Exercise Associated hyponatraemia


Exercise Associated Hyponatraemia (EAH) is a serum sodium level < 135 mmol/l in an individual engaged in prolonged physical activity (generally >4 hours), or within 24 hours of it ending.

Risk factors

  • low body weight
  • slow speed of running (ease of drinking)
  • access to fluids
  • NSAID use


  • Excessive intake of hypotonic fluid during prolonged exercise, resulting in dilutional hyponatremia with an increase in total body water
  • Inadequate antidiuretic hormone (ADH) suppression (due to factors such as over exertion, nausea, vomiting, exertional hypovolemia, pain and thermal stress)
  • failure to mobilize osmotically inactive extracellular sodium stores (e.g. in bone)


EAH has an exceptionally high mortality rate and presents:

No symptoms

Mild symptoms

  • Bloating, weakness, dizziness, headache, nausea and/or vomiting

Severe symptoms

  • Pulmonary oedema and/or encephalopathy (confusion, seizures, coma)
  • Cardiac Arrest: Ensure all Cardiac Arrests in Marathon Runners have U&E done before terminating resuscitation.


  • Resuscitation — attend to ABCDEFG (don’t ever forget glucose)
  • Secondary survey to check for injury (fall/ seizure
  • Supportive treatment and monitoring
  • Fluid restriction to correct dilutional hyponatremia, but ensure adequate urine output to avoid renal failure secondary to rhabdomyolysis.
  • Be aware that unabsorbed hypotonic fluids in the GI tract can lead to further dilutional hyponatremia.
  • Consider hypertonic saline (see below)
  • Referral to HDU/ ICU for ongoing monitoring in severe cases.

Indications Hypertonic NaCl in EAH?

  • Severe hyponatremia (Na <120 mmol/L)
  • Significantly symptomatic (e.g. encephalopathy or acute pulmonary oedema)

When confirmed, in severe cases with epileptic seizures or severe mental changes suggesting worsening cerebral oedema, exercise associated hyponatraemia should be treated with hypertonic saline (to correct sodium to a level of 125 mmol/l over 1-2 hours, and to normal level over the following 2-4 hours.)

There are no reports of cerebral pontine myelinoysis resulting from over-vigorous treatment of exercise-associated hyponatremia (an acute process) with hypertonic saline.

Prepare hypertonic saline

Preparation of 3% sodium chloride using 30% sodium chloride.

Hypertonic 3% sodium chloride is no longer available & so 30% sodium chloride vials must be used to produce a 3% solution.

Vol of 30% NaCl (ml) Vol of 0.9% NaCl (ml) Final vol of 3% NaCl (ml)
7.2 92.8 100
14.4 185.6 200
18 232 250
21.6 278.2 300
28.8 371.2 400
36 464 500

e.g. to produce 250ml of 3% sodium chloride remove 18mls from a 250ml bag of 0.9% sodium chloride & discard. Then add 18mls of 30% sodium chloride to the remaining 232mls in the bag. Final concentration = 2.995%


Don’t drink too much!

Athletes should drink only according to thirst: (generally no more than 400 to 800 ml/h).

Content By Dr Jason van dr Velde, Dr Íomhar O' Sullivan 21/05/2015. Last review Dr ÍOS 21/05/15.