Each month, Dr. Jeffrey Sankoff looks at a recent study or body of research to talk to the researchers, explain the process behind it, and break down the findings.
This month: A review looking at which tests could decrease the risk of sudden cardiac death in endurance athlete—and the pros, cons, and questions around genetic testing.
While accurate numbers for sudden cardiac death (SCD) in endurance sports are hard to come by for a variety of reasons, the impact these cases have and the publicity they garner ensure that they resonate in the minds of the public and especially in those of their fellow athletes. No one wants a sudden cardiac death at their event—however rare!
The total rate of deaths in endurance athletes is estimated at 0.75 per 100,000 athletes per year but this can vary across different sports. In one study of triathletes over a ten-year period the rate was reported as 1.74 per 100,000, with most of those occurring during the swim.
In most studies that have looked at SCD, about one-third of cases are found to have exhibited some signs and symptoms of ischemic cardiac disease prior to the event, while for most young athletes SCD is often the first and only manifestation of underlying cardiac pathology. In triathlon there has been a growing interest in the idea of swimming induced pulmonary edema as a contributor to SCD, but even this is felt to more often than not be related to underlying cardiac causes and rarely develop in isolation.
RELATED: Solving the Mystery of Swim Deaths
Given how dramatic and impactful these rare cases are, there has long been an interest in trying to identify those who might be at risk for SCD in advance of it occurring in a race. Time and again though, when researchers have attempted to develop strategies that utilize typical tools for evaluating the heart, such as electrocardiogram (ECG) or echocardiography, they have failed to accurately and reliably identify those endurance athletes at risk.
There are a variety of reasons for this.
First it helps to understand the causes of SCD. While autopsy results are not always available, even when they are they don’t always give a definitive answer, especially in young people, and this leaves us with a great degree of uncertainty as to what actually happened.
The vast majority of SCD, especially in older athletes, is attributable to ischemic heart disease. The next most common cause is cardiac dysrhythmia, though this is speculative in many cases as dysrhythmias are undetectable on autopsy.
Other common causes of SCD include congenital heart problems, such as hypertrophic obstructive cardiomyopathy (HOCM), pulmonary embolisms (in which a clot travels to the lungs and can interfere with both pulmonary and cardiac function), as well as the previously mentioned swimming induced pulmonary edema.
Unfortunately, resting ECG and even echocardiograms are not very good for detecting most of these conditions nor for identifying who might be at risk. In study after study, when the widespread application of these screening tests have been done, the yield has been incredibly low and the conclusions have repeatedly been that instead of screening everyone screening should be reserved for select populations who are more likely to be at risk.
You may be wondering: Why not just screen everyone anyways? Even if the yield is very low isn’t a low yield better than no yield? There are two problems with this approach. The first relates to cost. None of these tests are free and someone has to pay for them. In addition, testing hundreds of thousands of athletes would put a burden on the system and potentially displace patients who actually need these tests for diagnostic and treatment purposes.
The second issue relates to the results of the screening tests. When you begin with young healthy athletes, then the likelihood that they have a true hidden problem that can be identified by these screening tests is exceedingly low. Consequently, if an abnormal result is found it is far more often that this represents a false positive or is simply a normal variant. Is it fair to have these healthy athletes have to live with the stress of knowing they have some kind of ‘abnormal screening result’ that might not actually mean anything negative or, worse, have them undergo an additional battery of tests just to find out that they were fine in the first place?
RELATED: Understanding the Athlete’s Heart
All of which brings us to a new frontier in screening tests, the incorporation of genetic testing for inherited cardiac conditions.
There are many people who succumb to SCD during endurance exercise who are later found to have carried genes for specific cardiac conditions that likely played a role in their death. Long QT syndrome, hypertrophic cardiomyopathy, Brugada Syndrome, and arrhythmogenic cardiomyopathy are just a few of the conditions that we know of that are passed down from parents to children and if expressed as cardiac anomalies can place the individual at high risk of SCD during intense exercise. Over the past decades, the individual genes responsible for these problems have been identified and tests have been developed in order to detect their presence.
Now, researchers have begun to question if widespread genetic testing for these inherited cardiac conditions (ICC) might also have a role in preventing sudden cardiac death in young athletes.
A recent review article in the European Journal of Preventive Cardiology summarized what is known on these diseases, the tests that can help identify them, and how doctors, coaches, and athletes can think about assessing risk. Some of the issues raised by the authors are similar to those seen with more conventional screening tests like ECG and echocardiography. It turns out that in many cases the tests themselves are not that accurate. For example, for Brugada Syndrome, a condition in which fatal dysrhythmias can occur during exercise, genetic testing can only yield positive results in some 10% of cases. For other inherited cardiac conditions, the yield is 50%, so a negative test cannot be taken as truly negative in many cases.
Another important problem related to genetic testing has to do with the difference between genotype and phenotype. Genotype relates to the actual makeup of your genetic code. Your genotype is determined by what genes you inherited from your parents. Phenotype relates to which of those genes are actually expressed. In other words, you may carry a gene for a specific condition, but if you do not express that gene then the implications of having the gene are effectively moot.
Not all people who test positive for some of these conditions will actually express the conditions that those genes code for. However, it isn’t always so easy to know if the condition is present or not, only that the gene is there. How then should such a test be interpreted? Should the athlete be counseled to not participate in sport?
These are just some of the complicated issues that the authors raise in evaluating the utility of such genetic screening for athletes. In the end their conclusion was tempered. Similar to the guidelines that have been developed for more conventional cardiac screening, these authors believe that genetic screening for inherited cardiac conditions should be targeted to patients who clearly have a highly suggestive family history or phenotype for one of these.
Before genetic testing is undertaken, they argue that there should be extensive counseling of the athlete, their family, and their support team so that everyone understands exactly what the significance of any result is and so decision-making can be thought out in advance.
They also advise that such genetic testing be conducted by specialized teams made up of multidisciplinary professionals, all highly skilled in this area, and that athletes should not seek out commercial genetic screening on their own because of the incredibly variable quality of testing done in those environments and the lack of multidisciplinary team support.
As you can see this remains an area of significant uncertainty and that, even as technology and our understanding of the causes of SCD improves, clearer answers as to how to prevent it remain very elusive.
What remains of the utmost importance for all athletes, but especially those over 35, is that it is critically important to pay attention to your body and be mindful of any signs or symptoms that might indicate some underlying cardiac disease. New or unexpected exercise intolerance, shortness of breath, palpitations, and certainly any chest pain should always be taken seriously and investigated immediately. That continues to be the single best way to prevent SCD and allow for a lengthy career in endurance sport—regardless of any screening programs.