Exercise Associated Hyponatraemia

Exercise associated hyponatraemia is a serum sodium under 135 mmol/L during or up to 24 hours after prolonged physical activity. It is caused mainly by drinking more fluid than is lost in sweat, together with non-osmotic antidiuretic hormone secretion that limits water excretion.

Incidence is 5 to 15% of marathon finishers (up to 30% in ultra-endurance events), and 0.5 to 1% have symptoms. Severe cases with cerebral oedema (encephalopathy) and non-cardiogenic pulmonary oedema have been fatal. The condition is much commoner in people who gain weight during the event.

Risk factors:

• Prolonged exertion (marathon, ultramarathon, triathlon, long hikes, military training, cycling).
• Drinking 'by schedule' beyond thirst.
• Slower athletes (running for more than 4 hours), women, and lower body weight.
• NSAID use, hot or very cold conditions, and high sodium losses.

Pathophysiology. Excess hypotonic fluid intake, non-osmotic vasopressin release (nausea, pain, stress, exercise, inflammatory cytokines, hypoglycaemia, NSAIDs), sodium loss in sweat and a transient reduction of mobilised osmotically inactive sodium all dilute plasma sodium. Extra fluid in the gut is absorbed after exercise, causing a delayed fall.

• Mild: bloating, weight gain, swollen hands and ankles, nausea, headache, dizziness, tight rings.
• Moderate: vomiting, confusion, agitation, weakness, unsteady gait.
• Severe: seizures, coma, respiratory distress (non-cardiogenic pulmonary oedema), cerebral herniation.
• Weight gain during or after the event is typical, clear urine, no signs of dehydration, normal or cool temperature.
• Differentiate from heat stroke (core temperature over 40 degrees Celsius, hot skin) and dehydration (weight loss, thirst).

Red flags

• Confusion, seizure, vomiting, headache or collapse during or after an endurance event.
• Weight gain over 2 kg during exercise, or slow finishers with overhydration.
• Breathlessness or frothy sputum (pulmonary oedema).
• Sodium under 125 mmol/L.
• Persisting symptoms despite oral salt.

• Point of care or laboratory sodium (delay in treatment is dangerous when only the clinical picture is available and the athlete is severely ill); potassium, glucose, creatinine, creatine kinase, urea.
• Weight compared with pre-event weight; core (rectal) temperature to exclude heat stroke.
• Urine osmolality (over 100 mosmol/kg often), serum osmolality.
• Chest radiograph if breathless; CT head only if atypical features or no improvement.
• Exclude other causes: heat stroke, hypoglycaemia, head injury, cardiac events, rhabdomyolysis, adrenal insufficiency.

Differential diagnosis

• Heat stroke and heat exhaustion, dehydration, hypoglycaemia, cardiac arrhythmia, subarachnoid or other intracranial haemorrhage, concussion, rhabdomyolysis, drug use (MDMA), adrenal insufficiency, inappropriate antidiuresis from other causes.

Mild asymptomatic or minimal symptoms (field or medical tent)

• Stop fluid intake and observe in a supervised area.
• Give oral salt (salty snacks, 3 to 6 g of sodium chloride as bouillon, or 1.5 to 2 g salt in 200 mL water if tolerated).
• Do not give oral or intravenous hypotonic fluids.
• Recheck sodium and symptoms in 1 to 2 hours.
• Do not give 0.9% sodium chloride intravenously to an athlete with a high urine osmolality and normal volume, as it can lower sodium further.

Moderately symptomatic (headache, vomiting, confusion)

• Hypertonic saline 3% 100 mL intravenously over 10 minutes, repeated once or twice at 10 minute intervals until symptoms improve. Transfer to hospital.
• When intravenous access is not available, give oral concentrated bouillon (3 cubes in 125 mL hot water).

Severely symptomatic (seizure, coma, pulmonary oedema)

• 3% sodium chloride 100 mL intravenously over 10 minutes as a bolus, repeated up to two further times at 10 minute intervals. Aim to raise sodium by 4 to 6 mmol/L, enough to reverse the encephalopathy. Do not wait for laboratory sodium results if suspected.
• Protect the airway and give oxygen 10 to 15 litres/minute. Give lorazepam 4 mg intravenously for seizures and positive pressure ventilation for pulmonary oedema. Avoid hypotonic fluid.
• In hospital, check sodium every 1 to 2 hours. Use intensive care for coma or respiratory failure.
• Stop hypertonic saline when symptoms resolve. Limit rise to 10 mmol/L in 24 hours (8 mmol/L in each following 24 hours).
• Watch for dilute diuresis and overcorrection (desmopressin 2 microg intravenously and 5% glucose 3 mL/kg if sodium rises too quickly).

Prevention

• Drink to thirst during exercise (usually 400 to 800 mL each hour). Avoid drinking more than sweat loss.
• Weigh before and after training. A weight change of minus 2 to 0% is ideal; avoid weight gain.
• In events over 4 hours, take sodium (sports drinks or salt, 300 to 600 mg of sodium each hour).
• Avoid NSAIDs before and during events. Avoid thiazides and selective serotonin reuptake inhibitors in endurance athletes where possible.

Monitoring. Sodium until normal and the athlete is passing dilute urine; weight; neurological state hourly. Discharge only when sodium is at least 130 mmol/L and symptoms are resolved. Follow up with a sports medicine physician.

Referral and escalation. Emergency transfer for neurological symptoms, respiratory distress or sodium under 125 mmol/L. Nephrology or endocrinology if hyponatraemia persists beyond 24 hours; neurology for persisting deficits.

Special groups

• Women, slower athletes and people with low body weight: highest risk; individual hydration plans.
• Children and adolescents in sport: avoid forced drinking, particularly in summer camps and military training.
• Older athletes: thiazide or selective serotonin reuptake inhibitor use increases risk.
• Pregnancy: avoid prolonged endurance exercise with forced fluid intake; drink to thirst.