**4.5 TCD can recognize patients with extracranial ICA stenosis at a higher stroke risk, can assess both collateral pathways, and the vasomotor reactivity (VMR), which detects patients at higher risk of hemodynamic stroke**

Severe extracranial ICA stenosis may produce embolic or hemodynamic hemispheric infarct [48]. While the risk of an embolic ischemic stroke increases with the severity of ICA's stenosis, the hemodynamic risk correlates less well with the degree of stenosis because of the functional capacity of the collateral pathways [48]. A complete circle of Willis and the possibility to activate primary collaterals (anterior communicating artery-ACoA, posterior communicating artery-PCoA) or secondary collaterals (ophthalmic artery-OA, lepto-meningeal arteries) reduce the risk of hemodynamic infarct ipsilateral to the extracranial ICA disease [33, 36, 38]. In patients with collateral flow signals (reversed OA, anterior cross-filling, and PCoA flow) identified by TCD, proximal ICA occlusion is confirmed by subsequent neck CTA, MRA, or DSA [33, 36, 38].

Vasomotor reactivity (VMR) defines the autoregulatory vasodilation of cerebral vessels in response to a vasodilatory challenge, such as hypercapnia or acetazolamide (apnea test, breath-holding test, and Diamox test). VMR represents a measure of dynamic cerebrovascular reserve capacity. Its study recognizes patients at higher risk of hemodynamic stroke, in both intra and extracranial large vessel disease, thus allowing to select those patients who could benefit from a more aggressive treatment [38, 48]. According to Prabhakaran, the presence of MES, poor collateral flow, and impaired VMR predict the high risk of recurrence in intracranial atherosclerosis [41].

TCD can identify intracranial arterial blood flow steals (reversed Robin Hood syndrome).

Intracranial arterial blood flow steals can be detected in chronic disease (e.g., subclavian artery stenoses, arterio-venous malformations, and fistulas) but also in patients with acute ischemic stroke. Flow diversion is the hallmark of a steal and can appear at any level of the intracranial arteries (large proximal vessels and small distal vessels) [33–35].
