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Despite efforts to arrive at a diagnosis, the cause of infarction in a discouragingly large number of cases remains undetermined. Some cases may be unexplained because no appropriate laboratory studies are performed, while others are due to the improper timing of the appropriate laboratory studies. The most frequent circumstances are when normal or ambiguous findings are reached despite appropriate laboratory studies performed at the appropriate time. Results from the Stroke Data Bank indicated that large artery atherosclerotic occlusive disease was a less frequent cause of stroke; small vessel or lacunar and cardioembolic infarction were relatively frequent; and that the cause for the majority of the cases of infarction could not be classified into these traditional diagnostic categories. This forced the creation of a separate diagnostic category for cases whose mechanisms of infarction remained unproven, one known as ‘infarct of undetermined cause’ or ‘cryptogenic infarction.’
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Cases categorized as cryptogenic infarction have no bruit or TIA ipsilateral to the hemisphere affected by stroke, no obvious history suggestive of cardiac embolism and usually do not present with a lacunar syndrome. The CT or MRI scan performed within seven days may be normal, show an infarct limited to a surface branch territory or may show a large zone of infarction affecting regions larger than that accounted for by a single penetrant arterial territory. Viagra soft tabs canada. Non-invasive vascular imaging fails to demonstrate an underlying large vessel occlusion or stenosis. No cardiac source of embolism is uncovered by echocardiography, electrocardiography or Holter monitor. If an angiogram is performed, the study may be normal, show a distal branch occlusion or occlusion of a major cerebral artery stem, or the top of the basilar. Because these latter occlusions can be from embolus or thrombosis of an atherosclerotic vessel, their demonstration does not settle the mechanism. Image results for echocardiography

Many of these cases present with a hemispheral syndrome, a surface infarction by CT or MRI and a corresponding branch occlusion documented by angiography or normal angiogram. This constellation of findings has been considered suggestive of embolism. There is ample evidence for many occult sources of emboli, the difficulty proving their existence, and their role in the first or succeeding ischemic strokes. Emerging technologies have led to the suggestions that some cryptogenic infarcts may be explained by hematologic disorders causing hypercoagulable states from Protein C, free Protein S, Lupus anticoagulant or anti-cardiolipin antibody abnormalities. Others have implicated paradoxical emboli through a patent foramen ovale and aortic arch atherosclerosis.

Small vessel, Lacunar Infarction
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These strokes have distinctive clinical syndromes with a small zone of ischemia confined to the territory of a single vessel. They are understood to reflect arterial disease of the vessels penetrating the brain to supply the internal capsule, basal ganglia, thalamus, corona radiata and paramedian regions of the brainstem. There are disagreements about the pathogenesis of lacunar infarcts with some favoring the use of the term “lacune” to describe size and location without indicating a specific pathology. Only a handful have been pathologically studied by serial section and documented a tiny focus of microatheroma or lipohyalinosis stenosing one of the deep penetrating arteries. The arterial damage is usually the result of long-standing hypertension or diabetes mellitus. Less common causes include stenosis of the middle cerebral artery stem or micr0embolization to penetrant arterial territories. The vast majority of radiologically-defined, small’, deep infarcts do not have significant large artery atherosclerosis, lack even a potential cardiac source of embolism and occur in vascular territories less likely to be occluded by emboli.
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Many lacunar strokes are diagnosed by clinical characteristics alone. Clinical syndromes include pure motor hemiparesis, pure sensory, clumsy hand dysarthria, ataxic hemiparesis and sensorimotor stroke. When brain imaging is positive, a strategically placed small, deep infarct is usually found. Because the vascular lesion lies in small vessels, it is no surprise that cerebral angiography is normal. Incidental large vessel disease may be found in some series, but whether etiologically related to the site of infarction is often unclear.

Atherosclerotic Infarction
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Atherosclerotic plaque at a bifurcation or curve in one of the larger vessels leads to progressive stenosis with the final large artery occlusion due to thrombosis of the narrowed lumen. Arteriosclerotic plaques may develop at any point along the carotid artery and the vertebral-basilar system, either extracranial or intracranial. The most common sites are the bifurcation of the common carotid artery into the external and internal carotid arteries, the origins of the middle and anterior cerebral arteries, and the origins of the vertebral from the subclavian arteries. The site of infarction depends on the collateral flow, but is usually in the distal fields or border zones. Specifying the degree of stenosis that will lead to perfusion difficulty is dependent on multiple factors and is often not easily defined. Classification schemes have relied on stenoses greater than 70 to 80% as more predictive of impending hemodynamic compromise.
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Infarcts are also produced by emboli arising from the proximally-situated atheromatous lesions to otherwise healthy branches located more distal in the arterial tree. This mechanism of infarction has earned its own category, tandem arterial pathology. Embolic fragments may arise from extracranial arteries affected by stenosis or ulcer; stenosis of any major cerebral artery stem; the stump of the occluded internal carotid artery; and even from intracranial tail of the anterograde thrombus atop an occluded carotid. Making the diagnosis of an antherosclerotic infarction depends upon finding a stenosis or occlusion on carotid/vertebral Duplex Doppler, transcranial Doppler, magnetic resonance angiography, or even conventional cerebral angiography.

Cardiac Embolism

The most common sources of cardiac embolism include: atrial fibrillation, valvular heart disease (mitral stenosis, mitral regurgitation, rheumatic heart disease); intracardiac thrombus particularly along the left ventricular wall (mural thrombus) after anterior myocardial infarction or in the left atrial appendage in patients with atrial fibrillation; ventricular or septal aneurysm; and cardiomyopathies leading to stagnation of blood flow and an increased propensity for the formation of intracardiac thrombus.
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Embolism is inferred when the brain image demonstrates an infarction confined to the cerebral surface territory of a single branch, combinations of infarcts involving branches of different divisions of major cerebral arteries, or hemorrhagic infarction. The difficult problem in arriving at a diagnosis of embolism is the identification of the occluding particle and the source. Mural thrombi and platelet aggregates are remarkably evanescent, as has been inferred by findings on angiography. Embolic fragments are found in over 75% of cases angiogramed within 48 hours of onset of the stroke, and are then gone when angiogram is repeated later. Embolic obstruction of an arterial lumen is cleared most commonly by recanalization and fibrinolysis. The evanescent quality of emboli may explain the wide variation in the frequency with which this subtype is diagnosed in retrospective or prospective studies of stroke.
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There are multiple mechanisms which may lead to brain ischemia. Hemodynamic infarction originates when there is an impediment to normal perfusion usually caused by a severe arterial stenosis or occlusion due to atherosclerosis and coexisting thrombosis. Embolism occurs When a particle of thrombus originating from a more proximal source (arterial or cardiac) travels through the vascular system and leads to a distal occlusion. Small vessel disease occurs when lipohyalinosis or local atherosclerotic disease leads to an occlusion of a small penetrating artery. Less frequent conditions which lead to reductions in cerebral perfusion and result in infarction are: arterial dissection, primary or secondary vasculitis, hypercoagulable states, vasospasm, systemic hypotension, hyperviscosity, moyamoya disease, fibromuscular dysplasia, extrinsic compression of the major arteries by tumor and occlusion of the veins that drain the brain. The relative frequency of each major subtype of infarction varies from study to study. The proportion with cardioembolic or lacunar infarcts is approximately 15% to 30% each, atherosclerotic infarcts occur in 15% to 40%, and less common causes account for about 5%. Infarcts of indeterminate cause comprise as many as 40% of strokes in some series.
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The infarct subtype categories are far from ideal with many examples of controversies regarding classification, reliability and validity. The distribution of infarct subtypes depends upon the sample from which cases are drawn (hospital or population-based), the geographic region of the study, the demographic and risk factor profile of the cohort, the timing and sophistication of the work-up and the design of investigator-driven diagnostic algorithms. Part of the difficulty regarding classification of ischemic stroke stems from the inability to discriminate between infarct subtypes on clinical grounds alone. Some clinical characteristics are helpful in distinguishing subtypes, such as fractional arm weakness (shoulder different from the hand), hypertension, diabetes, and male gender which are more frequently found in the atherosclerotic category. Besides a greater frequency of cardiac disease, patients with cardioembolic infarction more often present with reduced consciousness and have an abrupt onset. However, clinical features at stroke onset are not reliable enough to lead to a definite determination of infarct subtype without confirmatory laboratory data.
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Brain imaging with head computerized tomography (CT) or magnetic resonance imaging (MRI) will clearly separate hemorrhage from infarction and small, deep infarcts from larger, cortical or deep infarcts. Differentiating the underlying mechanism of the brain infarct is rarely possible based solely on the infarct topography. Non-invasive duplex Doppler and transcranial Doppler helps identify those with large artery carotid, vertebral or intracranial stenosis or occlusion. However, the relevance of findings such as a 40-60% carotid stenosis is still questionable. Corroborating evidence regarding the status of the cerebral vasculature is obtained from magnetic resonance angiography and conventional cerebral angiography. Electrocardiography and transthoracic echocardiography identify potential cardioembolic sources such as atrial fibrillation, valvular disease, anterior wall myocardial infarction, and intracardiac thrombus. The etiologic significance of other findings such as mitral valve prolapse, mitral annular calcification and patent foramen ovale need to be decided based upon the presenting clinical syndrome. Newer technologies such as transesophageal echocardiography and hematologic testing have helped classify some patients who might have been classified as cryptogenic in the past. The final decision regarding the infarct subtype requires considering the presenting clinical syndrome in conjunction with the pertinent positive and negative findings from brain, vascular and cardiac imaging studies.
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Introduction
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Ischemic stroke continues to have a major impact on the public health of our nation. Ranking among the leading causes of death, stroke is far more disabling than fatal and results in enormous costs measured in both health-care dollars and lost productivity. Once considered untreatable, ischemic stroke has become subject to intensive scrutiny in recent years. Considerable research has led to a better delineation of risk factors, as well as an expanded understanding of pathophysiologic subtypes. Innovative acute therapies are being applied in a manner analogous to the treatment of acute myocardial infarction that emerged more than a decade ago. There are still many unanswered questions regarding the most effective therapies to prevent first or recurrent stroke, however recent clinical trials have helped clarify the best options for stroke prevention in a variety of settings.
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Definitions of TIA and Stroke
Ischemic stroke is characterized by the abrupt or ictal onset of neurologic dysfunction due to inadequate perfusion of the brain. By conventional clinical definitions, if the neurological symptoms continue for more than 24 hours, a person is diagnosed with ischemic stroke. Otherwise, a focal neurological deficit lasting less than 24 hours is defined as a transient ischemic attack (TIA). However, with the advent of more sensitive brain imaging, acute cerebral infarcts have been identified even when symptoms last less than 24 hours. The most recent definition of ischemic stroke for clinical trials has required either symptoms lasting more than 24 hours or imaging an acute, clinically-relevant infarct among those with rapidly vanishing symptoms. When the diagnosis of stroke is made, the duration and severity of the clinical syndrome helps classify it as minor or major ischemic stroke.
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