the blades paralleling the axis of the ICA. (c) Postoperative three-dimensional angiographic reconstruction showed the stacked angled clips and aneurysm occlusion. ICA, internal carotid artery; ON, optic nerve. (Reproduced with permission from Owen et al. ³)

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