Hyponatremia is defined as a serum sodium concentration < 135 mmol/L (or mEq/L). It is the most common electrolyte disorder, particularly in hospitalized patients, the elderly, and those with chronic illnesses.
Sodium: Role and Physiology
- Primary ECF cation; normal range: 135–145 mEq/L
- <10–14 mEq/L found intracellularly
Physiological Functions of Sodium
| Function | Description |
|---|---|
| ECF volume regulation | Maintains blood volume and pressure |
| Osmolality control | Major determinant of serum osmolality (normal: 275–295 mOsm/kg) |
| Nerve & muscle function | Facilitates action potentials in excitable tissues |
| Acid-base balance | Part of sodium bicarbonate (NaHCO₃) buffer system |
Sodium Regulation
- Intake: Primarily via diet; also from IV fluids, medications
- Minimum requirement: ~500 mg/day
- Excretion:
- Renal (primary)
- GI tract (vomiting, diarrhea, fistulas)
- Skin (sweating – typically negligible but ↑ with exercise/heat)
🧠 The kidneys can excrete nearly sodium-free urine when intake is low, or retain sodium during high losses.
Hyponatremia Classification
1. Based on Serum Osmolality
| Type | Serum Osmolality | Common Causes |
|---|---|---|
| Hypotonic (most common) | ↓ <275 mOsm/kg | Water retention, diuretics, SIADH |
| Isotonic (pseudohyponatremia) | 275–295 | Hyperlipidemia, hyperproteinemia |
| Hypertonic | ↑ >295 | Hyperglycemia, mannitol (osmotic shift) |
2. Based on Volume Status
| Type | Description | Causes |
|---|---|---|
| Hypovolemic hypotonic | Na+ and water loss; water loss < Na+ loss | Diuretics, GI losses, burns, adrenal insufficiency |
| Euvolemic hypotonic | Water gain only; no edema | SIADH, hypothyroidism, psychogenic polydipsia |
| Hypervolemic hypotonic | Na+ and water gain; water gain > Na+ gain | Heart failure, liver cirrhosis, nephrotic syndrome |
Pathophysiology
- Water moves into cells due to ↓ serum osmolality → cellular swelling
- Cerebral edema is the most dangerous complication
- In hyperglycemia: ↑ extracellular glucose → water shifts out of cells → dilutional hyponatremia
💡 High-Yield Causes
| Category | Specific Causes |
|---|---|
| GI losses | Diarrhea, vomiting, NG suction |
| Renal losses | Diuretics (esp. thiazides), adrenal insufficiency |
| SIADH | CNS injury, malignancy (e.g., small cell lung cancer), drugs (SSRIs, carbamazepine) |
| Psychogenic polydipsia | Excessive water intake |
| Post-op state | ↑ ADH secretion |
| CHF, cirrhosis, nephrotic syndrome | Hypervolemic states with effective volume depletion |
Clinical Features
Depends on onset and severity. More rapid ↓ in Na+ leads to worse symptoms.
| Sodium Level | Symptoms |
|---|---|
| 130–135 mEq/L | Often asymptomatic or mild GI symptoms |
| 125–129 | Nausea, headache, malaise |
| 115–124 | Lethargy, confusion, muscle cramps, weakness |
| <115 | Seizures, coma, respiratory arrest, death |
💡 Symptoms stem from cerebral edema due to water shifting into brain cells.
Diagnostic Workup
- Serum sodium
- Serum osmolality
- Urine sodium
- Urine osmolality
- Volume status assessment (clinical exam)
Example Diagnostic Patterns
| Type | Serum Osmolality | Urine Na+ | Urine Osmolality |
|---|---|---|---|
| SIADH | ↓ | >40 mEq/L | >100 mOsm/kg |
| Hypovolemic (e.g., diarrhea) | ↓ | <20 mEq/L | >450 mOsm/kg |
| Psychogenic polydipsia | ↓ | <20 mEq/L | <100 mOsm/kg |
| Hyperglycemia | ↑ | Variable | Variable |
💊 Management of Hyponatremia
General Principles
- Treat underlying cause
- Assess symptoms and rapidity of onset
- Avoid rapid correction to prevent osmotic demyelination syndrome (ODS)
Treatment by Severity:
| Severity | Treatment |
|---|---|
| Mild/asymptomatic | Fluid restriction, oral NaCl tablets |
| Moderate (symptomatic) | IV normal saline ± loop diuretic |
| Severe (e.g., seizures) | IV 3% hypertonic saline cautiously + furosemide |
⛔ Correct Na+ at ≤ 8–10 mEq/L over 24 hours
Specific Management
- SIADH: Fluid restriction, salt tablets, loop diuretics, vasopressin receptor antagonists (e.g., tolvaptan)
- Adrenal insufficiency: Glucocorticoid replacement
- Heart failure/cirrhosis: Fluid and sodium restriction, treat underlying disease
High-Yield Clinical Pearls
- Thiazide diuretics are a common cause of hyponatremia in elderly
- SIADH is the most common cause of euvolemic hyponatremia
- Psychogenic polydipsia presents with dilute urine and low serum osmolality
- Rapid correction of chronic hyponatremia risks central pontine myelinolysis
- Hyponatremia + low cortisol = suspect Addison’s disease
Mnemonics
“SALT LOSS” for symptoms of hyponatremia:
- Stupor/seizures
- Apprehension
- Lethargy
- Tendon reflexes ↓
- Limp muscles (weakness)
- Orthostatic hypotension
- Stomach cramps
- Seizures/coma
