Clinical guidelines and conventional medical wisdom all recommend oral anticoagulation (OAC) for patients with atrial fibrillation who are determined to have an increased risk of stroke according to widely used risk indicator tools. Usually the CHA2DS2-VASc risk assessment tool is used and for patients with a risk score of greater than or equal 2 ,OAC is generally recommended.European and Canadian and U.S. guidelines differ in regard to recommendations for risk score of 1. For risk categories 0 and 1 there are no randomized clinical trials demonstrating the net clinical benefit of OAC . Those recommendations are driven simply observational trials and expert option seems to vary geographically.
According to a 2016 article by Overvad et al :. "current guidelines
discrepancies also reflect the fact that the level of stroke risk among
men with a score of 1 and women with a score of 2 is on the borderline
of where the impact of anticoagulation treatment shifts from beneficial
to harmful."( ref 2)
In simple terms-it is a close call.
The clinical issue is the trade off between OAC reduction in ischemic stroke risk and the risk of serious bleeding complications the worse of which is intracranial hemorrhage. Importantly and perhaps ironically the higher the CHA2DS2 score , the higher the risk of major hemorrhage. Analysis (ref 1) of over 44 thousand in the Military Health System patients with non-valvular atrial fibrillation over a 2.5 year period treated with rivaroxaban demonstrated a strong relationship between the risk score and the risk of major bleeding.So the patients at the highest risk of stroke are the same people who have the highest risk of major hemorrhage.,especially those with the high vascular disease components of the risk score score.The data indicated that while the risk of hemorrhage increases with the risk score the stroke risk increases more so it is generally believed that there is net clinical benefit even at the highest hemorrhage risk levels.
Hess et al published a study focused on data from a outcomes registry for patients with atrial fibrillation in part to determine if patients with afib were being treated according to accepted guidelines.(American Journal of Medicine 2016 , Nov 22).See here for link to abstract.
There were 9555 patients with afib and 1846 of those were not being treated with OAC even though they had a CHA2DS2-Vasc score greater than 2.
Interestingly the stroke risk in those not treated compared with those who were treated was not statistically significantly elevated.. The adjusted HR was 1.18 with a 95% Confidence interval of 0.91--1.54.
It is often stated that risk of stroke in afib patients is increased by a factor of 3- 5 (based on Framingham data) and that OAC may decrease that risk by 60% or more.Yet in this large data base OAC did not seem to bring about that degree of benefit. However, absence of OAC was associated with a statistically significant increase in risk of death. ( HR 1..22 (1.05--1.41).
So how do patients do with atrial fibrillation not treated with OAC ? Based on this observational data- better than you might expect. On the other hand if we look at older data from a randomized clinical trial published in 1991 ( SPAF trial) the stroke risk per year for the placebo arm was 6.3% versus the warfarin arm which was 2.3% and surprisingly the aspirin arm was 3.6%.Later studies never confirmed the value of aspirin in stroke prevention in afib but the SPAF numbers conform with the broad brush comments that the risk of stroke is about three to five times and the risk reduction from OAC may be in the 40-60% range. The BAFTA study not only failed to show the value of aspirin in stroke reduction in afib patients but demonstrated that in older patient s ( over 75 years of age) aspirin caused a similar risk of intracranial bleeding as warfarin.
So the question remains why did the patients without treatment do so well in Hess's data or maybe the real life treatment of afib with OAC is much less impressive than it is in randomized clinical trials or maybe there are so many potential biases in observational big data exercises that solid "take home" messages are difficult to find and/or rely upon..
And things get even murkier or perhaps more clear when Dr. H. Kamel discusses challenges to the notion that the risk of stroke in afib is in fact solely due to thrombi in the left atrium and proposes that things are much more complicated including the notion that at least sometimes a stroke can cause an atrial thrombus. It is well known that stroke can precede the onset of afib.He and coauthors discuss a new model for stroke and afib in which both afib and emboli are downstream effect of abnormal atria substrate (an atrial myopathy).This model might explain why stroke risk is not eliminated by rhythm control strategies and why stroke can predate afib and generally the poor temporal relationship that exist between afib onset and stroke and why some reports do not show a dose response relationship between afib duration and stroke(.However, other reports such as the TRENDS data do show dose response relationship .)
Drs Akar and Marieb make similar comments and say in part "...AF may simply be a marker of underlying conditions that causes stroke as opposed to an active participant in the stroke pathogenesis"
In short, they are saying there is reason to believe there is much more to it than simply that afib cause thrombi in the left atrium and the clot embolizes to the brain. although no one is saying that does not happen (" Atrial Fibrillation and Thrombogenesis:Innocent bystander or guilty accomplice", JACEEP 2015, 2015;1(13) p 218.
Another article (ref 3) tends to support the notion that there is more to afib and stroke than the century old model that the former simply leads to the later.Boriani published a study evaluating the risk of stroke in men and women with pacemakers and found that atrial fibrillation of at least five minutes occurred in 44% of 2398 patients and that a higher atrial fibrillation burden was not associated with a higher stroke risk. Confirming other studies this analysis found the stroke and TIA risk in women was twice that on men.
1.Peacock,WF et al CHA2DS2-VASc Scores and Major Bleeding in Patients with Nonvalvular Atrial Fibrillation Who are Receiving Rivaroxaban.Annals of Emergency Medicine, published online, accessed 12/4/16
2. Overvad TF et al. "Treatment thresholds for stroke prevention in atrial fibrillation :observations on the CHA2DS2-VASC Score" Euro Heart Journal-Cardiovascular pharmacology Published on line August 2016
3. Boriani, G et al " The increased risk of stroke/transient ischemic attack in women with a cardiac implantable electronic device is not associated with a higher atrial fibrillation burden. Europace: 2016, dec 28.
"Truth is much too complicated to allow anything but approximations" John Von Neumann
Addendum: 12/22/2016. Unfortunately I became aware of the 2016 Circulation article by GR Quinn et al after the above commentary was published. That very important article provides good reason to question the dogma that the CHA2DS2-VASc score translate to fixed stoke rate. It is generally accepted that if a person's stroke rate is estimated to be 1-2% per year then treatment with an OAC offers a a net clinical benefit and that the risk score clearly relates to a quantitative stroke risk, e.g. a CHA2DS2-VASC score of 1 means the person has a risk of about 1 % per year and a score of 2 indicates a risk of 2%.
However, analysis of 34 studies of patients not treated with anticoagulants demonstrated that the stroke rate varies widely in various cohorts. For example, with a risk score of 2, 27% of the cohorts reported a stroke risk of less than 1% and 33% reported stroke risk greater than 2% per year. So the correlation between risk score and stroke risk varies with the cohort studied.The numbers from the Northern European studies formed the basis of the alleged relationship between the CHA2DS2-VASc score and annual stroke risk and the North American Cohort analyses indicate significantly lower ( about 1/3 of the European rate) stroke rates for untreated AF.
Quoting from the authors conclusions: " The majority of cohorts did not observe stroke rate that would indicate a clear expected net clinical benefit for anticoagulating AF patients with a CHA2DS2-VASc score of 1 or 2."