164 - Sodium and Water Balance

between the two spaces when water diffuses freely across the membrane. The predominant solute of the extracellular space is sodium. Various disease states may alter water balance and lead to abnormally high or low sodium levels that may result in significant disability or death from either the causative disease, the direct effects of the sodium concentration, or the ill effects of inappropriate treatment. The signs and symptoms range from subtle constitutional symptoms to seizure and coma. Suspicion for disorders of water balance depends on assessment of existing risk factors and the clinical information available at the time of arrival at the emergency department (ED). Hyponatremia is diagnosed when the serum sodium level is lower than 135 mEq/L, but clinical signs and symptoms most often occur when sodium falls below 130. Hyponatremia most commonly occurs in the very young and the very old, with prevalence increasing with advancing age. Hyponatremia is observed in infants given tap water as a home remedy for gastroenteritis and in elderly patients with a poorly regulated thirst mechanism or an inability to procure fluids because of immobility (or both). Hypernatremia, a plasma sodium level higher than 145 mEq/L, most commonly results from inadequate water intake. In the very young, this situation usually occurs secondary to water loss exceeding intake, such as with diarrheal illness; in the geriatric population, hypernatremia may result from a poor sense of thirst or an inability to obtain adequate fluids because of physical or mental impairment. Hypernatremia is less common than hyponatremia, but it is associated with a far greater mortality rate of approximately 50%, primarily from the causative disease states in elderly patients and from the direct neurologic effects of the high sodium concentration in the very young.