16.1 History and Initial Description

Extracranial–intracranial (EC–IC) bypass for the treatment of the symptomatic hypoperfused hemisphere in intracranial steno-occlusive diseases, secondary to atherosclerotic disease or moyamoya vasculopathy (MMV), is currently the treatment of choice. ^{1 ,​ 2} The use of a combined direct superficial temporal artery to middle cerebral artery (STA–MCA) and indirect encephalo-duro-arterio-synangiosis (EDAS), by performing an STA onlay with flipped dural flaps, is advocated as an optimal treatment option as it allows immediate augmentation of blood flow in the postoperative period, while providing the brain to acquire additional indirect flow in the long term. ³ Dual placement of direct and indirect bypasses within the same hemisphere has been demonstrated by formal diagnostic angiogram, regardless of patients’ age and the hemodynamic status, to be of benefit with the indirect graft serving as an adjunct to a direct bypass in order to maximize revascularization. ³ Quantitative magnetic resonance imaging (QMRI) studies have shown a reciprocal relationship between the direct and the indirect bypasses after combined bypass surgery, thus providing complementary revascularization. ⁴

The STA is the terminal branch of the external carotid artery (ECA). It supplies the anterolateral part of the scalp. This artery usually (in 71.4–95.7% of the vessels) bifurcates above the level of the superior margin of the zygomatic arch. The mean inner diameter of the STA at the level of the zygomatic arch is approximately 2.2 to 2.7 ± 0.5 mm. ^{5 ,​ 6} The STA commonly bifurcates into an anterior frontal branch that courses anterosuperiorly and a posterior parietal branch that courses posterosuperiorly. The vessel however can be highly irregular in its anatomy and therefore its course must be studied angiographically preoperatively to insure adequate mapping of its trajectory (Fig. 16‑1). The inner diameter of the frontal and parietal branches is approximately 1.4 to 2.1 and 1.4 to 1.8 at 7 cm respectively, distal to the zygomatic arch. ^{6 ,​ 7}

16.2 Indications

The operative benefits of combined cerebral revascularization have been previously demonstrated with combined hemispheric revascularization shown to decrease rates of both ischemic and hemorrhagic stroke. ^{8 ,​ 9} Increased attention has also been directed at cognitive function, an important determinant of the quality of life, in patients with MMD. Cognition in MMD has been shown to be affected, ¹⁰ possibly due to occult strokes, although it has also been shown in patients without stroke. ¹¹ Definitive benefits on cognition following revascularization have yet to be fully characterized. A recent systematic review concluded that combined cerebral revascularization was superior in producing favorable long-term clinical outcome. ¹² However, combining both techniques at the same procedure may be challenging and its nuances are critical for a good surgical and long-term outcome. In our previously published case series of patients treated with neurosurgical revascularization, the number of peri- and postoperative complications was acceptable, supporting evidence for the safety of these procedures in selected groups of patients. ¹³ To favor optimal cerebral revascularization of the affected hemisphere, combined direct and indirect bypass therapy should become a standard treatment modality among vascular neurosurgeons when treating symptomatic MMV. Although little is known about the optimal timing of surgical intervention, we prefer not to perform revascularization surgery in the acute phase, which is in accordance with common practice. ¹⁴ Patients presenting with hemorrhage are initially managed conservatively, with surgical consideration and radiographic workup performed following at least 2 to 3 months after recovery. Patients with transient ischemic attacks or those with small watershed ischemic infarction can be treated earlier following their full workup. Patients with larger recent infarctions are treated in more of a delayed fashion but usually within a month of their stroke, depending upon their neurological condition. We consider treating asymptomatic hemispheres in patients with bilateral MMV who have been revascularized on their symptomatic side, when there is evidence of hypoperfusion on cerebral hemodynamic studies.

16.3 Key Principles

The bypass procedure consists of (1) distal to proximal donor vessel preparation of the frontal STA branch (STAfb) for direct and the parietal STA branch (STApb) for indirect revascularization, followed by (2) a craniotomy in the region of interest (preferably a hypoperfused area as demonstrated on hemodynamic studies), (3) performing the anastomosis with a cortical M4 branch and verifying patency, (4) performing the EDAS, and (5) meticulous wound closure.

16.4 SWOT Analysis

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  Strength: Bypass surgery is the only surgical therapy for MMD and MMS that carries a low complication risk and favorable clinical outcome.

  
  
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  Weakness: Extensive surgical experience in bypass surgery and familiarity with the clinical key aspects of moyamoya are mandatory for successful treatment.

  
  
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  Opportunity: Due to a low caseload, this surgical therapy is only performed in a few centers worldwide, which is creating an inadequate number of experienced treating facilities.

  
  
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  Threat: Since the footprint of endovascular therapy is growing, fewer neurosurgeons are trained in microvascular surgery.

16.5 Contraindications

Surgery in patients who carry a high general anesthesia risk should be extensively studied and great care taken should be taken before recommending operative intervention. Asymptomatic moyamoya patients without hemodynamic compromise should not be operated on, but should be monitored over time instead. Bypass surgery in large infarcted areas should be avoided since it carries a great risk of intracerebral hemorrhage.

16.6 Special Considerations

16.6.2 Postoperative Considerations

The first 24 hours postoperatively patients are admitted to an intensive care unit and special attention is paid to maintain normotension to ensure the patency of the bypass and to prevent hyperperfusion syndrome, which may cause temporary neurological deficits. A postoperative conventional CT angiography or DSA is performed to confirm bypass patency. In the illustrative case presented here, the postoperative angiogram at 6 months shows a robust flow through the both the STAfb direct bypass in to the MCA branches and the STApb continues to flow distal to the craniotomy site. We also perform QMRA in the immediate postoperative period. We continue to follow the patient both clinically and by QMRA at 3 months and DSA at 6 months follow-up (Fig. 16‑2). The indirect graft is assessed and a contralateral “indirect only” surgery is considered on the asymptomatic hemisphere if robust collateral flow from the EDAS is seen on angiography. We maintain the patient on aspirin 81 mg per day unless there is a medical contraindication. Patients who have presented with hemorrhage have their aspirin stopped 3 months postoperatively.

Related posts:

14 Combined STA–MCA Bypass and Encephalo-myo-synangiosis 15 STA–MCA Bypass and EMS/EDMS 18 STA–MCA Bypass and Encephalo-duro-arterio-synangiosis 17 STA–MCA Anastomosis and EDMAPS 19 Individualized Extracranial-Intracranial Revascularization in the Treatment of Late-Stage Moyamoya Disease 7 Encephalo-duro-arterio-synangiosis: In Adults

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