Hyponatremia: A Systematic Approach to Diagnosis and Treatment
Hyponatremia is the most common electrolyte disorder in hospitalized patients — and it's almost always a question of figuring out why, because the 'why' determines the treatment. The systematic approach is what separates a correct answer from a dangerous one.
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Progress Note Editorial · Evidence-based
Updated July 2026
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The 60-second version
First step: check serum osmolality to classify (isotonic/hypertonic/hypotonic)
Second step: assess volume status (hypo/eu/hypervolemic)
SIADH: euvolemic, urine Na >40, urine osmol >300, treat with fluid restriction
SCLC is the #1 paraneoplastic cause of SIADH
Chronic hypo-Na: correct <8–10 mEq/L per 24h to prevent osmotic demyelination
Step 1: Serum Osmolality
Isotonic hyponatremia (osmol 280–290): pseudohyponatremia from hyperlipidemia or hyperproteinemia — lab artifact. Hypertonic hyponatremia (osmol >290): osmotically active solute drawing water into plasma — hyperglycemia (correct Na by adding 1.6 mEq/L for every 100 mg/dL glucose above 100), mannitol, contrast. Hypotonic hyponatremia (osmol <280): true hyponatremia — proceed to step 2.
Step 2: Volume Status
Hypovolemic (dry): urine Na <20 suggests extra-renal losses (vomiting, diarrhea, sweating); urine Na >20 suggests renal losses (diuretics, Addison's disease, salt-wasting nephropathy). Euvolemic: most commonly SIADH; also hypothyroidism, cortisol deficiency. Hypervolemic (edematous): urine Na <20 suggests cirrhosis/heart failure/nephrotic syndrome; urine Na >20 suggests advanced renal failure.
Chronic hyponatremia (>48h): correct at <8–10 mEq/L per 24 hours (some guidelines say <10–12). Too-rapid correction causes osmotic demyelination syndrome (central pontine myelinolysis): dysarthria, dysphagia, spastic quadriplegia, locked-in syndrome. Severe symptomatic hyponatremia (seizures, coma): give 100–150 mL 3% saline over 10–20 minutes; can repeat × 2 — target 5 mEq/L increase to stop seizures, not full correction.
Test yourself
Quick check — 3 questions
1. A 58-year-old patient with small cell lung cancer presents with Na+ 121, serum osmolality 256, urine osmolality 480, urine Na+ 55, no edema. Most likely diagnosis?
Explanation. SIADH: euvolemic (no edema, no dehydration signs), urine osmolality inappropriately concentrated (>100, here 480), urine Na >40, low serum osmolality. SCLC is a classic paraneoplastic cause of SIADH via ectopic ADH production.
2. A patient with chronic hyponatremia (Na 114) is treated. After 24 hours, Na is 130. The patient develops dysarthria and quadriplegia 3 days later. What occurred?
Explanation. The sodium was corrected by 16 mEq/L in 24 hours, exceeding the safe threshold of 8–10 mEq/L per 24 hours for chronic hyponatremia. Osmotic demyelination syndrome (central pontine myelinolysis) results, presenting days after correction with brainstem dysfunction.
3. In severe symptomatic hyponatremia with active seizures, the immediate treatment is:
Explanation. Seizures from severe hyponatremia require 3% hypertonic saline to raise sodium acutely by 4–6 mEq/L to stop seizure activity — not full correction. Normal saline is NOT appropriate here and may worsen SIADH. After stabilization, resume slow correction at <8–10 mEq/L per 24 hours.
Frequently asked questions
What is the most common cause of SIADH?
The most commonly tested paraneoplastic cause of SIADH is small cell lung cancer (SCLC), which produces ectopic ADH. However, in clinical practice, the most common causes are CNS pathology (stroke, SAH, meningitis/encephalitis) and pulmonary disease (pneumonia, tuberculosis). SSRIs are the most common drug-induced cause.
How do you treat SIADH?
First-line: fluid restriction (usually 1–1.5 L/day). If fluid restriction fails: salt tablets ± loop diuretics, demeclocycline (induces nephrogenic DI), or tolvaptan/conivaptan (V2/V1a-V2 vasopressin receptor antagonists). Severe symptomatic hyponatremia (seizures): 3% hypertonic saline — raise sodium 4–6 mEq/L acutely to stop seizures, then resume slow correction.
What is osmotic demyelination syndrome?
Osmotic demyelination syndrome (ODS), formerly called central pontine myelinolysis (CPM), occurs when chronic hyponatremia is corrected too rapidly (>10–12 mEq/L in 24 hours). Brain cells adapt to chronic hypo-osmolality by losing solutes; rapid correction creates an osmotic gradient that strips water from neurons, causing demyelination of the pons. Presents 2–6 days post-correction with dysarthria, dysphagia, quadriplegia, and potentially locked-in syndrome.
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