Key Points

• Life-threatening hyponatremia develops when excessive hypotonic fluid is consumed, with concomitant sodium sweat loss.

• Exertional hyponatremia most often occurs in women completing endurance races in over 4 hours who drink copiously throughout the race.

• Symptoms of exertional hyponatremia include mental status changes and peripheral edema, without significant elevation in core temperature.

Exertional hyponatremia (serum sodium <130 mmol/L), once considered a rare complication of exercise in the heat, is now recognized as more common and responsible for a number of exercise-related deaths (Almond et al., 2005). Controversy surrounds the exact pathophysiology, but the condition develops in the setting of excessive hypotonic fluid replacement while sodium is progressively lost in

Table 29-1 Exertional Heat Illness

Definition

Management

Prevention

Heat edema

Dependent edema usually occurring before acclimation.

Elevation of swollen extremity, rest, cooling. Diuretics contraindicated.

Gradual acclimation to heat.

Heat (exercise associated) muscle cramps

Painful spasms of single or multiple muscles. Likely sodium deficiency and salty sweaters most prone.

Rest, stretching, cooling, oral hydration with hypertonic sodium drink. Intravenous fluids (normal saline) if oral treatment limited or to expedite recovery.

Maintain hydration and increase salt intake. Add salt to fluids, especially for those with predisposition based on past history.

Heat syncope

Orthostatic dizziness at cessation of exercise, with prolonged standing, or after assuming upright posture.

Rest, cooling, place supine with legs elevated, monitor vital signs, and mental status. Oral fluid hydration.

Adequate hydration and acclimation. If occurs during exercise, requires cardiovascular evaluation.

Heat exhaustion

Inability to continue exercise in heat. Symptoms: weakness, fainting, dizziness, headache, nausea, vomiting, cramps, dehydration with low urine output. Minimal mental status symptoms; core temperature <40° C.

Immediate rest, rapid cooling (ice bath), close monitoring of mental status, vital signs (e.g., core temperature). Serum sodium if hyponatremia considered. Oral fluid hydration with IV fluids (normal saline) if hypotension present.

Adequate acclimation, monitor hydration, adjust training to climate, follow player's weight, and close monitoring for symptoms of heat illness.

Heat stroke

Heat exhaustion with core temperature >40° C and mental status alteration or central nervous system collapse.

As for heat exhaustion, with hospitalization as soon as possible.

As for heat exhaustion, be prepared with ice baths, monitoring equipment, and access to emergency medical services.

sweat (Levine and Thompson, 2005; Noakes, 2002). Typical victims are relatively inexperienced female marathon runners who tend to be light sweaters, finish in over 4 hours, and drink copiously throughout the race. Nonsteroidal anti-inflammatory drugs (NSAIDs) taken before the race may be a contributing factor (Hsieh, 2004). Athletes with a history of exercise-induced hyponatremia do not seem to be predisposed to water overload at rest, although there may be some physiologic mechanism beyond pure water overload that accounts for the condition in some susceptible individuals (Speedy et al., 2001). Symptoms are similar to those of heat exhaustion, including weakness, dizziness, headache, nausea, vomiting, and cramping, but the headache is more prominent and progressively severe, extremity swelling may be noted, and progressive mental status changes occur despite a core temperature lower than 40° C (104° F). Cerebral edema underlies the mental status changes, and pulmonary edema may also occur.

The serum sodium level must be assessed in athletes in whom the diagnosis is suspected, to differentiate hyponatremia from heat stroke. Intravenous (IV) fluids, often indicated in heat stroke, can actually worsen hyponatremia if related to excessive hypotonic fluid intake. Prompt hos-pitalization is indicated for any athlete with mental status changes or persistently altered vital signs once the diagnosis of hyponatremia has been established. Exertional hypo-natremic encephalopathy is treated with 3% hypertonic saline boluses (100 mL) (Hew-Butler et al., 2008). Prevention includes following an athlete's weight change with exercise to understand the fluid requirements more precisely, not deviating from established fluid intake on race day to avoid overhydration, incorporating sodium/electrolyte-containing fluids, and limiting fluid intake to 1 L/ hr unless higher fluid requirements have been established (Gardner, 2002).

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