Renal, Fluids, Electrolytes & Acid-Base · Topic 10 of 22

Hyponatremia

Fluids & electrolytes · sodium

Hyponatremia

Severe symptoms get a 3% saline bolus to raise sodium 4–6 mEq/L; otherwise correct slowly — no more than 8 mEq/L in 24 hours.

Key numbers

Hyponatremia: serum sodium below 135 mEq/LNa < 135 mEq/L

Severe symptoms: 3% saline 100 mL bolus↑ 4–6 mEq/L

Raise sodium no more than 8 mEq/L/day≤ 8 mEq/L in 24 h

01 · DoSevere symptoms: 3% saline 100 mL bolus

Seizures, coma or severe confusion: give 3% saline 100 mL IV over 10 minutes, repeated up to twice more if symptoms persist, to raise sodium 4–6 mEq/L in the first hours; monitor in the ICU.

02 · The ruleRaise sodium no more than 8 mEq/L/day

Chronic or unknown-duration hyponatremia: target a 4–8 mEq/L rise per 24 h (4–6 if demyelination risk is high); never exceed 8 mEq/L in the first 24 h, including the initial rise.

03 · DoTreat by volume status

Treat by volume status: hypovolemic — 0.9% saline; euvolemic — treat the cause (SIADH, hypothyroidism, adrenal insufficiency, drugs) with water restriction; hypervolemic — water restriction, loop diuretic and treatment of heart failure or cirrhosis.

What it is

Core idea

Hyponatremia: serum sodium below 135 mEq/L must knowHyponatremia is serum sodium < 135 mEq/L from excess water relative to solute; true hypotonic hyponatremia also has a low serum osmolality (< 275 mOsm/kg).

Causes & risk factors

Why it happens

Brisk dilute diuresis signals overcorrection riskOnce volume or the cause is corrected (e.g. saline for hypovolemia, stopping a thiazide), vasopressin falls and a large water diuresis can raise sodium too fast — watch urine output closely.

Thiazides: a classic cause in older adultsThiazide diuretics are a common cause, especially in older adults, through volume depletion, hypokalemia and impaired diluting capacity; other drugs include SSRIs, carbamazepine, opioids and MDMA.

Presentation

What you'll see

Symptoms: nausea, confusion, falls, then seizuresSymptoms are neurologic — nausea, headache, unsteadiness and falls, confusion and lethargy; below about 115 mEq/L, seizures, coma and death can occur, sooner when onset is rapid.

Diagnosis & monitoring

Work-up

Order serum osm, urine osm, urine sodium must knowWork up with serum and urine osmolality, urine sodium, volume status, TSH and cortisol: urine osm < 100 means appropriately dilute urine; urine Na < 20 suggests extrarenal loss, > 30 SIADH or renal loss.

Check sodium every 2–4 hours during correctionDuring hypertonic saline, check serum sodium every 2–4 hours (every 2 hours in acute symptomatic hyponatremia); formulas only estimate the response.

SIADH: euvolemic, concentrated urine, urine Na > 30SIADH is a euvolemic hypotonic hyponatremia with inappropriately concentrated urine (> 100 mOsm/kg), urine sodium > 30 mEq/L and low uric acid, with normal thyroid and adrenal function; restrict fluid.

Do right now

In this order

First moves

Severe symptoms: 3% saline 100 mL bolus↑ 4–6 mEq/L

→

Check sodium every 2–4 hours during correction

→

Order serum osm, urine osm, urine sodium

→

Treat by volume status

Why each step

Severe symptoms: 3% saline 100 mL bolusSeizures, coma or severe confusion: give 3% saline 100 mL IV over 10 minutes, repeated up to twice more if symptoms persist, to raise sodium 4–6 mEq/L in the first hours; monitor in the ICU.

Check sodium every 2–4 hours during correctionDuring hypertonic saline, check serum sodium every 2–4 hours (every 2 hours in acute symptomatic hyponatremia); formulas only estimate the response.

Order serum osm, urine osm, urine sodiumWork up with serum and urine osmolality, urine sodium, volume status, TSH and cortisol: urine osm < 100 means appropriately dilute urine; urine Na < 20 suggests extrarenal loss, > 30 SIADH or renal loss.

Treat by volume statusTreat by volume status: hypovolemic — 0.9% saline; euvolemic — treat the cause (SIADH, hypothyroidism, adrenal insufficiency, drugs) with water restriction; hypervolemic — water restriction, loop diuretic and treatment of heart failure or cirrhosis.

Management

Orders

Overcorrection: stop saline, give hypotonic fluidIf sodium rises too fast, stop the hypertonic saline and relower sodium with hypotonic fluid (D5W), with or without desmopressin, to prevent permanent neurologic damage.

Vaptans: never in hypovolemia or liver diseaseTolvaptan (oral) and conivaptan (IV) treat resistant euvolemic or hypervolemic hyponatremia; never use in hypovolemia, avoid with liver disease or advanced CKD, limit tolvaptan to 30 days, and watch for overcorrection.

Asymptomatic chronic hyponatremia: no hypertonic bolusAsymptomatic chronic hyponatremia, even with sodium below 121 mEq/L, is treated with water restriction and correction of the cause rather than hypertonic saline.

Red flags & don'ts

Avoid harm

Osmotic demyelination: dysarthria, dysphagia, quadriparesis complicationOsmotic demyelination appears days after overcorrection with dysarthria, dysphagia and flaccid paralysis, sometimes locked-in syndrome; risk is highest with alcohol use disorder, malnutrition, hypokalemia and liver disease.

Hypovolemic hyponatremia or SIADH?

Commonly confused

Volume status	Dry — vomiting, diarrhea, third-spacing
Urine sodium	< 20 mEq/L
Urine osmolality	Concentrated
Treatment	0.9% saline

Commonly confused

Tell apart

Exclude hyperglycemia and pseudohyponatremia firstHyperglycemia lowers sodium by about 1.6 mEq/L per 100 mg/dL of glucose (high osmolality); severe hypertriglyceridemia or paraproteinemia causes pseudohyponatremia with normal osmolality.

Acute onset: treat even mild symptomsHyponatremia of known rapid onset (< 24 h: marathon runners, MDMA, postoperative hypotonic fluid, primary polydipsia, hypotonic irrigation) causes cerebral edema — give hypertonic saline even for mild neurologic symptoms.

More detail

Reference

Show 6 reference facts

A desmopressin 'clamp' (1–2 mcg every 8 hours with hypertonic saline) prevents an unpredictable water diuresis and overcorrection; stop it after 24 hours of controlled correction.

Cerebral salt wasting (after subarachnoid hemorrhage or CNS injury) causes volume depletion with urine sodium > 40 mEq/L; give saline and do not fluid-restrict.

For chronic SIADH when fluid restriction fails, oral urea or tolvaptan are the most effective second-line options; salt tablets help in mild to moderate cases.

Sodium deficit (mEq) = TBW × (target Na − current Na), with TBW = 0.6 × weight (men) or 0.5 × weight (women); 3% saline contains 513 mEq/L.

In a 2025 meta-analysis of 26,710 inpatients, ODS occurred in 0.73% after rapid correction (defined as > 8–12 mEq/L in 24 h) versus 0.10% with slower correction.

In hypervolemic hyponatremia (heart failure, cirrhosis) low effective circulating volume drives vasopressin release; urine sodium is usually < 10 mEq/L.

Pearl

Fix the brain fast and the number slowly: 4–6 mEq/L stops the seizure; more than 8 mEq/L in a day risks osmotic demyelination.

Sources · Merck Manual Professional — Hyponatremia (2025) · Spasovski G. Hyponatraemia — treatment standard 2024. Nephrol Dial Transplant 2024;39:1583–1592 · Suppadungsuk S et al. Hyponatremia Correction and Osmotic Demyelination Syndrome Risk: A Systematic Review and Meta-Analysis. Kidney Med 2025 · Lawless SJ, Thompson C et al. The management of acute and chronic hyponatraemia. Ther Adv Endocrinol Metab 2022;13:20420188221097343 (summarizes the 2013 US expert panel and 2014 European guideline; PMC9109487) · Starter sheet built from the verified AGACNP fact map (facts reviewed 2026-10-15).