**5. Results surgical techniques of cerebral revascularization in moyamoya disease**

BR decreased both the haemorrhagic [115] and the ischemic stroke [164, 218], but it is more effective in the first than in the second [219–221]. On one-year follow-up, haemorrhagic stroke was 4.6% for those who underwent BR versus 18.6% for those managed conservatively [115, 116, 164, 219]. The mortality rate due to haemorrhagic stroke is four times higher in the conservative than in the BR group [116, 164, 220]. The medical treatment had even worse results in the paediatric than in the adult MMD age group [107, 222].

Younger age at BR surgery correlates with better results and long-term prognosis [62, 164, 205]. In children, indirect BR is associated with a 0.4%/year symptomatic haemorrhage and 0.2%/year infarction rates, with cumulative incidences of 1.8% at ten, 7.3% at 20 and 7.3% at 30 years [223, 224]. There are no statistically significative differences in clinical outcome between direct, indirect and combined BR procedures [99, 205]. The average good clinical outcome is 84.8–88% [143, 205] with a 6.4% 5-year risk of ischemic or haemorrhagic stroke [143]. Children seem to improve more than adults (93% versus 82.7%) [205].

Direct BR with extra-intracranial artery bypass significantly decreases the haemorrhagic stroke rate [115, 223], particularly in the patients with haemorrhages in the posterior half of the brain [85]. Compared to indirect procedures, it reduces the stroke risk [116, 205, 219, 221], particularly the haemorrhagic type in adults [225] and adolescents [196]. These differences are not so evident in children as adults due to the technical difficulty in performing a successful arterial bypass in the first group [140]. Both in adults and the paediatric population, direct BR is technically more challenging that indirect BR, demands a longer operating time, and there is always the risk of hyperperfusion syndrome [116]. There no statistically significative difference in the number of perioperative complications between direct and indirect BR techniques [219]. The direct bypass was accompanied by an annual stroke rate of 0–6% for children and 1.4% for adults, while the indirect techniques had a 1.6% stroke rate for the same period of time [196]. Direct BR is recommended whenever it is technically feasible [197, 221].

Indirect BR is easier to perform, but 40–50% of adult MMD patients do not develop an adequate collateral arterial circulation [53, 62, 141, 226]. These results have been improved with changes in the surgical technique and perioperative care [140], but a new surgical procedure with a combined approach will be needed if there is an unsatisfactory outcome [189, 207, 224]. Meanwhile, paediatric patients submitted to indirect BR have low middle and long term haemorrhagic stroke rates [227, 228].

Perioperative complications happen in 9.4–13.6% of BR procedures [116, 149, 194, 205, 219, 221, 226] with a 0–0.5% [188] mortality rate. Indirect BR has a significantly higher postoperative stroke rate than direct techniques [116, 205] but fewer haemorrhages and no hyperperfusion syndrome [140, 229, 230]. The

incidence of postoperative surgical complications is higher in Asian than other racial groups (6.51/100,00 inhabitants/year versus 5.21/100,000/persons/year) [198, 231]. They also show a different response to BR [198]. Patients with MMD associated with other diseases have a higher perioperative complication rate than regular MMD patients [188].

Preoperative infarction is related to a greater risk of postoperative complications [232, 233].

Direct or combined BR is associated with better outcomes than indirect BR, particularly in the ischemic type MMD [140, 190, 205]. Meanwhile, for haemorrhagic strokes, there are no differences between direct, indirect or combined BR, and thus the indirect techniques are recommended because they are more accessible to perform [205].

The best collateral vessel formation results are obtained using a superficial temporal to MCA bypass combined with an EDAMS and lowest with EDAS [140]. Nevertheless, EDAS combined with multiple burr-holes can provide similar outcomes with a less technically demanding procedure and fewer postoperative negative events [159, 177].

Surgical BR in MMD can prevent future cerebrovascular insults and avoid cognitive decline [116, 117, 227]. It is indicated in asymptomatic patients at the slightest sign of disease progression or patient neurological or mental impairment [103, 234, 235], particularly in the paediatric age group [62, 103]. If performed early before irreversible damage, it can improve the neurological status and prevent the cognitive decline [103], stopping or at least slowing the progression of this nasty disease [136, 206, 234]. Ideally, in children, the surgical BR should be performed not later than three months after the first symptoms appear at a young age [190], the best before six years of age [106].
