Exercise-associated Hyponatremia

Exercise-associated hyponatremia complicated by encephalopathy (confusion and seizure). There is also evidence of seizure-induced or exercise-induced rhabdomyolysis and mild renal impairment.

EAH is defined as a serum sodium level below the normal reference range (135 mmol/l) in an individual engaged in prolonged physical activity (generally >4 hours), or within 24 hours of it ending.

Risk factors for exercise-associated hyponatremia may include:

low body weight

prolonged exercise

slow speed of running associated with ease of drinking

access to fluids

NSAID use

female sex

Exercise-associated hyponatremia is caused by the excessive intake of hypotonic fluid during prolonged exercise, resulting in dilutional hyponatremia with an increase in total body water.

Other factors may also play a role:

Inadequate antidiuretic hormone (ADH) suppression
(due to factors such as exertion, nausea, vomiting, exertional hypovolemia, pain and thermal stress)

failure to mobilize osmotically inactive extracellular sodium stores
(e.g. in bone)

Exercise-associated hyponatremia may have:

no symptoms

mild symptoms — e.g. bloating, weakness, dizziness, headache, nausea and/ or vomiting

severe symptoms — acute pulmonary oedema and/ or encephalopathy (confusion, seizures, coma)

Death can occur.

A reasonable management plan would include:

Resuscitation — attend to ABCDEFG (don’t ever forget glucose) and vital signs.

Secondary survey to check for evidence of injury secondary to 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 Q6).

Referral to HDU/ ICU level care for ongoing monitoring in severe cases such as this.

Yes.

Most authorities consider the following as indications for hypertonic saline in the treatment of exercise-associated hyponatremia:

severe hyponatremia (Na <120 mmol/L)

significantly symptomatic (e.g. vomiting, encephalopathy or acute pulmonary oedema)

Most authorities advocate 3% hypertonic saline given at 1 to 2 ml/kg/h.

This may be increased to 3-4 mL/h if urine output is inadequate.

The infusion rate can be decreased following significant water diuresis.

Infusion should be stopped when the patient is asymptomatic with a normal level of consciousness.

Serum electrolytes need to be closely monitored (e.g. hourly initially).

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

Don’t drink too much!

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

Some authorities recommend estimating sweat loss by serial weight measurements during prolonged exercise, but this is impractical in most situations. There is no good evidence for adding salt to fluids or for commercial sport drinks (which are generally hypotonic).