What is Hyponatremia?
Hyponatremia is a low sodium concentration in your blood, defined as a serum sodium level below 135 mmol/L, making it the most common electrolyte disorder in clinical medicine. 1, 2
How It Develops
Hyponatremia most commonly results from water retention rather than actual sodium loss. 2 Your body retains too much water relative to sodium, which dilutes the sodium concentration in your bloodstream. This typically happens when a hormone called arginine vasopressin (also known as antidiuretic hormone or ADH) is elevated, causing your kidneys to hold onto water. 3
Three Main Types Based on Fluid Status
Hyponatremia develops through three primary mechanisms: 1, 4
Hypovolemic hyponatremia: You've lost both sodium and water from your body (through vomiting, diarrhea, excessive sweating, or diuretic medications), but proportionally more sodium than water 1, 5
Euvolemic hyponatremia: Your total body water is normal, but you have too much water relative to sodium, often due to syndrome of inappropriate antidiuretic hormone (SIADH) from medications, lung cancer, or brain disorders 1, 4
Hypervolemic hyponatremia: You have excess total body water and sodium, but even more excess water (seen in heart failure, liver cirrhosis, or kidney disease) 1, 5
Why It Matters
Even mild hyponatremia affects approximately 5% of adults and 35% of hospitalized patients, and is associated with increased hospital stay and mortality. 2 Mild chronic hyponatremia causes cognitive impairment, gait disturbances, and increased rates of falls and fractures—patients with hyponatremia report falls 23.8% of the time compared to 16.4% in those with normal sodium levels. 2 Hyponatremia is also a secondary cause of osteoporosis. 2
Symptoms Range from Mild to Life-Threatening
Symptoms depend on how quickly sodium drops, how long it's been low, and how severe it is: 2, 3
- Mild symptoms: Weakness, nausea, headaches, confusion 2, 6
- Moderate symptoms: Mental impairment, numbness 2, 6
- Severe symptoms: Seizures, coma, cardiorespiratory distress, or death 2, 6
Common Causes
- Medications: Thiazide and loop diuretics, SSRIs (antidepressants), carbamazepine, oxcarbazepine, chemotherapy drugs 1, 4
- Medical conditions: Heart failure, liver cirrhosis (affects ~60% of cirrhotic patients), kidney disease, SIADH from lung cancer or brain disorders 1, 4, 5
- Fluid losses: Vomiting, diarrhea, excessive sweating, blood loss 1, 5
How Doctors Diagnose It
Initial assessment includes serum and urine osmolality, urine sodium concentration, serum uric acid, and clinical assessment of your fluid volume status (whether you're dehydrated, normal, or fluid-overloaded). 1 A urinary sodium less than 30 mmol/L suggests volume depletion, while greater than 20-40 mmol/L with concentrated urine suggests SIADH. 1
Treatment Depends on the Cause
For most patients, treatment focuses on addressing the underlying cause and varies based on your volume status: 2
- Hypovolemic: Isotonic saline to restore volume 1, 5
- Euvolemic (SIADH): Fluid restriction to 1 L/day, sometimes with medications like urea or vaptans 1, 2
- Hypervolemic: Fluid restriction to 1-1.5 L/day, treating the underlying heart or liver disease 1, 5
Severely symptomatic hyponatremia with seizures, coma, or severe confusion is a medical emergency requiring immediate treatment with 3% hypertonic saline to increase sodium by 4-6 mmol/L within 1-2 hours. 2, 3 However, correction must not exceed 8 mmol/L in 24 hours to prevent osmotic demyelination syndrome, a rare but devastating neurological complication that can cause permanent brain damage. 1, 2, 3