Exercise as it Relates to Disease/ Hyponatremia in Marathon Runners

Exercise as it relates to Disease



What is Hyponatremia?
Hyponatremia in runners is known as water intoxication (1). The onset of this type of hyponatremia begins when sodium levels are below 135 mEq/L (milli equivalents per litre). This occurs when the blood sodium concentration is heavily diluted by water. Sodium promotes water retention so low levels of sodium means more water loss (2). The low concentration of sodium and high content of water means that water needs to be expelled from the body quickly. As such the kidneys swell (hydronephrosis) as the cannot process the fluids quick enough to expel them. This is when the water invades the other cells in the body causing them to swell (3). If the cells in the brain swell it can become dangerous as there is not enough room in the cranium for the enlarging brain. This is known as cerebral edema, and it can be fatal.

What Causes Hyponatremia to Occur?
Water intoxication occurs when a large volume of water is consumed quickly and without replacing the sodium that has been lost through sweat (4). This usually occurs towards the end of a marathon where runners are dehydrated and possibly overheating. As such they take in large volumes of water at the few opportunities they receive to do so. This is problematic as it drastically changes the sodium concentrations in the blood and causes the cellular swelling. This can cause a range of symptoms that a runner with hyponatremia will experience.

Symptoms of Hyponatremia During a Race
•	Slurred speech •	Confusion/disorientation •	Headache •	Muscle spasms/cramps •	Nausea/vomiting •	Weakness •	Restlessness •	Seizures •	Significant weight gain

If these symptoms persist they can result in coma, kidney (hydronephrosis) and heart failure and death (5)

How to Prevent Hyponatremia
The prevention of hyponatraemia is easily accomplished with a set plan of fluid and food consumption during an endurance event. (6)

Pre Event: During Event: Post Event:
 * Develop a plan of fluid and food consumption, considering the locations of the supplied drink stations on the race course.
 * Adequate hydration and food consumption to prevent dehydration before the event.
 * Consume 500ml per hour over, consuming small amounts at regular intervals.
 * Include a Carbohydrate-Electrolyte (CE) solution fluid to replace sodium lost through sweat
 * One large amount of fluid consumed to replace fluids lost (varies from person to person).
 * Smaller amounts (150mL every hour) over the next three hours.
 * Continue to consume CE fluid with water when rehydrating to reinstate sodium balance.



Treatment of Hyponatremia
The treatment of hyponatraemia for endurance athletes depends on the severity of the condition at the time of treatment. For accurate diagnosis and treatment the amount of fluid that was consumed must be known. A higher fluid intake means there will be a lower sodium concentration (mmol/L/h serum sodium concentration). This will then result in a higher escalation of treatment required. Intraveinous drips must be avoided at all costs as they can exacerbate the problem, and this can be fatal.

Suggested means of treatment for severely affected athletes include:
 * Hypertonic Saline: Solution with a higher sodium content than the body. Amount taken depends on individual characteristics and degree of hyponatremia. (7)
 * Loop Diuretics: Loop diuretics act upon the ascending loop of Henle in the kidneys nephron to inhibit sodium reabsorption. Thus, more sodium is kept for use of the cells. (8)
 * Mannitol: Mannitol is an osmotic diuretic as well as a renal vasodilator. It prevents the reabsorption of sodium and swells the kidneys with fluids with the aim of expelling fluids quicker. (9)

Further Information
PubMed - Hyponatremia: Cool Runnings - Hyponatremia a Concern for Marathon Runners MedicineNet - Hyponatremia (Low Blood Sodium)
 * http://www.ncbi.nlm.nih.gov/pubmedhealth/PMH0001431/
 * http://www.coolrunning.com/engine/6/6_1/1179.shtml
 * http://www.medicinenet.com/hyponatremia/article.htm

You should contact your health care professional to devise a suitable fluid intake program for you during a race.

Reference List

 * 1) Smith, S, 2011, http://www.wisegeek.com/what-is-water-intoxication.htm, Accessed 20/10/11
 * 2) Howley, E, and Powers, S, Exercise Physiology: Theory and Application to Fitness and Performance. 7th ed 2009. 496-99.
 * 3) Dugdale, D, http://www.ncbi.nlm.nih.gov/pubmedhealth/PMH0001431/, Accessed 18/10/11
 * 4) Almond, C, et.al. http://www.nejm.org/doi/full/10.1056/NEJMoa043901#t=article, Accessed 18/10/11
 * 5) Dugdale, D, http://www.nlm.nih.gov/medlineplus/ency/article/000394.htm, Accessed 18/10/11
 * 6) Burke, L, 1996, Rehydration Strategies Before and After Exercise, Australian Journal of Nutrition and Dietetics, 53 (4)
 * 7) Hsieh, M, 2004, Recommendations for Treatment of Hyponatraemia at Endurance Events, Sports Med, 34 (4): 231-238
 * 8) Dormans, T, Gerlag, P, Russel, F, Smits, P, 1998, Combination Diuretic Therapy in Severe Congestive Heart Failure, Drugs, 55(2): 165-172
 * 9) Haviv, Y, 2000, Hyponatraemia – Hyperosmolaroty Associated with Intravenous Mannitol, Clin Drug Invest, 20 (4): 287-289