Ischemic Stroke Subtypes

Hemodynamic factors, embolism and small vessel disease are the most common mechanisms by which ischemic strokes occur. The most frequent conditions leading to cerebral ischemia are atherosclerosis, embolism secondary to cardiac disease, small vessel disease and cryptogenic infarction.

Atherosclerosis

As previously noted, atherosclerotic plaques can lead to progressive vessel stenosis or occlusion. Ischemia leading to infarction is due to diminished blood flow distal to the site of stenosis or occlusion. The extent of infarction depends on collateral blood flow or re-establishment of blood flow through the affected region. Atherosclerosis can lead to infarct through dis-lodgement of an embolic fragment arising from an ulcerated vessel or from an unstable plaque.

Cardiac Embolism

Ischemic strokes caused by embolism usually have a cardiac source. It is estimated that car-dioembolic events account for 15-20% of all ischemic strokes [9]. It is also estimated that 75% of cardiac emboli travel to the brain. Once in the brain, emboli can lodge themselves in arteries that are too small for them to pass, leading to vessel occlusion. Mural thrombi can form after a myocardial infarction (MI) and ischemic events follow an acute MI in 2-5% of cases [10]. Other common sources of cardiac embolism include atrial fibrillation, atrial myxoma, dilated cardiomyopathy, patent foramen ovale, atrial septal aneurysms, prosthetic heart valves, infective endocarditis, mitral valve prolapse and mitral annular calcification.

Small Vessel Lacunar Disease

Lacunes are small infarcts, which occur as a result of arterial disease of vessels supplying the deep aspects of the cerebrum and brain stem, such as the basal ganglia, the internal capsule, the thalamus, the corona radiata and paramedian regions of the brainstem. They account for 12-15% of ischemic strokes [11]. They can be silent or they can present with neurological deficits, such as pure motor hemiparesis, pure sensory syndrome, clumsy hand dysarthria, ataxic hemiparesis and sensorimotor stroke. The arterial damage is usually due to the effects of long-standing hypertension and diabetes. Many patients with radiographic evidence of lacunar infarcts do not present with the classic syndromes. Most patients are awake and their intellectual functions are not compromised. The mortality associated with this disease is low, but the morbidity can be significant.

Infarcts of Undetermined Cause

Despite complete diagnostic evaluation, the cause of cerebral ischemia cannot be identified in as many as 40% of ischemic strokes. Patients with these "cryptogenic strokes" usually have no prior TIAs, no bruits on physical examination and, usually, normal angiography. CT scanning or MRI may be normal or may show an infarct limited to a surface brain territory. Some of these infarcts have been attributed to conditions such as sickle cell disease, hyperco-agulable states or protein C and protein S deficiencies, lupus anticoagulant or anticardiolipin antibodies.

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