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Source: Coyle JD, Matzke GR. Disorders of Sodium and Water Homeostasis. In: DiPiro, JT, Talbert RL, Yee GC, Matzke GR, Wells BG, Posey LM. Pharmacotherapy: A Pathophysiologic Approach, 8th ed. http://accesspharmacy.com/content.aspx?aid=7983523. Accessed August 18, 2012.

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

  • Result of excess extracellular fluid (ECF) relative to sodium because of impaired water excretion
    • Drug-induced (e.g., diuretics, psychotropics)
    • Ingestion of excessive quantities of fluids (e.g., marathon runners)
    • Administration of hypotonic fluids to hospitalized patients
  • Syndrome of inappropriate antidiuretic hormone (SIADH)
    • Associated with oncologic disease, especially small cell lung cancer.

  • 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. (CHF, congestive heart failure; EABV, effective arterial blood volume; SIADH, syndrome of inappropriate antidiuretic hormone; UNa, 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. 8th ed. New York: McGraw-Hill, 2012.

  • Most common electrolyte abnormality in clinical practice.
  • Nursing home incidence 2-fold higher than in similar aged-community dwelling individuals.

  • Athletes should consider oral fluids that contain electrolytes.

  • Increasing age
  • SIADH
  • Diarrhea
  • Thiazide diuretic use

Signs and Symptoms

  • Patients with chronic, mild hyponatremia (serum sodium >125–130 mEq/L [>125-130 mmol/L]) usually asymptomatic.
    • May be associated with impairment of attention, posture and gait, increasing risk of falls.
  • Moderate (serum sodium 115–125 mEq/L [115–125 mmol/L]) to severe (serum sodium <110–115 mEq/L [110–115 mmol/L]) or rapidly developing hypotonic hyponatremia present with range of symptoms.
    • Classic symptoms:
      • Nausea
      • Malaise
      • Headache
      • Lethargy
      • Restlessness
      • Disorientation
    • Which may progress to:
      • Seizures
      • Coma
      • Permanent brain damage
      • Respiratory arrest
      • Brainstem herniation
      • Death
  • Patients with hypovolemic hyponatremia present with, in addition to ...

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