This chapter deals with focal brain ischemia, either TIA or ischemic stroke. Causes, mechanisms and clinical syndromes of brain hemorrhage are described elsewhere. This chapter is divided into three parts. The first part focuses on an uncommon mechanism of focal brain ischemia, which is low flow. Most TIA and ischemic strokes are caused by embolism or in situ artery occlusion. Hemodynamic causes of focal brain ischemia are less common. Secondly, uncommon clinical presentations of focal brain ischemia are described. In the third part, uncommon causes of TIA and ischemic stroke are presented together with associated clinical syndromes.
Uncommon mechanism of stroke: low flow
Ischemic strokes and transient ischemic attacks caused by low cerebral flow – anterior circulation
Most ischemic strokes and transient ischemic attacks are caused by embolic and acute, in situ (usually thrombotic) occlusion of an artery in the brain. However, in some patients severe stenosis or occlusion of carotid or vertebral arteries may cause a critical reduction of blood flow, particularly when collateral circulation is compromised because the circle of Willis is incomplete or diseased. Mechanisms to compensate for the reduction of blood flow are vasodilatation by autoregulation and an increase of the oxygen extraction fraction. If the vascular bed is maximally dilated the supplied brain is particularly vulnerable to any fall in perfusion pressure. Under these circumstances a small drop in systemic blood pressure may cause transient or permanent focal ischemia.