Indirect Bypasses for Moyamoya Disease

Edward Smith

Keywords: moyamoya, revascularization, pial synangiosis, indirect, stroke

37.1 Introduction

Moyamoya is an arteriopathy of the intracranial internal carotid arteries (ICAs), characterized by progressive narrowing of the intradural ICA and proximal middle and anterior cerebral arteries with concomitant development of a network of collateral vasculature likened to a “puff of smoke” when seen on a catheter angiogram. Moyamoya syndrome refers to patients with an identified cause, such as cranial irradiation. The syndrome may be unilateral. Moyamoya disease is bilateral and idiopathic. The etiology of the disorder remains under investigation, with data suggesting that different genetic and epigenetic influences contribute to initiating a process that ends with a shared common pathology of smooth muscle cell overgrowth in the media of the vessels, leading to stenosis. ¹ This stenosis ultimately causes critical reductions in blood flow to the brain, leading to cerebral ischemia and hemorrhage.

Once moyamoya is diagnosed, evidence supports surgical revascularization as the primary therapy, even in most asymptomatic children. ^{2,​ 3,​ 4} Without intervention, there is an annual 13% risk of ischemic stroke coupled with a 7% risk of hemorrhage. ⁵ Indirect revascularization is the more common intervention in children, a surgical procedure (such as pial synangiosis), in which a pedicle of vascularized tissue is grafted to the brain and used as a source of new blood supply to reduce the risk of stroke. Done at high volume centers, this procedure can reduce the 5 year risk of stroke to 4.7%; a near 20-fold reduction. ^{2,​ 6}

37.2 Diagnosis

Once suspected, the evaluation of moyamoya should start with magnetic resonance imaging (MRI) and magnetic resonance angiography. Many patients will show watershed infarcts, evidence of narrowed branches of the ICA and up to 81% of symptomatic patients will have the ivy sign—sulcal hyperintensity on FLAIR imaging indicative of slow blood flow. ⁷ Angiograms should include injections of both carotid (both internal and external) and both vertebral arteries. Important features to note include the presence of spontaneous transdural collateral vessels, particularly those arising from vessels contained within the surgical field such as the superficial temporal artery (STA) and the middle meningeal artery.

Typically, patients are started on aspirin (81 mg daily) and counseled to avoid dehydration and hyperventilation. Referral to a specialist in moyamoya, ideally in a multidisciplinary practice at a high-volume center is an appropriate next step. ^{2,​ 8}

37.3 Indications and Contraindications

The primary objective of surgery is to create a new vascular supply to the brain to correct the ischemia caused by the arteriopathy. Guidelines from the Japan Ministry of Health and Welfare regarding indications for surgical treatment of moyamoya state the following: “In the cases with (1) repeated clinical symptoms due to apparent cerebral ischemia or (2) a decreased regional cerebral blood flow, vascular response and perfusion reserve, based on the findings of a cerebral circulation and metabolism study, surgery is indicated.” ⁹ In the United States, the American Stroke Association guidelines are similarly broad, suggesting that indications for revascularization surgery include “progressive ischemic symptoms or evidence of inadequate blood flow or cerebral perfusion reserve in an individual without a contraindication to surgery.” ¹⁰ Consequently, indications for surgery in many centers include: (1) radiographic evidence of moyamoya and either (2) symptomatic moyamoya at any Suzuki stage, or (3) asymptomatic moyamoya with Suzuki II–VI and/or evidence of progressive radiographic changes suggestive of ischemia (such as FLAIR changes or worsening perfusion on arterial spin labeling MRI). ^{2,​ 3,​ 8}

Contraindications include: (1) those patients with unclear diagnoses, (2) asymptomatic hemispheres with low Suzuki stages (I–II) without clear evidence of ischemia, or (3) patients medically unfit for the operating room (such as those with severe cardiac or pulmonary disease).

37.4 Selection of Approach

Surgical revascularization is typically accomplished using branches of the external carotid artery (which are unaffected by moyamoya) as donor vessels. Indirect approaches rely on the growth of new vessels from a transplanted supply, usually the STA, although any vascularized tissue (such as muscle, pericranium, omentum, or dura) has potential to work. ¹¹ There are a bevy of named indirect approaches, including encephaloarteriodurosynangiosis, pial synangiosis, encephalomyosynangiosis, dural inversion, and multiple burr holes. ¹² Indirect approaches have the advantages of working in any age, not being limited by donor vessel size and providing long-term durable revascularization (Video 37.1). The main limitation is that indirect approaches require weeks to grow new vessels, meaning that there is a period of time postoperatively during which the patient remains at risk of stroke.

Choosing the type of approach remains a controversial subject, often the outcome of institutional preference and surgeon comfort. There is a dearth of evidence-based data to drive decision making. Overall, children are predominantly treated with indirect approaches, about 75% indirect: 25% direct, with American Heart Association Guidelines supporting the use of indirect approaches in younger children. ^{10,​ 13} Adults, on the other hand, more commonly undergo STA–MCA bypass. ^{14,​ 15}

37.5 Surgical Procedure

Any moyamoya surgery requires meticulous perioperative attention to detail. While each surgeon will individualize his or her approach, there are general principles that can be applied to most cases. These principles will be illustrated through descriptions of one of the most common moyamoya operations, pial synangiosis ( ▶ Fig. 37.1, Video 37.1).

37.5.1 Preoperative (Day 1)

Ensure imaging is concordant with diagnosis, confirm side(s) and arrange anesthesia evaluation. Admit to hospital the night before surgery for intravenous hydration (usually 1–1.5 times baseline rate if otherwise healthy) and—if on aspirin—administer dose the day before (but not the day of) surgery.

37.5.2 Preoperative (Morning of Surgery)

Consider electroencephalography monitoring, place array if patient able to tolerate leads (ensuring leads are not in planned surgical field). Discuss plan with anesthesiologist, continue intravenous fluids, avoid stress of hypotension with induction, and administer antibiotics.

37.5.3 Equipment

Microscope, Doppler ultrasound (including micro-Doppler if direct bypass planned), fine curved snap, bipolar, craniotome, microdissection kit (jeweler’s forceps, microscissors, microneedle driver, tying instruments, arachnoid knife), heparin saline, rubber dam, 10-nylon sutures (BV needle), and Gelfoam.

37.5.4 Positioning

Once anesthetized, place Mayfield with pins along sagittal axis (one for forehead and two near inion). Position supine, head turned opposite to side to be treated, with shoulder roll to reduce excessive rotation of the neck. Elevate the head and ensure that the operative field is flat, parallel to the floor ( ▶ Fig. 37.2).

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