Acute Kidney Injury in Ultramarathons

This article originally ran in the March 2013 issue of UltraRunning Magazine

By Dr. Marty Hoffman

Running ultramarathons is generally safe and offers up plenty of physical and mental challenges, but the physical demands can possibly have adverse health consequences. One of the potentially serious medical issues we face as ultramarathon runners is exercise-associated hyponatremia, which has been the focus of much of our recent research and educational efforts at the Western States Endurance Run. This condition can indeed have serious consequences, as we are aware of a number of hyponatremiarelated deaths having occurred during marathons, and recently during an endurance canoe race, and hiking. Fortunately, to our knowledge, we’ve not had any deaths in ultramarathons from exercise-associated hyponatremia.

Another potentially serious medical issue for ultramarathon runners is rhabdomyolysis (muscle damage) resulting in acute kidney injury. In simple terms, the large molecules resulting from muscle damage can clog and damage the kidneys.

Photo Marty Hoffman, MD

In 2011, we began some studies to look at this issue, which evolved out of our recognition that we have had several runners requiring hospitalization for rhabodomyolysis and acute kidney injury in recent years at the Western States Endurance Run.

We described some of these in one of our recent scientific publications.

The treatment of exertional rhabdomyolysis and impending renal failure should include early and aggressive hydration. Unfortunately, such treatment is contraindicated and can result in disastrous complications, including death, when exercise-associated hyponatremia is present. In fact, part of the treatment of mild exercise-associated hyponatremia is fluid restriction – just the reverse of that for rhabdomyolysis.

Our recent work has demonstrated that the incidence of exercise-associated hyponatremia may be as high as 30-51 percent in 100-mile ultramarathons, so indiscriminate aggressive postrace hydration is potentially risky.

Furthermore, the reality is that most events do not have the resources for on-site measurement of blood sodium concentration. Therefore, proper management of athletes seeking medical attention after endurance events can present a clinical management dilemma.

If we think the problem is acute renal injury, we should attempt to save the kidneys with aggressive fluid replacement, whereas if we think the issue is exercise-associated hyponatremia, we should restrict fluids in order to save the brain by reducing the risk of cerebral edema (brain swelling). The wrong call and we’ve potentially put the other organ at risk.

To make matters worse, both conditions can present simultaneously.

During the 2011 Western States Endurance Run we examined whether a simple, fast and inexpensive test might be a useful tool to predict which runners might be most at risk for developing acute kidney injury. With such a tool, aggressive hydration could be reserved for only those who appear most in need of this intervention. This work was recently published in the Journal of Sports Science and will be summarized here.

The study examined the urine dipstick test. This test requires a small urine sample and the test sticks. We were primarily interested in the tests for protein, blood and specific gravity as these are most pertinent to kidney function. Participants in Western States underwent postrace blood and urine dipstick analyses. Of the 310 race finishers, post-race urine dipstick testing was completed on 152 (49 percent); post-race blood was obtained from 150 of those runners. Based on established “injury” and “risk” criteria for acute kidney injury, we found that four percent met the criteria for injury and an additional 30 percent met the criteria for risk of injury. Urine dipstick tests that read positive for at least 1+ protein, 3+ blood, and specific gravity of ≥1.025 predicted those meeting the injury criteria with excellent sensitivity and specificity.

In other words, this simple test was able to do a remarkable job at identifying those most at risk for developing kidney issues after the race.

So, what’s this mean? Well, pending further research to confirm the utility of this test, we have promising results suggesting this simple test could be useful when we are faced with the decision of whether aggressive hydration is warranted or not. As to our findings of 34 percent meeting the criteria for kidney injury or risk of injury, we must look at that cautiously.

A challenge we face in studying the physiology of ultramarathon runners is that the usual “normal” doesn’t apply. An obvious example is the post-race blood creatine kinase (CK, also commonly referred to as CPK) concentrations which we’ve found to typically be higher than what many “experts” have recommended should necessitate hospitalization. As we all know, most of us do not need hospitalization after an ultramarathon. For this reason, it becomes increasingly important to be able to distinguish between normal physiological responses, from which we recover without medical intervention, and those responses that are indicative of need for medical intervention.

In this regard, the human body generally has a remarkable ability to tolerate stress, recover, and adapt by becoming even more tolerant. However, there has been concern that repeated insults to certain organs may not result in full recovery, and may make that organ more vulnerable to future injury. The kidney might be one of those organs, but to date, we really don’t know if this is the case or not.

The best I can offer at this time is that we intend to continue to examine this using some new biomarkers to try to determine if the kidneys are vulnerable to these repeat injuries. Regardless of the results, I recognize that it probably won’t change the behavior of many ultrarunners given the degree to which we enjoy our sport.

Dr. Marty Hoffman is the Director of Research for the Western States Endurance Run, Professor of Physical Medicine & Rehabilitation at the University of California, Davis, and has been an ultramarathon runner since 1984.