Embolic Stroke – Common Causes and Prevention
Cardioembolic Stroke – Stroke in Atrial Fibrillation – Embolic Stroke Prevention – Embolic Stroke in Heart Attack and Endocarditis
What is Embolic Stroke?
An embolic stroke occurs when some brain artery is occluded by an embolus. An embolus is some object or mass traveling with a blood flow. Emboli may have gas, liquid or solid composition. Embolic strokes caused by fat (in bone fractures), gas, or liquid (amniotic fluid during labor) are relatively rare. The majority of embolic strokes are caused by blood clot fragments.
A blood clot formed somewhere within circulatory system may get fragmented or detached and enter the circulation. Eventually, this embolus reaches some smaller artery and plugs it. Occlusion of some brain or spinal cord artery by embolus is called embolic stroke.
Heart chambers are the most common sources of emboli. In this case, it is called a cardioembolic stroke. Under normal circumstances, blood can’t coagulate inside the heart. The heart lives in constant motion, which prevents blood clot formation. A thrombus (blood clot) builds up if the blood stops moving and/or if it comes in contact with damaged tissues. It makes perfect sense because blood coagulation is meant to prevent blood loss. A damage to the inner heart lining and or blood stagnation within the heart are responsible for cardioembolic strokes.
Symptoms of cardioembolic stroke are typically sudden with maximum function loss at the onset. Cardioembolic stroke often involves a large portion of the brain leading to severe disability or death.
Sometimes a large embolus disintegrates into smaller pieces and sends a shower of emboli into the brain circulation. The result is multiple smaller strokes throughout the brain.
Cardioembolic strokes tend to recur if nothing is done about stroke prevention.
Stroke in Atrial Fibrillation
Atrial fibrillation is a common type of cardiac arrhythmia. The chance or atrial fibrillation (AF) increases with age. In the United States, about 8% of people 80 years and older have AF. Atrial fibrillation is responsible for about half of cardioembolic strokes. Atrial fibrillation carries a 5% annual risk of stroke.
Presence of other risk factors, such as congestive heart failure, diabetes, hypertension, heart valve disease, or previous strokes, increase the chance of stroke.
In AF, smaller heart chambers, or atria, fail to orderly contract during the heart cycle. Instead of pumping blood into the ventricles (large chambers responsible for most of pumping function) at the right time, they contract in rapid and irregular fashion. Blood flow slows and blood clots are formed along some portion of the atrium wall.
Especially vulnerable portion is called an atrium appendage. It is a pouch attached to the left atrium. Slow blood flow within this pouch promotes clot formation inside of this structure. Pieces of the clot may get into the bloodstream and cause a stroke.
It is believed that return to normal heart rhythm after episodic atrial fibrillation is especially dangerous. Normal atrial contractions detach the thrombus from the wall and send its fragments into the bloodstream.
Diagnosis of Atrial Fibrillation
Chronic atrial fibrillation is easily detectable on examination and on EKG. The heart rate has an irregularly irregular pattern. This implies an irregular heart rate with no particular pattern of irregularity. Some patients have paroxysmal atrial fibrillation, when AF comes and goes. In this case, AF diagnosis requires prolonged monitoring.
Stroke Prevention in Atrial Fibrillation
The mainstay of stroke prevention in atrial fibrillation is a pharmacological delay of blood clot formation. It is called an anticoagulation therapy. Warfarin (Coumadin) reduces the chance of stroke 68%.
Treatment with Warfarin is associated with substantial risk of bleeding from any source. It requires regular monitoring of INR (international normalized ratio). Current standards of care for AF require INR to be between 2.0 and 3.0. Drop of INR below 2 increases the chance of stroke, while rise above 3.0 is associated with bleeding complications.
In case of a large stroke, Warfarin treatment is usually delayed for a few weeks, since it may cause bleeding into the stroke area.
In cases when Warfarin is contraindicated, Aspirin is used instead. Aspirin is substantially less effective. Stroke reduction rate for aspirin in atrial fibrillation is only 22% (vs. 68% for warfarin).
Warfarin related potential bleeding complications lead to development of other drugs for stroke prevention in atrial fibrillation.
Dabigatran Etexilate (Pradaxa), thombin inhibitor, 150 mg taken twice a day
Rivaroxaban (Xarelto) taken 20 mg once a day and Apixaban (Eliquis) taken 5 mg twice a day. They are both Factor Xa inhibitors.
All these medications have advantages of fast onset of action, less bleeding complications, and less monitoring required.
The disadvantages are: necessity of dose adjustments in liver disease and abnormal kidney function, expense, and no established protocols for reversal of the anticoagulation effects.
There are no laboratory tests for monitoring. No information available if acute stroke related thrombolytic therapy is safe in patients taking those drugs.
Anticoagulation effect is short-lasting. Skipped doses will nullify their beneficial effects within hours, therefore increasing the stroke risk.
At this point, these new medications are not recommended for embolic strokes caused by other than atrial fibrillation mechanisms.
Embolic Stroke due to Endocarditis
Endocarditis is an infection of the inner lining of the heart. Rheumatic heart disease also causes autoimmune endocarditis. It is not so common in the developed part of the world any more.
Irrelevantly of the cause, damage to endocardium promotes blood clot build up on the surface of the heart valves.
Endocarditis causes fever, weight loss, and increased heart rate. Infectious endocarditis is common in IV drug users and people who have combination of a heart valves disease with infection elsewhere. Endocarditis carries a high risk of embolic stroke. It is not unusual to see small strokes on brain MRIs in patients with endocarditis even without apparent stroke symptoms.
Even though the strokes are ischemic, they often convert into hemorrhagic (bleeding). Antibiotic treatment does decrease stroke risk in infectious endocarditis.
Anticoagulation with warfarin is not routinely used for infectious endocarditis. Anticoagulation is double-edged sword due to potential hemorrhagic transformation of ischemic strokes.
Stroke in Heart Attack
Heart attack (myocardial infarction) poses a risk of embolic stroke. A heart attack is caused by a blocked artery supplying the heart muscle. Some portion of the heart muscle loses ability to contract. Blood flow slows down locally leading to thrombus build up along the immobile part of the heart wall. These blood clots serve as sources of emboli.
If presence of a blood clot inside the heart is detected on diagnostic testing, at least 3 months of anticoagulation with warfarin is usually required.
Other risk factors for cardioembolic strokes are: metallic heart valves, congestive heart failure, any tumors or growth on the heart valves.
Treatment of Acute Cardioembolic Stroke
Acute ischemic stroke management is the same irrelevantly of the cause – thrombolytic therapy. In emergency setting it is not practical to differentiate the exact mechanism of ischemic stroke.
Preventive measures of future strokes are all cause dependent. See above.
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