Overview
This lecture explains hyponatremia, its classification based on serum osmolality and volume status, and the main causes for each category, highlighting key distinctions useful for exams.
Hyponatremia Basics
- Hyponatremia is defined as serum sodium less than 135 mEq/L.
- Many diseases cause hyponatremia, making it a frequent exam topic.
- First step in evaluation: Determine serum osmolality (amount of solute in blood).
Types of Hyponatremia by Serum Osmolality
Isotonic Hyponatremia
- Normal serum osmolality with low sodium.
- Main causes: hyperlipidemia, multiple myeloma, TURP syndrome.
- Hyperlipidemia and multiple myeloma cause pseudohyponatremia (lab artifact, sodium actually normal).
- TURP syndrome (post prostate surgery) causes dilutional hyponatremia due to water absorption.
Hypertonic Hyponatremia
- High serum osmolality with low sodium.
- Causes: hyperglycemia, mannitol, radiocontrast agents.
- Mechanism: Excess solute draws water into blood, diluting sodium.
Hypotonic Hyponatremia
- Low serum osmolality with low sodium (most common and complex).
- Requires assessment of volume status: hypovolemic, euvolemic, or hypervolemic.
Hypotonic Hyponatremia by Volume Status
Hypovolemic (Low Volume)
- Causes: fluid loss outside kidney (vomiting, diarrhea, dehydration, burns, bleeding).
- Extra-renal causes: Low urine sodium as sodium is lost with the fluid outside kidneys.
- Renal causes: diuretics, ACE inhibitors, Addison’s disease, cerebral salt wasting, ATN.
- Renal losses: High or normal urine sodium as sodium lost through kidneys.
Euvolemic (Normal Volume)
- Causes: SIADH, hypothyroidism, psychogenic polydipsia, adrenal insufficiency, medications, beer potomania.
- Mechanism: inappropriate water retention due to impaired free water excretion.
Hypervolemic (High Volume)
- Causes: severe fluid overload like congestive heart failure, cirrhosis, nephrotic syndrome, end-stage renal disease.
- Mechanism: Fluid retention exceeds body’s ability to excrete water.
Key Terms & Definitions
- Hyponatremia — low serum sodium (<135 mEq/L)
- Serum osmolality — concentration of solutes in the blood
- Pseudohyponatremia — falsely low sodium due to high lipids or proteins
- TURP syndrome — hyponatremia from water absorption during prostate surgery
- SIADH — Syndrome of Inappropriate Antidiuretic Hormone, causing water retention
- Euvolemic — normal body fluid volume
- Hypervolemic — excess body fluid volume
- Hypovolemic — decreased body fluid volume
- Urine sodium (Una) — helps distinguish renal from extra-renal causes
Action Items / Next Steps
- Review and memorize causes and mechanisms of each hyponatremia type.
- Practice distinguishing hypotonic hyponatremia using urine sodium values.
- Study common diseases linked to each category for clinical correlations.