Hyponatremia & Hypernatremia
ABCs
red flags
- seizures, coma, or depressed consciousness
- rapid or large change in Na
- Na <120 or >160 mmol/L
- check vitals
- assess → neurological status + volume status
- confirm the sodium, compare with previous levels
- correct for hyperglycemia
- every ↑ glucose 5.6 mmol/L → Na ↓ ~1.6 mmol/L
hyponatremia
- mild: fatigue, weakness, nausea
- moderate: confusion, ataxia
- severe: seizures, coma
work it up
- serum osmolality
- < 275 mOsm/kg → true hypotonic hyponatremia
- urine osmolality
- < 100 → excess water intake (e.g. polydipsia)
- > 100 → ADH active → check urine Na
- urine sodium
- < 30 mmol/L → hypovolemia (GI losses, dehydration)
- > 30 mmol/L → SIADH, renal loss, endocrine
- then determine acute vs chronic + volume status
treat by volume status
| volume status | treatment |
|---|---|
| hypovolemic | IV normal saline 0.9% |
| euvolemic | fluid restriction 500–1000 mL/day |
| hypervolemic | fluid restriction ± loop diuretic |
hypertonic saline (3%), acute + symptomatic only
- neurological symptoms (seizures, coma)
- bolus 100 mL over 10 min · repeat ×3
- or 150 mL over 20 min · repeat ×2
- do not start alone → call a senior first
- monitor Na hourly during correction
- avoid rapid correction → risk of osmotic demyelination
hypernatremia
- confusion, altered mental status
- weakness, fatigue
- nausea, vomiting
causes
- water loss (most common), GI losses, insensible (fever, burns)
- renal loss, diabetes insipidus, osmotic diuresis
- sodium gain, hypertonic fluids, NaHCO₃
replace free water
- free water deficit = TBW × (Na/140 – 1); TBW ≈ 0.6 × kg
- oral / NGT where possible
- most cases → D5W
- hypovolemic → 0.45% NS then D5W
correct slowly
- avoid rapid shifts → risk of cerebral edema
- consult a senior before initiating correction
correction limits, do not exceed
hyponatremia
- ≤ 8 mmol/L per 24 h
- ≤ 4–6 mmol/L per 24 h if high-risk: chronic, malnourished, liver disease, hypokalemia
hypernatremia
- ≤ 10 mmol/L per 24 h (~0.5 mmol/L/h)
overcorrection
- too-fast hyponatremia correction → osmotic demyelination
- too-fast hypernatremia correction → cerebral edema
calculator Na correction rate · MDCalc ↗︎
monitoring & handover
- recheck sodium + RFT every 4–6 h
- clear handover with the ongoing plan documented
MOC pearl
sodium is a water problem, not a salt problem, check osmolality first, let volume status drive treatment, and correct slowly: osmotic demyelination if you over-correct a low Na, cerebral oedema if you over-correct a high one.
studying for the IM exam? the IM Rapid Review covers electrolytes in the same format. see the sample chapter.
Last reviewed · June 2026