What Proportion of Stroke Is Not Explained by Classic Risk
Ischemic stroke is a complex entity with multiple etiologies and variable clinical manifestations. The most frequent cause of stroke is atherosclerosis of cerebral vasculature followed by cardioembolism. Two thirds of strokes are explained by identifiable risk factors. Age, hypertension, and nonvalvular atrial fibrillation are by far the most frequent and well documented ones. Approximately 5% of strokes are caused by conditions other than atherosclerosis or heart disease, i.e., cervical arteries dissections, nonatherosclerotic vasculopathies, infectious or systemic vasculitis, and collagen vascular diseases. In spite of a thorough diagnostic evaluation, 30% of strokes remain cryptogenic, i.e., no specific cause is identified and the classic risk factors are not present. Identification of unknown environmental or genetic risk factors should be the subject of further research.

Ischemic stroke is a complex entity with multiple etiologies and variable clinical manifestations. Atherosclerosis of cerebral vasculature is the most frequent cause of ischemic stroke. Large artery disease causes stroke through multiple mechanisms. Severe stenosis can lead to distal hypoperfusion or superimposed thrombosis with occlusion and/or distal artery-to-artery embolization. Sometimes the atherosclerotic aortic plaques are the source of an embolism. Atherosclerotic disease of small cerebral arteries, such as microatheroma and lipohyalinosis, causes lacunar infarcts. Atrial fibrillation and other cardiac sources of emboli lead to cardioembolic strokes. The well documented or classic risk factors for stroke are defined based on a class I or class II epidemiological evidence of association with an increased stroke risk. The less well documented stroke risk factors are defined based on class III, IV, or V epidemiologic evidence.