Revascularization Techniques in Pediatric Cerebrovascular Disorders

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Chapter 61 Revascularization Techniques in Pediatric Cerebrovascular Disorders

Edward Smith, R. Michael Scott

This chapter focuses on methods used to revascularize the brain in the setting of treating pediatric cerebrovascular disease. While there are many situations that might require some form of surgical revascularization, there are three conditions in particular—atherosclerotic carotid disease, intracranial aneurysms, and moyamoya syndrome—that are most commonly encountered by neurosurgeons. However, age-specific differences in disease presentation mean that the spectrum of cerebrovascular disease encountered by neurosurgeons who treat children is notably different than what is seen in adult patients. Atherosclerotic carotid disease is not present in the pediatric population and using revascularization techniques to treat the rare pediatric intracranial aneurysm is rarely feasible. Since these conditions are rare in children, the use of revascularization techniques in these diagnoses will not be discussed here and the reader is referred to the relevant descriptions elsewhere in this textbook. This chapter will thus primarily center on the surgical revascularization of children with moyamoya syndrome.

Moyamoya Syndrome

Moyamoya syndrome is an arteriopathy characterized by progressive stenosis of the distal internal carotid arteries as they enter the cranial vault.1,2 With narrowing of the internal carotids, cerebral blood flow is reduced, cerebral ischemia develops, and collateral blood vessels develop in the region of the carotid bifurcation, on the cortical surface, and from branches of the external carotid artery (ECA). This alternative blood supply—comprised of maximally dilated pre-existing arteries and growth of new vessels—provides circulation to the region formerly supplied by the internal carotids. Although usually limited to the anterior circulation, this process may involve the posterior circulation as well; including the basilar and posterior cerebral arteries. The appearance of this basal collateral network on angiography has been compared to a hazy cloud or puff of smoke: the disease defined by the Japanese word “moyamoya.” Because of the natural propensity in patients with moyamoya for collateral vessels to the brain to develop from branches of the external carotid and because these arteries and those on the surface of the brain are not involved by the moyamoya arteriopathy, most of the surgical revascularization techniques in this condition utilize the external carotid circulation as a donor source of new blood flow to the ischemic brain.

Surgical Treatment of Moyamoya

Two general methods are employed: direct and indirect. In direct revascularization, a branch of the ECA (usually the superficial temporal artery [STA]) is anastomosed end to side to a cortical artery (usually a distal branch of the middle cerebral artery [MCA]), the so-called “STA-MCA bypass.” In contrast, indirect techniques involve mobilizing vascularized tissue supplied by the ECA (dura, muscle, pedicles of the STA) and placing it in contact with the brain to promote in-growth of new vessels to the cortex.

Historically, direct procedures have been used in adults, with immediate increase of blood flow to the ischemic brain cited as a major benefit of the procedure. Augmentation of cerebral blood flow usually does not occur for several weeks with indirect techniques. However, direct bypass is often technically difficult to perform in children because of the small size of donor and recipient vessels; making indirect techniques appealing. Nonetheless, direct operations have been successful in children as have indirect procedures in adults.3–5 Considerable debate exists regarding the relative merits and shortcomings of the two approaches; in fact, some centers advocate combinations of both approaches.5–7

Numerous indirect revascularization procedures have been described: encephaloduroarteriosynangiosis (EDAS) whereby the STA is dissected free over a course of several inches and then sutured to the cut edges of the opened dura; encephalomyosynangiosis (EMS) in which the temporalis muscle is dissected and placed onto the surface of the brain to encourage collateral vessel development; the combination of both, encephalo-myo-arterio-synangiosis (EMAS), a variant of EDAS in which the STA is sutured to the brain; pial synangiosis, described in detail below; and the drilling of multiple burr holes without vessel synangiosis.8–14 Dural inversion, carrying out a craniotomy, opening the dura, and turning the dural flaps inward over the surface of the brain has also been described as a revascularization technique.13 Cervical sympathectomy and omental transposition or omental pedicle grafting have also been described.2 We have found the technique of pial synangiosis particularly effective in the pediatric moyamoya population, and in a review of 143 patients treated with pial synangiosis, demonstrated marked reductions in stroke frequency following surgery.14 Regardless of the revascularization procedure utilized, the perioperative strategies for complication avoidance are relevant to all moyamoya patients, regardless of surgical technique employed.

Pial Synangiosis

We have recently published a specific perioperative protocol for patients with moyamoya15 (Table 61-1). This protocol has been adapted from our practice for all patients with moyamoya and highlights general strategies we have found useful in the surgical management of this condition.

Table 61-1 Perioperative Management Protocol Used at Our Institution for Patients with Moyamoya

At 1 Day Before Surgery
Continue aspirin therapy (usually 81 mg once a day orally if <70 kg, 325 mg once a day orally if ≥70 kg).
Admit patient to hospital for overnight intravenous hydration (isotonic fluids 1.25–1.5 × maintenance).
At Induction of Anesthesia
Institute electroencephalographic monitoring.
Maintain normotension during induction; also normothermia (especially with smaller children), normocarbia (avoid hyperventilation to minimize cerebral vasoconstriction, pCO2 > 35 mm Hg), and normal pH.
Placement of additional intravenous lines, arterial line, Foley catheter, and pulse oximeter.
Place precordial Doppler to monitor for venous air emboli (relevant with thicker bone resulting from extramedullary hematopoiesis).
During Surgery
Maintain normotension, normocarbia, normal pH, adequate oxygenation, normothermia, and adequate hydration.
Electroencephalographic slowing may respond to incremental blood pressure increases or other maneuvers to improve cerebral blood flow.
Postoperatively
Avoid hyperventilation (relevant with crying in children); pain control is important.
Maintain aspirin therapy on postoperative day 1.
Maintain intravenous hydration at 1.25–1.5 × maintenance until child is fully recovered and drinking well (usually 48–72 hours).

Revised from Smith ER, McClain CD, Heeney M, et al. Pial synangiosis in patients with moyamoya syndrome and sickle cell anemia: perioperative management and surgical outcome. Neurosurg Focus. 2009;26:E10.

Indications for Surgery

In the setting of patients with radiographically confirmed moyamoya syndrome, surgery is indicated in the cases with (1) history of neurologic symptoms such as stroke, TIA, seizures, and so on, due to cerebral ischemia; and (2) progressive moyamoya changes detected on surveillance scans in susceptible populations (e.g., following radiation therapy for brain tumor). We use cerebral blood flow and metabolism studies, such as MRI perfusion sutides, PET, and SPECT, only in problematic cases where the indications for surgery are not as well-defined.

Surgery is relatively contraindicated in patients who are at significant operative risk (severe cardiac disease, advanced debilitation from stroke burden, or other severe comorbidities). In addition, patients with an unclear diagnosis or whose arteriopathy is very mild, with a low Suzuki grade (I or, rarely, II), are occasionally observed closely with serial imaging before we consider surgical revascularization.

Overall, we generally advocate surgical treatment for patients with radiographically confirmed moyamoya, even if asymptomatic, as the preponderance of evidence suggests that moyamoya is a relentless, progressive process in children. Surgical treatment, both direct and indirect, has been shown to be safe when performed by experienced surgeons and confers long-lasting, durable protection from stroke relative to medical therapy alone.

Preoperative Strategy and Imaging

Preoperative management of moyamoya patients is critical to the success of surgery. Strategy is based on the utilization of appropriate imaging for planning and the maintenance of hypervolemia, normocarbia, and prevention of thrombosis. A full five- or six-vessel (both ICAs, both ECAs, and one or both vertebrals as indicated) diagnostic angiogram is critical to the planning of the procedure for:

1. Accurate identification of disease status

2. Identification of transdural collaterals so that they may be preserved during surgery

3. Confirmation of the presence of a suitable donor scalp vessel (usually the parietal branch of the superficial temporal artery [STA])

Once the decision to operate has been made, we follow a standardized perioperative protocol. Dehydration is a significant risk given the hypoperfused intracranial circulation. To minimize shifts in blood pressure during the induction of anesthesia, we routinely admit patients to the hospital on the evening prior to surgery for intravenous hydration. If there are no underlying cardiac or renal limitations, isotonic fluids are run at 1.5 times maintenance rate. Barring medical contraindication, patients are treated with daily aspirin therapy from the time of their diagnosis of moyamoya in order to minimize the risk of thrombosis in the slow-flowing cortical vessels. Dosing is continued up to and including the day prior to surgery (and restarted the day after surgery). Pain and anxiety must be aggressively managed, especially with children since hyperventilation, as occurs with crying, can induce cerebral vasoconstriction; leading to stroke. Steroids, cerebral dehydrating agents such as mannitol and anticonvulsants are not administered on a routine basis.

Anesthetic Issues and Monitoring

An experienced team of anesthesiologists is critical to the success of the operation. Generally, premedication is useful to minimize crying in children to prevent cerebral vasoconstriction and possible ischemic events. Hypotension, hyperthermia, and hypercarbia are to be avoided at all times, especially during induction. Muscular blockade is established by a non-depolarizing muscle relaxant prior to intubation. Anesthesia is maintained with low-dose isoflurane (a cerebral vasodilator) and a balanced nitrous oxide/oxygen mixture with fentanyl. End-tidal CO2 is usually kept on the high-normal side (35 to 40 mm Hg) to minimize cerebral vasoconstriction. Normotension is maintained. Diuretics (mannitol and Lasix) are usually avoided due to the possibility of hypotension.

We routinely supplement routine anesthetic monitoring with the use of intraoperative EEG. EEG is employed during surgery to identify focal slowing, indicative of compromised cerebral blood flow, so that immediate compensatory measures can be instituted by the operative team. EEG technicians must communicate changes in the EEG promptly to allow the team to respond immediately with appropriate titration of blood pressure, pCO2, and anesthetic agents.

Operative Technique and Setup

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