Hyponatremia: Pathophysiology of different etiologies.

Hyponatremia is the most frequently encountered electrolyte disorder in clinical practice. Understanding the pathophysiology of the many different etiologies of hyponatremia is essential for appropriate diagnosis and therapy of hyponatremic disorders. Hyponatremia can occur by an increase in total body water (TBW), a decrease in body Na+ or K+, or any combination of these. In many cases, more than one of these mechanisms is operant. Therefore, a classification system to separate the various etiologies of hyponatremia should be based on factors other than the level of serum [Na+] itself. An assessment of extracellular fluid volume (ECF) volume provides the most useful working classification of the etiology of the hyponatremia, because a low serum [Na+] can be associated with a decreased, normal, or increased ECF volume. This chapter describes the known pathophysiologies associated with the three major classes of hyponatremia: Hyponatremia with decreased ECF volume (hypovolemic hyponatremia); Hyponatremia with increased ECF volume (hypervolemic hyponatremia); and Hyponatremia with normal ECF volume (euvolemic hyponatremia). For each disorder, both preclinical (translational) and clinical research studies is summarized to fully understand the factors involved with each disorder. Understanding the pathophysiology responsible for the development of hyponatremia with different etiologies is essential for informed evaluation and effective treatment of hyponatremic patients.