II. Pathophysiology
- Total Body Sodium Deficit exceeds water losses
- Decreased Extracellular fluid volume
- Increased proximal tubule fluid reabsorption
- Decreased distal segment flow where dilution occurs
-
Hypovolemia stimulates non-osmotic fluid conservation
- Thirst
- ADH secretion
III. Differential Diagnosis
- Often difficult to distinguish Iso- from Hypovolemic
- See Isovolemic Hypoosmolar Hyponatremia
IV. Labs
- Non-Renal Sodium Loss
- Urine Sodium < 20 meq/L
- Urine Osmolality >400
- Fractional Excretion of Urea <35%
- Use instead of Urine Sodium in patients on Diuretics
- Carvounis (2002) Kidney Int 62(6): 2223-9 [PubMed]
- Renal Sodium Loss
- Urine Sodium > 20 meq/L
- Urine Osmolality <400
V. Causes: Non-Renal Losses (Sodium appropriately conserved)
- Gastrointestinal losses
- Third space losses
- Skin Losses
- Severe burns
VI. Causes: Renal Losses (Renal inappropriate Sodium losses)
- Diuretics (e.g. chronic Thiazide Diuretic use)
-
Renal Tubular Acidosis
- Hyperchloremic Metabolic Acidosis
- Increased urinary pH
- Fractional Excretion of Bicarbonate >15-20%
- Salt-losing Glomerulonephritis
- Chronic Renal Insufficiency on Low Sodium Diet
- Severe interstitial Kidney disease
- Polycystic Kidney Disease
- Medullary cystic disease
- Chronic Pyelonephritis
- Mineralocorticoid and Glucocorticoid deficiency
- Osmotic Diuresis (Bicarbonate, Glucose, Ketones)
- Excess osmotically active solutes in urine
- Draws increased Sodium and water renal losses
- Cerebral salt wasting (head injuries, Intracranial Hemorrhage)
- Diagnosis of exclusion
VII. Management
- See Hyponatremia Management
- Stop all Diuretics
- Correct non-renal fluid losses
- Replace Sodium deficit
- Calculate Total Body Sodium Deficit
- Use Normal Saline (0.9% = 150 meq/L)
- Replace one third Sodium deficit over first 6-8 hours
- Replace remaining Sodium deficit in next 24-48 hours
VIII. References
- Kone in Tisher (1993) Nephrology, p. 87-100
- Levinsky in Wilson (1991) Harrison's IM, p. 281-84
- Rose (1989) Acid-Base and Electrolytes, p. 601-38
- Braun (2015) Am Fam Physician 91(5): 299-307 [PubMed]