A research team from Johns Hopkins University has discovered evidence that abnormal function of the left atrium may lead to an increase in stroke risk. As the study showed, the risk for stroke was not dependent on the presence of atrial fibrillation. The findings go against the universally accepted notion that irregular rhythm in the upper chambers of the heart during atrial fibrillation (AF) causes blood clots to form, eventually finding their way to the brain.

The results were published in the Journal of the American Heart Association. The study was motivated by the knowledge that the current reasoning for stroke doesn’t fit in with what clinicians see on a daily basis – not all individuals with atrial fibrillation have strokes and not all stroke victims have atrial fibrillation. So this means that people with abnormal function of the left atrium are at risk for stroke, not only those with AF. 

Lead author and cardiologist Dr. Hiroshi Ashikaga, PhD, and his team from Johns Hopkins used a 1.5 Tesla MRI scanner (Magnetom Avanta, Siemens Healthcare) with motion­tracking software called multimodality tissue­tracking (MTT) made by Toshiba to study the movement of the myocardium (cardiac muscle).

"The good thing about this imaging technique is that it is noninvasive," Ashikaga, an assistant professor of medicine and biomedical engineering at Johns Hopkins, noted. "Traditionally, for people who have atrial fibrillation, we estimate the risk of a stroke and decide whether or not to start [them on] a blood thinner to prevent a stroke. Sometimes, there has to be a diagnosis of atrial fibrillation to start the [treatment] process, but this kind of approach does not help people who present with a stroke as the first manifestation of atrial fibrillation".

"So if we can look at the left atrial function in anyone, it is very easy because we don't have to wait for atrial fibrillation to occur," Ashikaga said. "If we can screen people who are at higher risk of a stroke, we can discuss therapy based on the result".

The study included 169 patients with prior history of atrial fibrillation. Eighteen of the patients had a stroke or transient ischemic attack (TIA) in the past and 151 had no history of stroke or TIA, the control group.  The results showed that the left atria of patients in the first group emptied a significantly less amount of blood into the left ventricle.

When reviewing the MR images, the researchers also noticed that the volume of left atria in patients with prior history of stroke or TIA was larger on average, 52 mL/m2, compared to the control group, which had 44 mL/m2 on average. Another interesting find was that the heart muscle was found to be less elastic and had overall less ability to handle strain.

Motion-­tracking MRI of the left atrium

Motion-­tracking MRI shows the heart wall of the left atrium highlighted in color (representing 0% to 40%) based on how much it has stretched from baseline. Image courtesy of Dr. Hiroshi Ashikaga, PhD. "Our findings are particularly important because, to our knowledge, this is the first report to demonstrate the significant contribution of the left atrium reservoir function to stroke in atrial fibrillation patients during sinus rhythm [or the normal beating of the heart]", the authors wrote.

Who benefits?

The potential benefits of using such a technique are still unknown and there are a lot of unanswered questions, which is why the researchers plan on performing several studies prospectively. They want to study people with atrial fibrillation, those at high risk for atrial fibrillation and those who have no signs of atrial fibrillation.