Increasingly recognized among Marthonan and Ultra-Distance runners.
A recent study of a Houston Marathon showed an incidence of symptomatic hyponatremia of 0.31% of runners78
Hyponatremia was found in 18% of race finishers in the New Zealand Ironman Triathlon.
The most likely explaination for athletic hyponatremia is overhydration. This theory is well documented. Athletes who lose the least weight, or even gain weight, are the most likely to experience hyponatremia. This is supported also by apparent increases in plasma volume in athletes who experience hyponatremia.
A second theory postulates that hyponatremia is caused by a sodium deficit, due to sodium loss in sweat. There is little support for this theory. Theoretical calculations reveal that a sodium deficit alone cannot cause hyponatremia. In addition, calculations reveal that hyponatremia can occur even when sodium ingestion exceeds sodium loss.101
It is unclear why the kidneys fail to excrete excess water ingested during prolonged exercise. A recent study found no evidence of Arginine Vasopressin or Aldosterone changes as a cause.101
Note that it is difficult to measure the true magnitude of dehydration. Simply measuring weight before and after exertion is not accurate. Weight loss from exercise include not only fluid loss but also irreversible loss of glycogen, used for fuel. One author suggests that an athlete who loses 2kg of weight during a marathon may be dehydrated by only 200g of water, the rest of the weight loss due to glycogen loss.130
It is thought that replacement of sodium only (salt tablets) is not beneficial unless all of the fluid losses are also replaced. Full replacement of sodium with partial fluid replacement may be no more effective than fluid replacement alone in reducing the risk of hyponatremia.
Excessive consumption of hypotonic fluids. In general, hyponatremic runners may consume almost twice as much fluid as non-hyponatremic runners.
Slow finishing times are common. Many hyponatremic runners are from beginning running groups. Times over 4hrs 20 min.
High ambient temperatures are also a risk factor. This may be more likely to lead to overhydration as runners interpret fatigue as dehydration.
Females may be more likely than males to be symptomatic.
It is possible that Non-Steroidal Anti-Inflammatory medications play a role. NSAIDS are a known cause of hyponatremia among neonates and among the elderly. NSAIDS act to promate Natriurisis. However, evidence of NSAID use as a cause on athletic hyponatremia is lacking.
Traditionally athletes were encourage to drink copius amounts of fluids as dehydration was reported to be a significant factor for decreased performance. However, studies often show that the fastest runners are also the MOST dehydrated; suggesting that moderate dehydration is not ergolytic.120
Excess fluid intake
Symptoms and signs of hyponatremia include lightheadednesws, nause, vomiting, malaise, exhaustions, altered mental status, headache, seizures, and death.
Pathognomonic historical features: Altered mental status or seizures in the absence of high rectal temperature or hypoglycemia.
Vomiting is common. The mechanism is uncertain.
True heat stroke is rare, but may occur. Diagnosis of heat stroke requires a rectal temperature of more than 40c-41c, an altered level of consciousness, AND failure to improve without active cooling. There are very few documented cases of true heat-stroke among marathon runners. The true incidence of heatstroke among marathon runners is unknow but appears to be extremely low. Note that there is no evidence that increasing fluid intake will decrease the risk of heatstroke.
Collapse occuring after the finish of the race, is usually due to postural hypotension. During the run, muscular beds are dilated, leading to increased blood flow to muscles. This also leads to decreased total peripheral resistance. Despite decreased resistance, blood pressure is maintained by increased cardiac output. In addition, the rhythmic motion of running, and intermittent muscle contraction aid in return of blood to the heart. When the athlete stops running, the cardiac output may decrease quickly before the vascular tone returns to the muscular bed. This, combined with the absence of cyclical muscular contraction may lead to decreased blood pressure, decreased blood flow to the brain, and collapse. Note that treatment is simple: the athlete can lie down with the head lower than the legs/body and will usually recover quickly. Intravenous fluids are rarely needed.
Fluid intake should be restricted to that amount which repletes the athletes sweat and urine production. In the New Zealand ironman triathlon, the incidence of hyponatremia fell from 22% to 3% in one year after athletes were encouraged to drink conservatively and after the number of water stations was reduced.120
Although the American College of Sport's Medicine recommends an intake of 0.6 to 1.2 litres per hour of fluids, even this may be to high in some circumstances, such as cold weather. A study of ultramarathon activity in cold weather revealed that hyponatremia occurred in athletes drinking as little as 0.3 litres per hour.101
The International Marathon Medical Directros Association (IMMDA) recommends that athletes drink when thirsty, to a maximum of 400 to 800 ml/hr.130
The American College of Sport's Medicine recommends an intake of 0.5 to 0.7 grams of sodium per litre of fluid in events greater than 1 hour duration. Note that most sport drinks contain only 0.4g/l.
For athletes who only partially replace there fluid losses, additional sodium supplementation is not needed to prevent hyponatremia.95