Frequently Asked Questions about EAH and Hydration

What is EAH?

Hyponatremia is defined by the sodium (Na+) concentration in the blood plasma falling below a normal range (135-145 mmol).  EAH (exercise associated hyponatremia) is a form of hyponatremia that develops either during or shortly (within 12 hours) after expended exercise. The primary culprit behind EAH is fluid intake exceeding urinary, sweat and respiratory losses. This may be exacerbated by the body's reduction in kidney (urine) output, something it tends to do during extended physical exertion.

1. Fluid intake in excess of output.
2. Fluid collects in the gut and is then absorbed from the gut into the blood stream.
3. Blood plasma is diluted by the incoming water and the sodium concentration decreases.
4. A dilution gradient (difference) is created between the blood (more dilute) and other areas of the body (less dilute).
5. The dilutional gradient causes osmotic pressure. The greater the gradient, the greater the pressure.
6. Osmotic pressure srives to eliminate the dilutional gradient by moving water from the blood into other areas of the body/
7. Water is absorbed by the cells in these other areas and the cells swell.
8. The swelling of brain cells largely enclosed by a non-expandable skull causes an increasing breakdown of brain function (see signs and symptoms)

Is there a gender difference and why?

Possibly.  There have been suggestions within the medical and scientific community that females may be behaviorally more inclined to EAH or more susceptible or may be more prone to a severe symptomatic form.  Some study results have shown a gender difference in incidence levels and severity (with females more at risk) while others have not.  To date no one has specifically designed a study to look at gender differences or underlying mechanisms.

In the absence of a definitive answer, there are several factors that could plausibly suggest mechanisms for female predisposition. 

• Females could be more likely to overdrink either because they more conscientiously adhere to misinformed drinking advice 
• or because they take in the same amount of fluid that males do, but suffer disproportionately because of relatively lower body weight. 
• The hormonal changes through the menstrual cycle could influence fluid and salt regulation and create periods of increased susceptibility to EAH.
• There may be other inherent gender differences in fluid and salt regulation.

Certainly the incidence of EAH along with documented deaths amongst males indicate that they are at risk as well, so for now the same advice should be adhered to by both males and females.

  Who is most at risk for EAH (risk factors)?

A number of risk factors have surfaced in the published literature on EAH.  Keep in mind that risk factors do not cause EAH, but the presence of any of them increases the likelihood that EAH will occur.

• Race length.  EAH is not quite exclusively a phenomena of endurance events, but almost.  Events in which the mean finishing time is under 2 1/2 hours (about a half marathon) rarely see a case of EAH.  When mean finishing times reach 4 1/2 - 5 hours (as in most large marathons these days) or longer, EAH becomes a common occurrence.
• Athlete inexperience.  This is likely a risk factor for several reasons.  First because inexperienced participants have a more difficult time gauging how much fluid they should consume.  Second because their bodies may be less adapted physiologically to the stresses of the event. And this group of athletes is more likely to spend a longer than average time on the race course (see the following risk factor).
• Longer than average (mean) finishing times.  EAH is absent among top finishers of athletic events, but becomes increasingly common among the slower cohort of participants.  As previously noted, slower participants are more likely inexperienced.  Additionally a slower pace becomes more conducive for pausing to drink at fluid stations
• Female gender.  (see FAQ on gender)
• Lower body weight.  This likely plays a role in that some people fail to tailor their body weight into their hydration strategy.  A 50 kg (110 pound) woman drinking the same as a 100 kg (220 pound) man will have ingested proportionately twice as much fluid and is much more likely to be overhydrated.
• Unchanged weight or weight gain during the endurance event.  A modest amount of weight loss during the event is recommended.  Although there is some debate about how much is ideal,  1 - 3% is a safe and desirable range.  Keep in mind that most athletes lining up for the start of an endurance event are already somewhat heavier than their lean, mean training weights.  Likely they have tapered their training and reduced caloric output over the previous couple of weeks; they have probably been "carbo loading" for the prior few days and have likely been aggressively hydrating for several hours.  So the pre-race weight is not necessarily a weight that would "normally" be maintained and it certainly shouldn't be increased during a race.
• Overdrinking. This is a primary risk factor for EAH.  Note that the intake of even moderate amounts of fluid may lead to EAH if the athlete's ability to excrete water is impaired.
• Non-steroidal Anti-inflammatory (NSAID) use.  There are a lot of unknowns with this risk factor, but it has been reported in a small number of studies and may play a role through interaction with renal (kidney) function.

Most cases of EAH are considered mild (showing blood serum sodium [Na+] levels between 130 and 135 mmol) and are largely asymptomatic.   In cases where EAH progresses, early signs and symptoms include:

Unfortunately these symptoms are not specific to hyponatremia or EAH and may occur where EAH is not present.   Anyone suffering from these symptoms should obtain medical attention, particularly if they have recently consumed large quantities of fluid or have other EAH risk factors.

Although EAH is clinically defined as having a blood serum sodium (Na+) level of under 135 mmol, severity of the symptoms will be determined by the amount and rapidity of serum sodium decline rather than the specific serum sodium value.  Someone with a pre-race value of 135 mmol and a post race value of 130 mmol  may be completely asymptomatic while another person starting with a value of 145 mmol dropping to 130 post race will be very sick indeed.

As EAH progresses, more serious symptoms can develop as a result of cerebral edema (brain swelling) and pulmonary edema (collection of fluid in the lungs).  Immediate medical attention is absolutely critical.  

alteration of mental status (e.g. confusion, disorientation, agitation, delirium)
obtundation
respiratory distress
seizures
coma


death (an increasing likelihood if treatment is delayed or absent)