"The importance of being external": review of the literature of the rare phenomenon of common carotid occlusion with bulb reverse-crossed stenosis and external collaterals activation. Is still correct speaking about carotid steal?

Common carotid artery occlusion (CCAO) is a rare phenomenon associated with the development of neurological symptoms. In CCAO, diversion of blood flow from the external carotid artery (ECA) to the internal carotid artery (ICA) via the carotid bulb (CB) may occur. This pathway activation has been called "carotid steal". Starting from a particular case we describe the ECD finding of a complete occlusion of CCA with patency of ICA and ECA. In case of occlusion of CCA, inversion of the ECA flow may occur, towards the ICA, and it can be damped by a significant stenosis crossed in retrograde direction that may concur to maintain the pressure balance between the two circulations. Usually, this particular compensation can guarantee normal flow velocities in middle cerebral arteries without signs of activation of anterior collateral pathways. In this review we underline the protective role of ECA and we propose a new definition for this phenomenon. The ECA may provide blood to the cerebral circulation through several anastomotic secondary channels. Finally, only with an extensive knowledge of hemodynamic information of all intracranial and extracranial arteries, including ECA, we can estimate cerebral ischemic risk of the patient and choose the correct management of this occlusion.