Approach to Hyponatremia in Hospitalized Patients: Diagnosis & Management Guide

Introduction
Hyponatremia is the most common electrolyte abnormality in hospitalized patients. Defined as a serum sodium level below 135 mEq/L, it can range from mild and asymptomatic to severe with life-threatening neurological effects. A systematic approach is crucial to avoid misdiagnosis and dangerous complications like osmotic demyelination.

What Causes Hyponatremia?

Hyponatremia is best classified by volume status and serum osmolality:

• Hypovolemic hyponatremia happens when both sodium and water are lost, but more sodium is lost than water. Common causes include vomiting, diarrhea, diuretics, or adrenal insufficiency.

• Euvolemic hyponatremia occurs when total body water increases but sodium remains normal. This is often caused by SIADH (Syndrome of Inappropriate ADH Secretion), hypothyroidism, or excessive water intake.

• Hypervolemic hyponatremia is due to conditions like heart failure, liver cirrhosis, or nephrotic syndrome, where both water and sodium are retained, but water retention exceeds sodium.

How to Diagnose Hyponatremia

1. Start with serum osmolality

   • Low osmolality confirms true hypotonic hyponatremia.

   • Normal or high osmolality may indicate pseudohyponatremia (e.g. due to high lipids or proteins) or hyperglycemia.

2. Assess the patient’s volume status

   • Clinical signs like blood pressure, heart rate, edema, or mucous membranes can help categorize the patient as hypovolemic, euvolemic, or hypervolemic.

3. Check urine sodium and urine osmolality

   • Low urine sodium suggests the kidneys are trying to conserve salt (as in hypovolemia).

   • High urine osmolality indicates impaired free water clearance (as seen in SIADH).

Treatment Strategy

The management of hyponatremia depends entirely on the cause and severity:

• For hypovolemic hyponatremia, the treatment is intravenous normal saline to restore volume and stop sodium loss.

• In euvolemic hyponatremia, especially due to SIADH, fluid restriction is the first step. In more resistant cases, medications like tolvaptan may be used.

• For hypervolemic hyponatremia, both fluid and salt restriction are key, often alongside loop diuretics to reduce fluid overload.

In severe cases with neurological symptoms like seizures or coma, hypertonic saline (3%) is used cautiously under close monitoring.

Important Note: Sodium correction should be slow—no more than 8–10 mEq/L in 24 hours—to prevent central pontine myelinolysis (osmotic demyelination syndrome).

Quick Mnemonic for Hypovolemic Causes: SALT LOSS

• Sweating

• Adrenal insufficiency

• Loop diuretics

• Third-spacing (e.g. pancreatitis, burns)

• Laxatives

• Overuse of diuretics

• Sodium restriction

• Serum (vomiting/diarrhea)