18.1 History and Initial Description

The general approach of performing combined direct and indirect revascularization evolved from observations of moyamoya disease (MMD) cases in the 1980s where indirect procedures, especially encephalo-duro-arterio-synangiosis (EDAS) alone, failed to provide sufficient collaterals. ^{1 – 3} Descriptions of combining direct superficial temporal artery (STA)–middle cerebral artery (MCA) bypass with various indirect procedures including EDAS, encephalo-myo-synangiosis (EMS), and encephalo-duro-arterio-myo-synangiosis (EDAMS) emerged in subsequent years, and demonstrated superior results in terms of revascularization. ^{4 ,​ 5} The STA–MCA bypass with EDAS is one of the simple and elegant options for performing combined direct and indirect revascularization.

18.2 Indications

STA–MCA with EDAS can be considered the standard operation for treatment of MMD in adults. The procedure is indicated in these patients, if the following criteria are present:

1. 
   

   Symptomatic patient, that is, patients presenting with stroke (hemorrhagic or ischemic), transient ischemic attack, or progressive cognitive decline.

   
   
2. 
   

   Evidence of hemodynamic compromise with poor cerebrovascular reserve, which can be evaluated using a number of imaging modalities. Our institutional protocol relies on a combination of magnetic resonance (MR) imaging consisting of quantitative MR angiography with and without diamox challenge, global blood oxygen level dependent (BOLD) imaging with hypercapnic challenge and regional BOLD imaging using functional MR imaging paradigms. The aggregate information is used to determine if there is reduced or absent cerebrovascular reserve.

   
   
3. 
   

   Adequate donor and recipient vessels to perform the STA–MCA direct anastomosis; this can be an issue in very young pediatric patients, in which case, indirect procedure alone can be pursued.

The combined STA–MCA with EDAS can also be considered the preferred option for pediatric patients if direct bypass is feasible; the greatest barrier to performing direct bypass is posed by the very young pediatric patients (< 5 years old), dependent on donor and recipient vessel size, in which case, indirect procedure alone can be pursued. For pediatric patients, even an asymptomatic hemisphere affected by moyamoya warrants consideration for surgery if there is presence of hemodynamic compromise on imaging, as progression to symptoms and stroke is prevalent in this population.

The advantage of the STA–MCA bypass in combination with EDAS, as opposed to in combination with other indirect procedures such as EMS or EDAMS, relate to the relative disadvantages of harvesting and using muscle on the brain surface. Although muscle can be a very effective source of collaterals, its use typically entails a larger craniotomy to expose more brain surface area for muscle contact, entails potential cosmetic issues related to transposition of the muscle underneath the bone flap, and has a higher risk of postoperative hematoma from oozing of muscle fibers over the brain surface, particularly when patients are on aspirin to optimize the patency of direct bypass. The EDAS procedure is more straightforward, combines naturally with the STA–MCA bypass requiring little alteration to the approach that would already be applied for the direct procedure, and provides effective collaterals without the potential disadvantage of muscle use.

18.3 Key Principles

The STA–MCA bypass with EDAS provides both immediate revascularization through direct bypass and longer term growth of additional collaterals through indirect EDAS. In fact, there is frequently a reciprocal relationship between direct STA bypass flow and indirect EDAS collaterals that provide durable temporally complementary revascularization. ⁶ Key concepts that are critical in this procedure are as follows:

• 
  

  Identification of the vascular territory in most need of flow augmentation, and choosing the skin incision, craniotomy, and donor vessel choice and configuration to optimize direct and indirect bypass.

  
  
• 
  

  Preserving the middle meningeal artery (MMA) when performing the craniotomy and opening the dura, particularly if it is already providing extracranial–intracranial (EC–IC) collaterals (as evident from preoperative angiographic imaging).

  
  
• 
  

  Use of flow-assisted surgical technique ^{7 ,​ 8} (as described further) to measure cut flow and cut flow index in order to assess direct STA–MCA bypass.

18.4 SWOT Analysis

18.5 Contraindications

The primary contraindication is recent stroke within the last 7 to 14 days, primarily due to higher risk of reperfusion hemorrhage with the direct STA–MCA portion of the procedure, and also in general due to heightened risk of anesthesia and surgery in the setting of a recent ischemic event. Additional contraindications for direct component of the procedure would be poor quality or caliber of the STA donor vessel, or existing spontaneous collaterals from the donor STA which would be disrupted by harvesting and cutting the vessel for direct anastomosis. In such a scenario indirect arterio-synangiosis, using the intact STA which is kept in continuity and apposed to the brain surface, would still be feasible, as long as harvesting the vessel in situ would not directly result in disruption of collaterals.

18.6 Special Considerations

For the STA–MCA with EDAS operation the preoperative imaging is critical in planning. Catheter angiography allows the best assessment of the course and caliber of the donor STA vessels, and allows evaluation of spontaneous EC–IC collaterals from the MMA or other sources which must be preserved while performing the operation. The angiogram should provide dedicated internal and external carotid injections to allow the optimal evaluation of these features. The angiogram, in conjunction with blood flow imaging, is also important in determining the vascular territory that is most compromised and most in need of flow augmentation.

In the preoperative arrangements, admission of the patient prior to day of surgery to ensure overnight intravenous hydration while the patient is not allowed oral intake is an important safety measure to reduce the risk of hypovolemia and hypotensive episode during induction of anesthesia. Preoperative administration of aspirin, if the patient is not already routinely taking the medication, is advisable as a maneuver to enhance patency of direct STA–MCA anastomosis, and checking aspirin sensitivity assay to ensure that the medication is effective preoperatively can be performed. For patients with comorbid diseases, it is essential to optimize the management of the condition prior to surgery, for example, in patients with sickle cell disease, use of exchange transfusions and reduction of hemoglobin S levels, or in patients with diabetes, strict glucose regulation, and controlled hemoglobin A1C levels. Otherwise, such conditions can affect either the success of the revascularization or healing and recovery from the surgery.

18.7 Pitfalls, Risk Assessment, and Complications

The primary risk associated with STA–MCA bypass with EDAS is similar to any revascularization surgery in patients with MMD, namely the risk of perioperative ischemic stroke. The addition of direct STA–MCA bypass to the EDAS operation ought not to increase this risk, given that temporary occlusion of the cortical MCA branch required for direct anastomosis is extremely well tolerated. However, it does add some additional operative time which could potentially elevate risk, as period of time under anesthesia may be a risk contributor. Direct bypass also can engender a risk for postoperative hyperperfusion or reperfusion hemorrhage which is not encountered with indirect EDAS alone, and requires postoperative vigilance and blood pressure management to avoid. Other general risks include postoperative seizures, and short-term prophylactic anticonvulsants are thus reasonable.

One feature of the STA–MCA with EDAS operation is the dual harvest of both STA branches, using one for direct anastomosis and the other for indirect arterio-synangiosis. The dual harvest however can lead to devascularization of the scalp tissue and incisional healing problems, which in turn can result in wound dehiscence or infection.

18.8 Special Instructions, Position, and Anesthesia

As noted above, aspirin is administered preoperatively to patients, typically at full dose of 325 mg for adults and 81 mg for pediatric patients, and aspirin sensitivity is confirmed by platelet function assay prior to the surgery. During the surgery, low-dose heparin (10 units/mL) is used for flushing the donor STA and recipient MCA vessel for direct anastomosis, but no systemic anticoagulation is used.

General endotracheal anesthesia is utilized. The primary considerations are maintenance of blood pressure at or above the patient’s baseline blood pressure throughout, especially during the induction of anesthesia when there is most risk of lability and hypotension. Arterial line placement is mandatory for monitoring of the blood pressure during the surgery and for access to draw arterial blood gas samples. End-tidal carbon dioxide levels should be monitored and correlated with arterial levels, to avoid hyperventilation, which reduces cerebral blood flow. The anesthetic regimen is chosen to allow titration to achieve burst suppression during the temporary clipping time for direct anastomosis. Both electroencephalography (EEG) and somatosensory-evoked potential (SSEP) leads are placed prior to positioning for neuromonitoring. EEG allows determination of burst suppression when needed during the operation, and along with SSEP can also alert to hypoperfusion and brain ischemia as both reflect integrity of cortical activity. It is important to assess baselines in these modalities, as prior strokes may affect the symmetry, amplitude, or latency; the relevant concern would be changes noted relative to baseline. Since the EEG and SSEP monitoring utilize scalp electrodes, it is advisable to map the STA prior to placement of these needle or corkscrew electrodes to avoid inadvertent injury to the STA.

During the surgery, the anesthesiologist must also maintain euvolemia for the patient; use of the typical brain relaxation agents such as mannitol and furosemide must be avoided due to risk for hypovolemia, hypotension and subsequent hypoperfusion in the already tenuous and compromised vascular territory undergoing surgery, or even the contralateral side in patients with bilateral disease.

The patient is positioned with the surgeon at the head of the bed, anesthesiologist to the left of the patient and the operating nurse to the right of the patient so that instruments can be easily handed to the right hand of the surgeon during the operation. The patient’s head is turned to the right or left dependent on the side that needs to be operated.