2 General Principles of Direct Bypass Surgery

10.1055/b-0039-172616

2 General Principles of Direct Bypass Surgery

Marcus Czabanka and Peter Vajkoczy

Abstract

Bypass surgery for treating moyamoya vasculopathy (MMV) is often regarded as treatment of choice. In contrast to extraintracranial bypass surgery in atherosclerotic disease, different revascularization techniques are proposed for the treatment of MMV which may be differentiated into direct, indirect, and combined bypass procedures. The superiority of direct bypass surgery in comparison to indirect strategies includes immediate additional blood flow to the ischemic brain leading to reduction of stroke risk in these patients. The following chapter focuses on the general principles of bypass surgery for the treatment of MMV, focusing on general ideas and surgical concepts which are important for indication and planning of the surgical strategy.

2.1 History and Initial Description

After the first description of an arterial end-to-end anastomosis using suture by the French surgeon Alexis Carrel in 1902 it took another 70 years until Yasargil described the most influential direct revascularization technique with the introduction of the superficial temporal artery to middle cerebral artery (STA–MCA) bypass for internal carotid artery occlusion and moyamoya vasculopathy (MMV). ¹ In contrast, indirect revascularization techniques had been performed already in the 1940s representing the first attempts to revascularize the ischemic brain. Since the introduction of both revascularization strategies there remains significant debate about the superiority of indirect versus direct revascularization techniques in MMV. ² Current treatment protocols recommend direct revascularization techniques aiming at immediate supply of additional blood flow to the brain for adult patients as the anatomic difficulties encountered especially in MMV (small and fragile donor and recipient vessels that impose difficulties for the microsurgical anastomosis) are less pronounced in adult patients compared to pediatric MMV patients. ³ ^– ⁵ Consequently, in pediatric MMV patients indirect revascularization techniques have been described to be very effective for treating ischemic symptoms and restoring cerebrovascular reserve capacity. ³ ^, ⁶ Therefore, in adult MMV patients, direct STA–MCA bypass has become the workhorse for revascularizing the ischemic brain, in pediatric patients indirect procedures are regarded as equivalent revascularization strategy often resulting in a combination of both procedures. ⁷

2.2 Analysis of Hemodynamic Compromise for Direct Bypass Surgery

The most important aim of direct bypass surgery represents correction of hemodynamic compromise and reduction of stroke risk. Hemodynamic compromise is assessed using PET measurements of cerebral blood flow with/without acetacolamide stimulation analyzing cerebrovascular reserve capacity and calculating oxygen extraction fraction (OEF). There are currently different tracers used including C¹⁵O PET for cerebral blood volume assessment, H₂ ¹⁵O for cerebral blood flow, and ¹⁵O₂ to measure OEF and cerebral metabolic rate of oxygen. ⁸ Using positron emission tomography (PET) analysis MMV patients have been characterized as patients with high cerebral blood volume due to maximal vasodilation and reduced OEF as a sign of hemodynamic compromise. Single-photon emission computed tomography (SPECT) analysis allows detection of hemodynamic compromise using nonquantitative, relative measurements comparing the healthy hemisphere with the diseased one, which imposes methodological limitations to a bilateral disease as MMV. Even though it remains to be determined whether PET analysis is superior to other imaging modalities analyzing cerebrovascular reserve capacity, our experience comparing SPECT and H₂ ¹⁵O PET for detecting hemodynamic compromise indicate improved sensitivity and specificity for PET techniques in this regard ⁹ (Fig. 2‑1).

Another highly reliable tool is Xenon-CT measurement of cerebrovascular reserve capacity. Xenon-enhanced CT has been shown to correlate with increased risk of ischemic stroke in the presence of reduced cerebrovascular reserve capacity and therefore represents a gold standard for detecting hemodynamic compromise. ¹⁰ ^, ¹¹ Correspondingly, direct bypass surgery significantly improves cerebrovascular reserve capacity in MMV patients. ¹² However, significantly reduced availability of this technique due to approval restrictions avoid a broad application of Xenon-CT for detecting hemodynamic compromise in MMV patients. Novel MRI techniques may contribute to the hemodynamic assessment preoperatively. Dynamic susceptibility contrast-weighted bolus-tracking MRI, arterial spin labeling MRI, and blood oxygen level–dependent MRI have been shown to potentially identify tissue at risk for cerebral ischemia, hemodynamic compromise, and restoration of hemodynamic compromise after direct revascularization. ¹³

Moreover, novel data imply that quantitative analysis of cerebral blood flow or OEF may not yet represent the only decisive indicator for future stroke in MMV as Zipfel et al demonstrate a 10% stroke risk for MMV patients with normal OEF in PET studies. ¹⁴ Therefore, other factors must be included in the surgical decision-making process. These include the potential progressive character of MMV and the extraordinarily high risk of ischemic stroke if both hemispheres are affected by the disease (> 80% stroke risk in 5 years). ¹⁵ Following the Berlin moyamoya grading system, MMV may be graded according to angiography, presence of ischemic lesions in MRI, and the associated hemodynamic compromised into three different grades that correlate with the presence of ischemic symptoms and the associated risk for cerebral revascularization ¹⁶ ^, ¹⁷ (Fig. 2‑2).

**Fig. 2.1** Analysis of cerebrovascular reserve capacity after acetazolamide stimulation demonstrating highly significant reduced cerebrovascular reserve capacity in H₂ ¹⁵O positron emission tomography imaging (left) compared to single-photon emission computed tomography (right) in the left hemisphere of a moyamoya patient.

**Fig. 2.2** Description of the Berlin moyamoya grading system.

Especially, high-grade MMV patients impose a higher ischemia risk during cerebral revascularization than low-grade MMV patients, indicating that early revascularization may be reasonable in these patients avoiding a high complication profile while resulting in a significant reduction of stroke risk ¹⁶ . Moreover, the risk for cerebral hemorrhage must be included as STA–MCA bypass significantly reduces the risk for cerebral hemorrhage. ¹⁸ The ethnic background of the patient and the risk for cerebral hemorrhage, that varies among MMV populations with a higher incidence of hemorrhagic MMV in Asia as compared to the predominantly ischemic populations in North America and Europe, ² ^, ¹⁹ ^– ²¹ in combination with the presence of fragile collateral vessels and microaneurysms further factors that should be considered in the surgical decision-making process for direct revascularization surgery.

2.3 Key Principles of Direct Revascularization Surgery

2.3.1 Graft Choice

In MMV, the STA is usually used as a donor vessel because standard STA–MCA bypass is regarded as the treatment of choice for direct revascularization procedures. High-flow bypass procedures are associated with a high-risk profile for hyperperfusion syndrome and therefore represent only rescue strategies in cases of failed low-flow anastomosis and/or failed indirect procedures. ²² In this regard, bypass flow in STA–MCA bypass has been described to range between 10 and 60 mL/min while flows above 30 mL/min are associated with increased risk for hemorrhage and stroke. ²³ Other direct anastomosis techniques including STA–ACA bypass, occipital artery-posterior cerebral artery (OA–PCA) anastomosis, multiple insertion procedures, and the use of the auricular artery as donor vessel have been described by different bypass-experienced groups, yet all of these procedures do not display superiority to regular STA–MCA anastomosis and they are often associated with distinct disadvantages. Therefore, they play a minor role in direct revascularization of MMV patients and predominantly serve as rescue strategies in cases for failed primary revascularization procedures.

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