II. Definition

  1. Hyponatremia manifests within 24 hours of Physical Activity (esp. in endurance events)

III. Epidemiology

  1. Common in Triathalons and Ultramarathons
  2. Symptomatic Hyponatremia is found in 23% of those seeking medical attention after endurance events
    1. Speedy (1999) Med Sci Sports Exerc 31(6): 809-15 [PubMed]

IV. Pathophysiology

  1. See Sodium and Water Homeostasis
  2. Excessive free water intake (overhydration) around the time of an endurance event
    1. In healthy patients, maximal renal excretion: 500-1000 ml/hour
    2. Additional losses during Exercise with sweating and respiration: 500 ml/hour
    3. Fluid intake above 1000-1500 ml/h results in fluid retention and decreased Serum Sodium
  3. Impaired urinary water excretion
    1. Failed ADH suppression due to pain, nausea Vomiting, intense Exercise, Hypoglycemia, sweating
    2. High ADH results in high Urine Osmolality (inadequate free water excretion) and decreased Serum Sodium
  4. Other factors
    1. Glycogen utilization releases water, with decreased Serum Sodium
    2. Cellular Lactic Acid accumulation draws water out of extracellular compartment
      1. After Exercise, water moves extracellularly as Lactic Acid is cleared
    3. Gastrointestinal water has reduced absorption due to decreased intestinal motility during Exercise
      1. Sodium (and chloride) are drawn into intestinal lumen, resulting in decreased Serum Sodium
      2. After Exercise, intestinal water is absorbed as motility increases, and Serum Sodium drops
    4. Brain Natriuretic Peptide (BNP, NT-BNP) increases during Exercise
      1. Results in urinary Sodium excretion and further fall in Serum Sodium

V. Risk Factors

  1. High fluid intake surrounding the time of exertional Exercise
  2. Long duration Exercise (>4 hours)
  3. High availability of fluid during event
  4. Higher Body Mass Index (often correlated with longer Exercise times)

VI. Symptoms

  1. Asymptomatic or unrecognized in up to 50% of cases
  2. Symptoms of mild Hyponatremia may be confused with other Exercise related causes (e.g. dehydration, Heat Illness)
    1. Light headedness
    2. Nausea
    3. Weakness
    4. Dizziness
    5. Oliguria

VII. Signs

  1. Findings that distinguish exertional Hyponatremia from dehydration, heat-related illness
    1. Edema
    2. Weight gain
  2. Later signs of significant Hyponatremia
    1. Pulmonary edema (Dyspnea, frothy Sputum)
    2. Neurologic progression
      1. Headache, lethargy, Ataxia
      2. Seizure
        1. Also consider other causes for Seizure (e.g. Hypoglycemia, Trauma, Epilepsy)
      3. Coma
      4. Brainstem relexes (Gag Reflex, Pupil Dilation) lost
      5. Brainstem Herniation

VIII. Labs

  1. See Hyponatremia
  2. Bedside Glucose
  3. Serum electrolytes (esp. Serum Sodium)
  4. Additional labs to consider
    1. Urine Osmolality
    2. Serum ADH

IX. Management

  1. Hyponatremia (Serum Sodium <130 mmol/L) and mild symptoms
    1. Salt tablets or bouillon cubes (3-4 cubes in 1/2 cup water)
    2. Observation by medical personnel until athlete urinates
  2. Hyponatremia (Serum Sodium <125-130 mmol/L) and severe symptoms (e.g. Seizures, ALOC, pulmonary edema)
    1. Precautions
      1. Normal Saline is often inadequate in athletes as Kidney retains free water despite NS
      2. Hypertonic Saline is more osmotic than urine and results in free water loss
      3. Hypertonic Saline volumes should be carefully administered
        1. For each kg body weight, 1 ml Hypertonic Saline raises Serum Sodium 1 mmol/L
        2. For a 50 kg person, 100 ml bolus will raise Serum Sodium 2 mmol/L
        3. Three boluses (max adult dose) will raise Serum Sodium up to 6 mmol/L (50 kg person)
        4. Acute loss correction is unlikely to cause Central Pontine Myelinolysis (but be careful)
    2. Hypertonic Saline (3% Saline, 513 mmol/L)
      1. Adults: Give 3% saline 100 ml every 10 min up to 3 doses until neurologic symptoms to improve
        1. Alternative: After first 100 ml bolus, start 3% saline at 2-3 ml/kg/hour
      2. As noted above, expect each 3% saline 100 ml bolus to raise Serum Sodium 1-2 mmol/L
    3. Disposition
      1. Transport all patients with severe symptoms warranting Hypertonic Saline bolus
  3. Seizures
    1. See Status Epilepticus
    2. Benzodiazepines (e.g. Lorazapam, Midazolam, Diazepam)
    3. Correct Hypontremia emergently with Hypertonic Saline protocol as above
    4. Rapid transport to emergency facility
  4. Other supportive measures
    1. Serum Glucose
    2. Supplemental Oxygen as needed
    3. Furosemide (Lasix) may be considered for Hyponatremia, esp. if pulmonary edema

X. Prevention

  1. Participants
    1. Educate on the risks of overhydration (e.g. drink to thirst)
    2. Electrolyte replacement solutions (however these do not eliminate Hyponatremia risk)
    3. Race coordinators collect height, weight, BMI and underlying conditions that my predispose to Hyponatremia
  2. Medical Tent and EMS
    1. Avoid hypotonic fluids for Resuscitation
    2. Onsite electrolyte testing (especially seryum Sodium) is ideal
    3. Have available Oral Rehydration Solutions (or bouillon cubes in water)
    4. Have Hypertonic Saline available for Resuscitation
  3. Event organization
    1. Consider weight monitoring stations with scale
    2. Avoid having too many rehydration stations

XI. References

  1. Anderson, Tomberg, Eastley (2017) Crit Dec Emerg Med 31(8): 3-10

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