**1. Introduction**

Takeuchi and Shimizu first described Moyamoya disease (MMD) in 1957 [1]. It is an idiopathic cerebral vasculopathy characterised by progressive stenosis of the terminal internal carotid artery and its branches, usually on both sides, and the development of a compensatory network of abnormal collateral vessels [2]. Unilateral involvement does not rule out this disease [3]. It affects mainly the middle (MCA) and anterior cerebral (ACA) arteries, less commonly the posterior cerebral or the middle meningeal arteries, and in a few cases the arteries that supply other organs [4], like the lungs or kidneys [5]. Some MMD patients suffer from pulmonary artery hypertension, which usually starts in adolescence or young adulthood and progresses slowly [5].

The collateral vessels that develop as the disease progresses [6] have a thin and weak non-elastic wall with aneurysm formation prone to haemorrhages [7–9]. These aneurysms' prevalence varies in the different series between 1.9 [10], 2.8% [11], 3.9 [12], 8.3 [6] and 14% [13]. They are usually located in deep areas [11], small in size and with very fragile walls, making their treatment, endovascular or surgical, extremely challenging [11, 12, 14]. They can also be found at the circle of Willis [9, 11, 15, 16], where they usually have a fusiform shape [11, 17] and can lead to a subarachnoid haemorrhage [11]. The prognosis in the case of this type of haemorrhage is poor [11].

Their recommended treatment is brain revascularization (BR) [12, 18, 19] as many regress spontaneously after their haemodynamic stress is reduced [11, 20–24].

MMD can be associated with other diseases like Down syndrome (trisomy 21) [25], sickle cell disease [26], neurofibromatosis type 1 [27], thalassemia [28], Graves' disease [29] and after head- and neck-radiotherapy [30]. In this case, it is known as moyamoya syndrome [3].

MMD patients have impaired cerebral haemodynamics with a low cerebral blood flow (CBF) or a poor brain vasodilatory capacity, making them prone to cerebrovascular events [31], particularly at the frontal lobe [32]. The CBF and the cerebral vascular reserve (CVR) capacity are used to evaluate the need for surgical treatment [33–36].

Although it is a rare disease, it is the leading cause of ischemic stroke in the paediatric population in Korea and Japan [37–39].

Digital subtraction angiography (DSA) is the gold standard for diagnosis, but Magnetic Resonance Angiography (MRA) is also helpful [40, 41]. More specific methods but not currently used because of the costs and equipment involved are xenon-enhanced CT, DSC, MRI, H2[ [15]O]-PET and [42] I aromatic amines SPECT [43–48]. These diagnostic techniques are used for preoperative evaluation to decide the best surgical approach. Their drawbacks are that they require contrast agents' intravenous injection and entail radiation exposure that can be harmful to patients, particularly in the paediatric age group. Preoperative transcranial colour-duplex sonography is a non-invasive method used to evaluate the degree of vascular impairment in MMD and monitor the results after surgical BR [49–51].

The patients' mean age at diagnosis peaks at ten and 30–50 years [9, 12, 52] with a ratio of women-to-men 1.00:1.03 [12].

MMD two basic clinical manifestation types are ischemic (30.4%) and haemorrhagic (70.9%) stroke [12]. The ischemic type is more common in children and the haemorrhagic in adults [13, 53]. Other clinical symptoms are headache [54], epilepsy [55], chorea movements [56] and cognitive decline [53, 57]. In comparison, haemorrhages are more frequent in the adult MMD population [58] and ischemic events in the paediatric age group [59–62]. Patients starting with ischemic symptoms like transitory ischemic attack (TIA) are more prone to develop an ischemic than a haemorrhagic stroke [63, 64]. In the paediatric population, multiple ischemic strokes, significantly if both hemispheres are affected, can lead to severe cognitive impairment and developmental delay [65–67]. Brain infarcts present in the ACA and MCA territories and less commonly in the posterior cerebral artery (PCA) covered area and vertebrobasilar system [68]. Posterior circulation involvement leads to more severe symptomatology and worse outcomes [69]. Haemorrhagic strokes are often followed by rebleeding and infarction in short to medium follow-up [70].

Unless surgical BR is undertaken, the stroke rate is 18% for the first year and 3.2–5% for the following years [71–73]. Once patients suffer an ischemic or haemorrhagic stroke, the risk of a new cerebrovascular insult in the following five years is 65% [74, 75].

In MMD, the PCA provides the collateral flow to the anterior circulation [32, 76–79] with choroidal anastomosis and hypertrophic collateral vessel formation [76, 80]. This increased blood flow through the PCA creates haemodynamic stress on the vertebrobasilar system and facilitates aneurysm formation [76, 81]. The choroidal collateral vessels are potential sources of haemorrhages and rebleeding [82]. A common complication is a brain haemorrhage [80, 83, 84], usually intraventricular [80, 85], particularly in the rear brain areas [80]. Moreover, MMD patients with haemorrhages in the posterior part of the brain have a higher risk of rebleeding than those with haemorrhages located in the anterior part [85]. PCA stenosis is typical of juvenile-onset MMD [86].
