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
- 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
- Administration of hypotonic fluids to hospitalized patients
- Syndrome of inappropriate antidiuretic hormone (SIADH)
- Associated with oncologic disease, especially small cell lung
- 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
mechanisms for controlling blood volume focused on controlling sodium
- 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.
chloride, and bicarbonate comprise >90% of total osmolality
- Intracellular osmolality depends primarily on concentration
- Arginine vasopressin (AVP), commonly known as antidiuretic hormone, released from
posterior pituitary when plasma osmolality increases by 1–2% or
- 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
- 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:
- Congestive heart failure (CHF)
- Nephrotic syndrome
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
- Athletes should consider oral fluids that contain electrolytes.
- Increasing age
- Thiazide diuretic use
- Patients with chronic, mild hyponatremia (serum sodium
>125–130 mEq/L [>125-130 mmol/L]) usually
- 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:
- Which may progress to:
- Permanent brain damage
- Respiratory arrest
- Brainstem herniation
- Patients with hypovolemic hyponatremia present with, in addition