Hyponatremia

SOURCE

Source: Chessman KH, Haney J. Disorders of sodium and water homeostasis. In: DiPiro JT, Talbert RL, Yee GC, Matzke GR, Wells BG, Posey LM, eds. Pharmacotherapy: A Pathophysiologic Approach. 10th ed. New York, NY: McGraw-Hill; 2017. http://accesspharmacy.mhmedical.com/content.aspx?bookid=1861&sectionid=146061710. Accessed April 18, 2017.

DEFINITION

• Serum sodium <135 mEq/L [<135 mmol/L])

ETIOLOGY

• Result of excess extracellular fluid (ECF) relative to sodium because of impaired water excretion.

  • Drug-induced (eg, diuretics, psychotropics)

  • Ingestion of excessive quantities of fluids.

  • Administration of hypotonic fluids to hospitalized patients.

• Syndrome of inappropriate antidiuretic hormone (SIADH)

  • Associated with oncologic disease, especially small cell lung cancer.

PATHOPHYSIOLOGY

• Homeostasis: Physiologic processes that maintain a relatively stable equilibrium between interdependent elements.

  • Mechanisms for regulating blood volume and plasma osmolality involve control of sodium and water balance.

    • Homeostatic mechanisms for controlling blood volume focused on controlling sodium balance.

    • Plasma osmolality largely determined by serum sodium concentration and controlled by water balance.

    • Sixty percent of total body water distributed intracellularly; 40% contained in extracellular space.

      • Sodium, chloride, and bicarbonate comprise >90% of total osmolality of ECF.

      • Intracellular osmolality depends primarily on concentration of potassium.

• Arginine vasopressin (AVP), commonly known as antidiuretic hormone, released from posterior pituitary when plasma osmolality increases by 1–2% or more.

• Classified as isotonic, hypertonic, or hypotonic depending on serum osmolality (Figure 1).

  • Hypertonic hyponatremia associated with increased serum osmolality, most commonly due to hyperglycemia.

  • Hypotonic hyponatremia most common with many potential causes.

    • Hypovolemic hypotonic hyponatremia associated with loss of ECF volume and sodium, with loss of more sodium than water.

      • Relatively common in patients taking thiazide diuretics and typically develops within 2 weeks of initiation of therapy (urine sodium >20 mEq/L [>20 mmol/L]), Extrarenal sodium loss with diarrhea (urine sodium <20 mEq/L [<20 mmol/L]).

    • Euvolemic hypotonic hyponatremia associated with normal or slightly decreased ECF sodium content and increased total body water and ECF volume.

      • Most commonly result of SIADH release.

    • Hypervolemic hypotonic hyponatremia associated with increase in ECF volume in conditions with impaired renal sodium and water excretion, such as:

      • Cirrhosis.

      • Congestive heart failure (CHF)

      • Nephrotic syndrome.

EPIDEMIOLOGY

• Most common electrolyte abnormality in clinical practice.

• Nursing home incidence 2-fold higher than in similar aged-community dwelling individuals.

PREVENTION

• Athletes should consider oral fluids that contain electrolytes.