Tuesday, May 01, 2007

More on marathoner's hyponatremia

A recent article in the American Journal of Medicine (AJM) provides some new data on the mechanisms(s) at work in the condition of marathoner's hyponatremia. I become more interested in this condition particularly as I fall deeper and deeper in the back of pack running group of aging marathoners. It seems that the really slow folks are more likely to end up in the medical tent with low serum sodium values-and with other problems as well- although it has also been reported in elite runners. ADH was singled out as the one of the likely suspects by Dr. Noakes, a long time guru of long distance running physiology, and I referenced some of his research and thoughts here.

Just as I was reviewing the subject the ever alert DB posted this entry.

Over the years as more folks ran marathons, adequate hydration was first emphasized and then perhaps overemphasized and now overhydration seems to be a major factor in causing runner's hyponatremia. (endurance event participants is a more accurate term as it is not running per se that is necessary to bring this on.) The early zeal for encouraging fluid intake was the notion that dehydration was the principal driving determinative factor in heat stroke in endurance runners. More recently and on the heels of case reports of severe hyponatremia in a few marathoners including at least seven reported fatalities and less severe decrements in serum sodium values in others, less enthusiastic fluid replacement has been emphasized for the runners and apparently fewer cases of runner's hyponatremia are being seen. For example, a recent version of the New York Marathon Handbook recommends 8 oz every twenty minutes . An intake of 400 to 800 ml/hour has been recommended by the International Marathon Medical Directors Association (IMMDA) as opposed to the older mantra of drink "as much as possible".

It is suggested by the authors of the AJM article than this may be a form of SIADH or the syndrome of inappropriate secretion of ADH and while excessive ingestion of water may be necessary it is not sufficient to cause the condition. The back of the pack runners and those who drink too much are most likely to lower their serum sodium values to dangerous levels.The current AJM article reported inappropriate levels of ADH placing the blame on ADH while an earlier report by Noakes did not find elevated ADH levels in endurance athletes who were hospitalized with severe hyponatremia. Perhaps differences in the sensitivity of the various assays used or some aspect of timing or some other procedural matters may be found to explain this apparent discrepancy. Regardless, there seems to be some combination of excessive fluid intake and fluid retention by whatever mechanism(s)-and I think a SIADH type picture is an appealing possible mechanism-resulting in a clinical situation where what you do not need is normal saline.

Since starting normal saline might well seem like the proper thing to do in collapsed runners,the suggestion has been made to have equipment on hand in the emergency treatment facilities set up to service marathons to quickly check the serum sodium level. This advice makes sense and the availability of bedside sodium testing makes it workable. Interestedly, Noakes who has finished more than 70 marathons himself and probably has more hands-on experience than anyone reports than most collapsed runners need only to be placed supine with their feet elevated and things get OK very quickly. This is not intended to diminish the importance of the recognition (by blood sodium measurements) and appropriate treatment of hyponatremia.

Interesting data from the 2003 Boston Marathon is available.Of the 17,548 runners who began the race 17,030 finished and 140 collapsed runners were evaluated and treated .Sodium measurements were done and 30 were hypernatremia (GL 146) while 9 were hyponatremic ( LT 135). The pre race fluid replacement advice was the 400-800 ml /hr so it appears that folks were cutting back on the fluid intake during the race. You cannot know what the serum sodium is in a collapsed runner without measuring it.

A different picture emerged from a study done at the Hawaii Ironman Triathlon, where hyponatremia was seen in 30% of those requiring treatment. The temperature for that race was in the high 80s for much of the race while at the 2003 Boston race the end of race high temp was only 71.The Hawaii race was done at a time before the new hydration advice was published and and is a much longer event than a 26.2 mile run.

Point-of-care sodium measurements really have to part of the medical facilities available in endurance events and the large number of hypernatremic runners noted at Boston might mean that further fine tuning of the hydration advice may still be needed.