Renal, Fluids, Electrolytes & Acid-Base · Topic 11 of 23

Hyponatremia

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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.

01 · Severe symptoms3% saline 100 mL IV over 10 min

02 · Correction limitNa⁺ rise ≤ 8 mEq/L in the first 24 h

03 · Treat by volumeHypovolemic: saline · SIADH: restrict water

What causes it

Excess water relative to solute — sort by volume status

Hypovolemic

VomitingDiarrheaThird-spacing

Euvolemic

SIADHHypothyroidismAdrenal insufficiency

Hypervolemic

Heart failureCirrhosis

Drugs

Thiazides (classic in older adults)SSRIsCarbamazepineOpioidsMDMA

Acute onset (under 24 h)

Marathon runningMDMAPostoperative hypotonic fluidPrimary polydipsiaHypotonic irrigation

Hypervolemic hyponatremia: low effective circulating volume drives vasopressin; urine Na⁺ is usually under 10 mEq/L. Cerebral salt wasting (after subarachnoid hemorrhage or CNS injury) causes volume depletion with urine Na⁺ over 40 mEq/L — give saline, don't fluid-restrict.

What you'll see

Neurologic — worse and sooner when onset is rapid

Vasopressin keeps water in; water shifts into brain cells → edema, especially when acute

Nausea, headache, unsteadiness and falls early

Confusion and lethargy

Below ~115 mEq/L: seizures, coma, death cerebral edema

Diagnosis & labs

Confirm it's hypotonic, then read the urine and the volume status

Hyponatremia: serum Na⁺ under 135 mEq/L

True hypotonic: serum osmolality under 275 mOsm/kg

Hyperglycemia: Na⁺ falls ~1.6 per 100 mg/dL glucose

Pseudohyponatremia: high triglycerides or paraprotein, normal osm

Order serum and urine osm, urine Na⁺, TSH, cortisol

Urine osm under 100 = appropriately dilute urine

Urine Na⁺ under 20 = extrarenal loss · over 30 = SIADH or renal loss

SIADH: euvolemic, urine osm over 100, urine Na⁺ over 30

Under 135 mEq/L is the board and guideline convention; Merck uses under 136. SIADH also has low uric acid and needs normal thyroid and adrenal function.

Red flags

Seizures, coma or severe confusion 3% saline bolus

Acute onset with even mild neurologic symptoms hypertonic saline

Na⁺ rising faster than 8 mEq/L in 24 h relower Na⁺

Brisk dilute diuresis once the cause is corrected overcorrection

Days later: dysarthria, dysphagia, flaccid paralysis demyelination

Vaptan in hypovolemia never give

Osmotic demyelination can progress to locked-in syndrome. Highest risk: alcohol use disorder, malnutrition, hypokalemia, liver disease.

Treatment

Fix the brain fast, the number slowly

Severe symptoms — first moves, in order

3% saline 100 mL IV over 10 minrepeat up to twice more · goal +4–6 mEq/L

→

Admit to the ICU, Na⁺ every 2 hevery 2–4 h on hypertonic saline

→

Send serum and urine osm, urine Na⁺plus TSH, cortisol, volume status

→

Then treat by volume statustotal rise ≤ 8 mEq/L in 24 h

Hypovolemicrestore volume

0.9% saline

Isotonic saline for hypovolemic hyponatremia

Volume back → water diuresis can overcorrect

Euvolemictreat the cause

Water restriction plus treatment of the cause

SIADH, hypothyroidism, adrenal insufficiency, drugs

Hypervolemicremove water

Loop diuretic

Water restriction, loop diuretic, treat HF or cirrhosis

Chronicgo slow

Target rise 4–8 mEq/L per 24 h (4–6 if high risk)

Asymptomatic, even Na⁺ under 121: no hypertonic bolus

Overcorrectionrelower

D5W Desmopressin

Stop hypertonic saline; relower Na⁺ with D5W

With or without desmopressin

Vaptansresistant cases

Tolvaptan (oral) Conivaptan (IV)

Resistant euvolemic or hypervolemic hyponatremia only

Avoid in liver disease, advanced CKD; tolvaptan ≤ 30 days

Bolus, clamp, formula and second-line

Desmopressin clamp 1–2 mcg every 8 h with hypertonic salineprevents an unpredictable water diuresis; stop after 24 h of controlled correction

Sodium deficit (mEq) = TBW × (target Na⁺ − current Na⁺)TBW = 0.6 × weight (men), 0.5 × weight (women) · 3% saline = 513 mEq/L

Chronic SIADH, restriction failed: oral urea or tolvaptanmost effective second-line; salt tablets help in mild to moderate cases

Demyelination 0.73% after rapid correction vs 0.10% slower2025 meta-analysis, 26,710 inpatients; rapid = over 8–12 mEq/L in 24 h

Hypertonic bolus: sources differ on volumeEuropean 150 mL over 20 min; Merck 100 mL over 15 min, repeated once; the 2024 standard prefers intermittent boluses over continuous infusion

Merck: ≤ 8 mEq/L in the first 24 h for every patient, including the initial rise. The 2013 US panel allows 10–12 in standard-risk patients (8 if Na⁺ under 105, hypokalemia, alcohol use disorder, malnutrition, advanced liver disease); European guidance uses 10; a rise ≤ 8 satisfies all. Formulas only estimate the response.

Hypovolemic hyponatremia or SIADH?

	Hypovolemic (extrarenal loss)	SIADH
Volume status	Volume depleted — vomiting, diarrhea, third-spacing	Euvolemic
Urine sodium	Under 20 mEq/L	Over 30 mEq/L
Urine osmolality	Concentrated	Inappropriately concentrated (over 100 mOsm/kg)
Treatment	0.9% saline	Fluid restriction; treat the cause

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 · Hyponatremia (Merck Manual Professional, 2025) · Hyponatraemia — treatment standard 2024 (Nephrol Dial Transplant) · Hyponatremia correction and osmotic demyelination risk: meta-analysis (Kidney Med 2025) · The management of acute and chronic hyponatraemia (Ther Adv Endocrinol Metab 2022) · Follow your institution's protocols.