II. Definition
- Hyponatremia with Measured Serum Osmolality <280 mOsm
- Hyponatremia is a water excess state
III. Pathophysiology
- Impaired renal water excretion with continued water intake
- Identify the cause of Hyponatremia by identifying why the Kidney can not excrete excess water
IV. Causes: Hypoosmolar Hyponatremia (impaired water excretion)
- Glomerulus: Reduced Glomerular Filtration Rate
- Renal Failure (GFR <20% of normal)
- Proximal Tubule: Increased water reabsorption
- Gastrointestinal losses (especially Vomiting) with free water replacement
- Extracellular fluid volume depletion results in Antidiuretic Hormone release, and fluid retention
- Edematous State (e.g. CHF, Cirrhosis, Nephrosis)
- Sodium retention and continued free water intake
- Gastrointestinal losses (especially Vomiting) with free water replacement
- Distal Convoluted Tubule: Impaired water excretion
-
Medullary collecting tubule: ADH-mediated water retention
- Syndrome of Inappropriate ADH or SIADH (e.g. CNS disease, lung disease, cancer, medications, postoperative)
- Miscellaneous mechanisms
- Endocrine cause (Hypothyroidism, Adrenal Insufficiency)
- Excessive free water intake (or excessive or prolonged hypotonic infusion)
- "Tea and Toast" Diet (elderly) or excessive beer (Alcoholism)
- Low solute diet of Carbohydrates with too little Protein and Sodium
- Kidney requires solute to effectively excrete water
- Carbohydrates are metabolized to carbon dioxide and exhaled
V. Evaluation
- Is Renal Failure present (with GFR <20% of normal)?
- Check Serum Creatinine and calculate Glomerular Filtration Rate (GFR)
- Identify source of free water intake
- Excessive oral free water
- Hypotonic fluids (Hypotonic Saline, medication IV solutions)
- Identify related medications
- Thiazide Diuretics (e.g. Hydrochlorothiazide, Chlorthalidone)
- Trimethoprim (causes high renin and high Aldosterone levels)
- Consider Medication Causes of SIADH
- Consider endocrine causes
- Assess for Extracellular Fluid Overload (Edematous State)
- Peripheral Edema
- Jugular Venous Distention
- Pulmonary rales
- IVC Ultrasound for Volume Status
- Third spacing of fluid (Pleural Effusion, Ascites)
- Assess for Extracellular Fluid Depletion
- Orthostatic Blood Pressure and Pulse
- Decreased Skin Turgor
- Dry mucous membranes
- Serum markers increased with Dehydration (Uric Acid, BUN, Serum Creatinine)
- What is Patient's Volume Status?
- Hypervolemic Hypoosmolar Hyponatremia (Edematous State)
- Non-Hypervolemic (Iso- or Hypovolemic)
- May be difficult to distinguish Isovolemic from Hypovolemic state
- Hypovolemic Hypoosmolar Hyponatremia (volume depletion, esp. Gastrointestinal losses)
- Isovolemic Hypoosmolar Hyponatremia (e.g. SIADH)
- Obtain Urine Sodium
- Differentiate renal causes (e.g. Renal Failure) in which Urine Sodium >20 mEq/L
- Urine Sodium <20 mEq/L suggests Sodium retention
- Volume depletion (Hypovolemic Hypoosmolar Hyponatremia)
- Volume overload or Edematous State (Hypervolemic Hypoosmolar Hyponatremia)
- Water Intoxication or tea and toast diet (Isovolemic Hypoosmolar Hyponatremia)
- Consider Urine Osmolality
- Urine Osmolality (normally 300-900 mOsm/L) is decreased with impaired renal dilutional function
- Do not use specific gravity in place of Urine Osmolality (does not accurately correlate)
- Voinescu (2002) Am J Med Sci 323(1):39-42 +PMID:11814141 [PubMed]
- Urine Osmolality (normally 300-900 mOsm/L) is decreased with impaired renal dilutional function
- Consider 24 hour urine for total solute excretion
- Total solute excretion is consistent with Tea and Toast Diet (high Carbohydrate, low Protein)
VI. Management
- See Hyponatremia Management
- See specific protocols based on fluid status above
VII. References
- Kone in Tisher (1993) Nephrology, p. 87-100
- Levinsky in Wilson (1991) Harrison's IM, p. 281-84
- Preston (2011) Acid-Base, Fluids and Electrolytes, Medmaster, Miami
- Rose (1989) Acid-Base and Electrolytes, p. 601-38
- Braun (2015) Am Fam Physician 91(5): 299-307 [PubMed]
- Miller (2023) Am Fam Physician 108(5): 476-86 [PubMed]