Atrial fibrillation is one of the causes of stroke. When the atriums fibrillate they don’t contract and pump effectively. This means that the flow of blood through the atriums slows down. When blood slows down or becomes static it has a tendency to clot. If a blood clot forms inside the atriums it may subsequently dislodge, break away and be pumped out of the heart, a process called embolisation. As a large proportion of blood flow goes to the brain, this is the most common area for the blood clot to end up. A blood clot that is pumped into the brain will block the blood flow to the part that the blood vessel serves, causing injury. In its mildest form this may not be noticeable. There may be neurological symptoms, such as a loss of vision or speech, muscle weakness etc which recover completely in 24 hours. This is called a transient ischaemic attack (TIA). In the most severe form, permanent damage or death can occur. This particular type of stroke is called thromboembolic, meaning it is due to a blood clot that travels from the heart to block a blood vessel in the brain. Strokes that result from atrial fibrillation tend to be bigger and more dangerous than strokes from other causes.
The left atrial appendage, which looks like a pouch or sock that sticks out from the left atrium, is the area where clots form and embolise from in >90% of atrial fibrillation patients who suffer a thromboembolic stroke. As it is on the left side of the heart, any clot that is pumped out will go to the brain or other parts of the body, rather than the lungs.
The risk of a stroke is believed to be the same whether someone has paroxysmal, persistent or permanent atrial fibrillation. Anyone whose episodes last longer than 5 minutes has an increased risk. It is less certain what risk very short attacks, lasting less than 5 minutes, provide. It is safest to assume that documented short episodes may be a sign that at other times there are longer episodes, or that longer episodes may develop in the near future.
The stroke risk is the same whether someone has symptoms from atrial fibrillation or is asymptomatic and it is discovered by chance. Unfortunately, the development of a stroke or TIA is often the event that leads to the discovery of asymptomatic atrial fibrillation. This fact is important when considering the role of drugs, cardioversions and ablation to treat atrial fibrillation symptoms. If the treatment is successful and symptoms are abolished it doesn’t mean that the stroke risk has gone – it may just be that the atrial fibrillation is silent and the symptoms are masked. Even if monitoring shows a normal rhythm it is very difficult to guarantee that atrial fibrillation won’t return.
Atrial flutter carries the same risk of stroke as atrial fibrillation.
Even young, healthy adults with no heart rhythm problems have a risk of stroke; it is just that the risk is extremely low. Although atrial fibrillation can increase an individual’s risk of stroke by up to 5 times compared to an identical individual who doesn’t have the arrhythmia, the overall yearly risk of a stroke is heavily influenced by many other factors. If a fit and healthy man under the age of 60 develops atrial fibrillation, multiplying his baseline risk by a factor of 5 still produces a very low risk of stroke (estimated to be 1.3% or 1 in 77 over a 15 year period, equating to less than 1 in 100 per year).
Certain other medical and physical conditions increase the baseline risk of stroke. The more of these that are present the greater the impact of adding atrial fibrillation. It is possible to estimate the yearly risk of stroke using a scoring system based on the most common and influential risk factors, which are Congestive Heart Failure, Hypertension (high blood pressure, even if it is now well treated), Age more than 75 years, Diabetes and having a previous Stroke or TIA. The first letters from each of these risk factors provides the acronym CHADS. Having a previous stroke or TIA is a particularly strong risk factor, so it counts double. The CHADS2 scoring system assigns 1 point for each of heart failure, hypertension, age more than 75 and diabetes and 2 points for previous stroke or TIA. The table to the right shows the average yearly risk of stroke for each score if no stroke prevention treatment is given. It is important to realise that this is only an estimate.
Recently a more sophisticated scoring system, CHA2DS2-Vasc, has been developed. It takes into account the fact that having one age category of 75 years and older is unrealistic as peoples risk gradually increases as they get older than 65. This scoring system assigns 1 point if 65 to 74 years of age and 2 points if over 75. It has also been discovered that women are slightly more at risk than men, although the reasons why are complex and still not fully understood. Female gender therefore scores a point. It also takes into account the impact of vascular disease, such as angina, heart attacks and peripheral vascular disease, which also scores a point. The yearly risks of stroke for each of the CHA2DS2-Vasc scores are shown in the table to the left. Note that although it may be more sophisticated at the lower scores, the small number of people who have very high risk scores means that the yearly risk numbers are less reliable.
For people with a CHADS2 or CHA2DS2-Vasc score of 0 it is reasonable to take no preventative medication. Those with a CHADS2 of 1 or more or a CHA2DS2-Vasc score of 2 or more are recommended to take an oral anticoagulant as the benefit in stroke reduction is greater than the risk of serious bleeding with the medication and so there is a net benefit, even if that benefit is only in the order of 1 or 2% per year. Those in the grey zone with a CHA2DS2-Vasc score of 1 could take aspirin or an oral anticoagulant, but an oral anticoagulant is preferred.
Aspirin is an antiplatelet drug, not an anticoagulant. It is rapidly losing its role in thromboembolic stroke prevention for patients with atrial fibrillation. It may still be of use in people who have TIAs or strokes from other causes, such as vascular disease, hypertension and diabetes and it still offers some protection against ischaemic heart disease, however it probably does not prevent left atrial clots from forming and embolising. Aspirin also carries a very small risk of causing serious bleeding problems, particularly in the elderly. For that reason, aspirin is very rarely an option for stroke prevention in atrial fibrillation. Nowadays the choice should be between an oral anticoagulant (such as warfarin or dabigatran etc) or nothing. The option of percutaneous left atrial appendage occlusion is discussed separately. Oral anticoagulants reduce the risk of stroke down to 1/3 of what it would be with no treatment. The higher the baseline risk (i.e. the higher the CHADS2 score) the greater the benefit from taking the oral anticoagulant.
There are challenges with oral anticoagulation. Warfarin is the most commonly used drug. It requires regular monitoring using blood tests to make sure the blood is thinned enough to protect against stroke but not so much that the risk of serious bleeding shoots up. People naturally get concerned about coming to harm if they cut themselves, fall or get injured. Warfarin can interact with other drugs, affecting its potency. Newer drugs such as dabigatran overcome some of these issues, but there is less experience with them. To find out more about the different anticoagulant drugs, click here
A very new approach to preventing thromboembolic stroke in atrial fibrillation patients is percutaneous left atrial appendage occlusion. Rather than thin all the blood in the body to stop clots from forming in the left atrial appendage, this procedure blocks off and seals the left atrial appendage, taking it out of the equation and avoiding the need for long-term anticoagulation. It may be suitable for people at high risk of stroke who should be on warfarin but cannot be prescribed it as they also have a high risk of bleeding or have had serious haemorrhages in the past. For others it may become a lifestyle choice – they would rather have a single operation than take medications every day for the rest of their life. In a randomised trial, left atrial appendage occlusion with the Watchman device was shown to be at least as effective as warfarin and preventing strokes in people who could have either treatment. To find out more about left atrial appendage occlusion, click here.
Sometimes a combination of drugs is used as each offers different benefits. After a heart attack or coronary stent procedure patients with atrial fibrillation often need to combine warfarin with aspirin and clopidogrel for a short while and then stop one of the antiplatelet drugs. If it has been more than a year since any heart problem however, aspirin can often be stopped and people stay on warfarin alone. This area is complex and decisions should be made by the cardiologists involved.