Complex Aneurysms and Cerebral Bypass

Indications for Bypass

The natural history and treatment indications for both ruptured and unruptured cerebral aneurysms have been well described in previous chapters. Complex aneurysms may require bypass for optimal treatment for a number of reasons: Intracavernous and basilar artery aneurysms that are not amenable to treatment by endovascular procedures are best treated with revascularization when open surgical reconstruction is necessary. Aneurysms with significant organized intramural thrombus and serpentine vascular channels or with atherosclerosis or calcifications at the neck are safer to treat with bypass and proximal occlusion or trapping, rather than risk clip-associated stenosis or thromboembolism. Nonsaccular aneurysms, such as blister aneurysms, dissecting aneurysms, and fusiform aneurysms often require bypass. Bypass is also well applied in complex aneurysms where endovascular treatment requires use of a stent or flow diverter but there is a contraindication to antiplatelet therapy (medication resistance, concern with patient noncompliance, or ruptured aneurysms) or as a salvage treatment for cases where endovascular treatment was not successful. As endovascular technologies continue to advance, it is important to remain cognizant of the evolving treatment options for complex aneurysms. Overlapping indications between cerebral bypass and endovascular treatment using intrasaccular devices, flow-diverting stents, or stent-assisted coiling require the surgical team to carefully weigh the relative safety and efficacy of each approach to arrive at the most appropriate treatment strategy.

Preoperative Planning

Patients undergo computed tomographic angiography (CTA) scan, magnetic resonance imaging (MRI), and magnetic resonance angiography (MRA) to evaluate the underlying disease. Selective four-vessel angiography is required to assess the anatomy of the involved artery and the collateral circulation. Three-dimensional (3D) rotational angiography with image reconstruction is helpful in surgical planning and visualization. External and common carotid artery injections demonstrate the caliber of donor vessels (eg, superficial temporal artery and occipital artery) for extracranial-intracranial bypasses and also are valuable in planning for proximal graft inserting sites in the cervical carotid artery. In select cases, cerebral blood flow (CBF) study or single photon emission computed tomography (SPECT) may be performed as well.

A general medical evaluation, especially a thorough cardiac evaluation, is critical because the agents used for temporary metabolic suppression and those used to induce temporary hypertension can affect cardiac function. Preoperatively, the patient is started on aspirin, 325 mg, by mouth 2 days before surgery and postoperatively. If the patient is allergic to aspirin, clopidogrel may be used at a small dose (75 mg).

Duplex ultrasonography of the saphenous veins and radial arteries with Allen testing are routinely performed. The choice of graft depends on four factors: (1) size of the recipient vessel, which is the major determinant; (2) availability of an adequate donor vessel; (3) availability of graft material; and (4) extent of blood flow augmentation required. The measured blood flow immediately after anastomosis in superficial temporal artery (STA)−middle cerebral artery (MCA) anastomosis is 20 to 60 mL/min. In radial artery grafts (RAGs) it averages 133 mL/min ± 70 mL/min (lower when the recipient is in a posterior circulation artery), and for saphenous vein grafts (SVGs) it averages 160 mL/min ± 50 mL/min. The flow rate depends on the donor vessel, the recipient vessel, and the diameter of the graft. For radial artery grafts, for each 1 mm increase of diameter, the flow rate increases by 33 mL/min; radial grafts should have a diameter of > 0.22 cm and a length of > 20 cm. The problem of vasospasm in RAGs has been mostly solved by the use of the pressure distention technique, but occasionally endovascular angioplasty may be needed. SVGs provide the largest volume of flow, but the potential for turbulence leading to occlusion at the anastomotic sites (especially recipient) and for hyperemia and hemorrhage in chronically ischemic patients is a potential drawback. The risk of hyperemia increases when the flow rate exceeds 200 mL/min. SVGs should be > 0.3 cm in diameter and also > 20 cm in length. The anterior tibial artery may be used in cases where no other donor vessel is viable.

A prophylactic antibiotic, usually ceftriaxone 2 g, is administered intravenously 1 hour prior to the incision and repeated every 4 hours during the operation. Prophylactic anticonvulsants (typically fosphenytoin) and mannitol are routinely given. A balanced anesthetic technique is used. Neurophysiologic monitoring consists of the somatosensory evoked potential (SEP), motor evoked potential (MEP), and the electroencephalogram (EEG). Cranial nerves may be monitored as necessary. The patient is maintained in a normocapnic state throughout the procedure. Immediately prior to beginning the anastomosis, 2500 units of heparin are given intravenously. In unruptured aneurysms, the blood pressure is raised 20% above the baseline during temporary arterial occlusion, and the patient is placed in electroencephalographic burst suppression using propofol. In the case of ruptured aneurysms, the blood pressure (BP) is kept at or below 120 mm until the aneurysm is occluded. In such patients, during temporary occlusion, the blood pressure can still be raised further if necessitated by MEP changes, after the inflow to the aneurysm is occluded. Intraoperative indocyanine angiography (ICG angio) and a micro-Doppler evaluation are used in all cases. Intraoperative angiography is performed in select cases.

Postoperatively, the patients are maintained on heparin administered subcutaneously, 5000 units two to three times daily for a period of 5 to 7 days. Patients with vein grafts are subsequently placed on aspirin, 325 mg, once daily, for life. Patients with RAGs and other types of arterial grafts are maintained on aspirin for at least 6 weeks postoperatively.

Reconstructive procedures can be classified into local bypasses and extracranial to intracranial (EC-IC) bypasses. Local bypasses consist of (1) reimplantation (end to side), (2) end-to-end reanastomosis, (3) side-to-side anastomosis, and (4) short interposition graft. These procedures take advantage of the arteries already present inside the cranium and are usually optimal for the replacement of small and medium-sized arteries. EC-IC bypasses are classified as low-flow, moderate-flow, or high-flow bypasses. Low-flow bypasses provide < 50 cc/min of blood flow (eg, STA-MCA and OA-PICA); moderate-flow (50–90 cc/min)—usually RAGs into the posterior circulation; and high-flow bypasses (>100 cc/min blood flow) that typically consist of RAG or SVG bypasses used to replace a large vessel such as the inferior cerebellar artery (ICA) or basilar artery.

In general, local bypasses are performed for unexpected vascular injuries during aneurysm and during the excision of distally placed aneurysms. Examples include middle cerebral artery (MCA) branch reimplantation, MCA branch repair, anterior cerebral artery (ACA) to ACA or posterior inferior cerebellar artery (PICA) to opposite PICA side-to-side anastomosis, and PICA to (ipsilateral) anterior inferior cerebellar artery (AICA) anastomosis. For similar circumstances, if direct resuture, implantation, or side-to-side anastomosis is not possible because the gap is too long, then a short interposition graft using the STA, occipital artery (OA), or even the superior thyroid artery (SThyA) can be utilized. STA-MCA or OA-MCA anastomosis is employed if the blood flow augmentation requirement is small, and the vessel being replaced is a small one such as the PICA. RAG is used when some collateral circulation is present, the recipient vessel is not large enough (≥2 mm), or if a moderate increase or replacement of flow is required. The volume of flow increases with the actual diameter of the radial artery graft. An SVG is employed if a very high flow replacement is needed. Alternatively, two bypasses (two RAGs or one RAG and one SVG) may be used one after the other.

Technique