**4. Indication of surgical techniques for cerebral revascularization in moyamoya disease**

The hemisphere with the worse vascularization is operated first [12]. If both hemispheres are equally affected, the recommendation is to start with the dominant one and revascularize the other six months latter [178].

In unilateral involvement, if the patients' symptoms disappear with the unilateral BR and an asymptomatic contralateral hemisphere, no further brain vascularization is advised for the time being [12] as contralateral hemisphere surgical BR in patients with unilateral involvement is controversial [99]. In an ischemic or haemorrhagic stroke, surgical BR is delayed for at least six weeks [188], and ideally, three months [12] and the BR of the contralateral cerebral hemisphere postponed at least 4–6 weeks [148]. Nevertheless, delaying surgical treatment is not advisable in late Suzuki stages [2], as BR improves brain collateral vascularization but not the stroke

rate [178]. So, once the diagnosis is made, it is better to avoid unnecessary delays, particularly in children [178].

Direct BR is particularly indicated in adult and adolescent MMD patients [189, 190] but not recommended before ten years of age [191]. An STA to MCA bypass is strongly advised as it corrects the MMD related hemodynamic insufficiency [71, 121]. For a successful result, both the donor and recipient arteries must be at least 0.8 mm in outer diameter [192].

Indirect techniques are much less demanding [193] but are not as valuable for adults as for children [99, 149, 150, 152, 159, 179, 194, 195]. Its main drawback is that BR takes time, on average 3–4 months [196], during which there is a continued risk of cerebrovascular events [140]. In the paediatric age group, these indirect surgical techniques are preferred because the vessels are often of insufficient size and maturity to safely allow a direct arterial bypass [197], significantly below ten years of age [198]. One of the main advantages of indirect BR is the possibility of improving the anterior and PAC territories' blood perfusion apart from the area covered by the MCA [146, 199]. This possibility of a wider area covered by the BR is crucial in children [99, 152, 158] as they often develop long-term symptoms secondary to hypoperfusion of the whole hemisphere [99]. The BR that use the temporalis muscle is not recommended in children [200] because it thickens with time, compressing the brain and inducing ischemia [201] and because it adheres to the brain and when it contracts can cause long-term neurological damage [202]. Additionally, it generates an unsightly cosmetic head [200].

A combination of direct and indirect BR procedures is often used to profit from the advantages of each of them [34, 114, 136, 150, 160, 203], because the reported results are better than isolated direct or indirect techniques [204–207]. The treatment strategy has to be tailored to the specific needs of each patient [136, 160, 208]. Combining more than one indirect BR techniques has similar mortality and morbidity as any of them isolated [136].

During the surgical procedure, it is essential to avoid hypotension, hyperthermia, hypocarbia, hypercarbia, and epileptic seizures as they all increase the brain metabolism and thus the chance of an ischemic event [148, 149, 188, 200, 209–211] and perioperative morbidity [212]. It is vital to control the pain and cry in the postoperative period to avoid hyperventilation as this will produce hypocarbia and an increased risk of ischemia [149, 213]. It is recommended to provide intravenous fluids at 1.5 times the regular maintenance rate for 48–72 hours [3, 149]. Platelet counts and prothrombin time must be monitored and controlled with transfusions if needed [148]. The blood haemoglobin must be kept above the 12–13 g/dL range [148].

Some improvements in the surgical technique have eased the surgical manoeuvres and improved the clinical results. Among them is placing 10-0 Proline sutures to the arachnoid membrane in both sides of the brain sulcus. When some retraction is applied, the recipient artery is brought to the surface, and the STA to MCA bypass is made more easily [192].

While it seems intuitive that arachnoid removal will improve the collateral vessel formation between the donor artery or tissue and the brain [153], it is no longer recommended in indirect BR because it is associated with a significantly higher complication risk with no improvement in the final clinical outcome [148]. Among these complications are the postoperative ischemic strokes secondary to the vasospasm induced by the arachnoid dissection [148]. Preserving the arachnoid membrane reduces the operating time at an average of 30 minutes [148].

The middle meningeal artery provides a vital source of collateral circulation, so during the surgical procedure, this artery and its main branches should be preserved as much as possible [191]. Preservation of this crucial source of collateral
