Introduction to Stroke

Stroke is a sudden impairment of cerebral circulation in one or more blood vessels supplying the brain. Stroke interrupts or diminishes oxygen supply and commonly causes serious damage or necrosis in brain tissues.
The sooner circulation returns to normal after a stroke, the better the chances are for complete recovery. However, about half of those who survive a stroke remain permanently disabled and experience a recurrence within weeks, months, or years.

Incidence
Stroke is the third most common cause of death in the United States today and the most common cause of neurologic disability. It affects 500,000 people each year; half of them die as a result.

Causes
Factors that increase the risk of stroke include history of transient ischemic attacks (TIAs), atherosclerosis, hypertension, electrocardiogram changes, arrhythmias, rheumatic heart disease, diabetes mellitus, gout, postural hypotension, cardiac or myocardial enlargement, high serum triglyceride levels, lack of exercise, use of hormonal contraceptives, cigarette smoking, and family history of stroke.

The major causes of stroke are thrombosis, embolism, and hemorrhage.

Thrombosis
In middle-aged and elderly people—among whom there’s a higher incidence of atherosclerosis, diabetes, and hypertension—thrombosis is the most common cause of stroke. Obstruction of a blood vessel causes the stroke. Typically, the main site of the obstruction is the extracerebralvessels, but sometimes it’s the intracerebral vessels.
Thrombosis causes ischemia in brain tissue supplied by the affected vessel as well as congestion and edema. The latter may produce more symptoms than the thrombosis itself, but these subside with the edema.
Thrombosis may develop while the patient sleeps or shortly after he awakens; it can also occur during surgery or after a myocardial infarction. The risk increases with obesity, smoking, or the use of hormonal contraceptives. Cocaine-induced ischemic stroke is now seen in younger patients.

Embolism
The second most common cause of stroke, embolism is an occlusion of a blood vessel caused by a fragmented clot, a tumor, fat, bacteria, or air. It can occur at any age, especially among patients with a history of rheumatic heart disease, endocarditis, posttraumatic valvular disease, or myocardial fibrillation and other cardiac arrhythmias or after open-heart surgery or placement of a mechanical heart valve.
The embolus usually develops rapidly—in 10 to 20 seconds—and without warning. When it reaches the cerebral vasculature, it cuts off circulation by lodging in a narrow portion of an artery, most commonly the middle cerebral artery, causing necrosis and edema.
If the embolus is septic and infection extends beyond the vessel wall, an abscess or encephalitis may develop. If the infection is within the vessel wall, an aneurysm may form, which could lead to cerebral hemorrhage.

Hemorrhage
The third most common cause of stroke is hemorrhage. Like an embolism, it may occur suddenly, at any age. Such hemorrhage results from chronic hypertension or aneurysms, which cause sudden rupture of a cerebral artery. The rupture diminishes blood supply to the area served by this artery. In addition, blood accumulates deep within the brain, further compressing neural tissue and causing even greater damage.

Stroke classification
Strokes are classified according to their course of progression. 

1. The least severe is the TIA, or little stroke, which results from a temporary interruption of blood flow, usually in the carotid and vertebrobasilar arteries. 
2. A progressive stroke, or stroke-in-evolution (thrombus-in-evolution), begins with slight neurologic deficit and worsens in a day or two. 
3. In a completed stroke, neurologic deficits are maximal at onset and don’t progress.

Signs and symptoms
Signs and symptoms of stroke vary, depending on the artery affected (and, consequently, the portion of the brain it supplies), the severity of damage, and the extent of collateral circulation that develops to help the brain compensate for decreased blood supply.
If the stroke occurs in the left hemisphere, it produces symptoms on the right side; if it occurs in the right hemisphere, it produces symptoms on the left side. However, a stroke that causes cranial nerve damage produces signs of cranial nerve dysfunction on the same side as the hemorrhage.

Symptoms are usually classified according to the artery affected:

• middle cerebral artery: aphasia, dysphasia, visual field cuts, and hemiparesis on the affected side (more severe in the face and arm than in the leg)
• carotid artery: weakness, paralysis, numbness, sensory changes, and visual disturbances on the affected side; altered level of consciousness; bruits; headaches; aphasia; and ptosis
• vertebrobasilar artery: weakness on the affected side, numbness around the lips and mouth, visual field cuts, diplopia, poor coordination, dysphagia, slurred speech, dizziness, amnesia, and ataxia
• anterior cerebral artery: confusion, weakness and numbness (especially in the leg) on the affected side, incontinence, loss of coordination, impaired motor and sensory functions, and personality changes 
• posterior cerebral arteries: visual field cuts, sensory impairment, dyslexia, coma, and cortical blindness. Usually, there’s no paralysis.

Symptoms can also be classified as premonitory, generalized, and focal. 

• Premonitory symptoms (such as drowsiness, dizziness, headache, and mental confusion) are rare.
• Generalized signs and symptoms (such as headache, vomiting, mental impairment, seizures, coma, nuchal rigidity, fever, and disorientation) are typical. 
• Focal symptoms (such as sensory and reflex changes) reflect the site of hemorrhage or infarction and may worsen.

Diagnosis
Confirmation of stroke is based on symptoms, a history of risk factors, and the results of diagnostic tests.

• Computed tomography scan shows evidence of hemorrhagic stroke immediately but may not show evidence of thrombotic infarction for 48 to 72 hours.
• Magnetic resonance imaging may help identify ischemic or infarcted areas and cerebral swelling.
• Ophthalmoscopy may show signs of hypertension and atherosclerotic changes in retinal arteries.
• Angiography outlines blood vessels and pinpoints atherosclerotic plaques, vessel occlusion, or the rupture site.
• EEG helps to localize the damaged area.
• Other baseline laboratory studies include urinalysis, coagulation studies, complete blood cell count, serum osmolality, and electrolyte, glucose, triglyceride, creatinine, and blood urea nitrogen levels.