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.

FIGURE 1.

Diagnostic algorithm for the evaluation of hyponatremia. (HF, heart failure; EABV, effective arterial blood volume; SIADH, syndrome of inappropriate antidiuretic hormone; U_Na, urine sodium concentration [values in mEq/L are numerically equivalent to mmol/L]; UOsm, urine osmolality [values in mOsm/kg are numerically equivalent to mmol/kg].) Reprinted with permission from Wells BG, DiPiro JT, Schwinghammer TL, et al. Pharmacotherapy Handbook. 10th ed. New York, NY: McGraw-Hill; 2017.