Hypernatraemia
Summary
Hypernatraemia (Serum Sodium >145 mmol/L) is a state of hyperosmolality representing a deficit of free water relative to sodium. It almost never occurs in an alert patient with access to water (as the thirst response is extremely potent). Therefore, it is typically a disease of the vulnerable: the elderly (dementia/immobility), infants, or the unconscious. Management balances the need to replace water with the critical risk of causing Cerebral Oedema through overly rapid correction. [1,2]
Clinical Pearls
The "Tea and Toast" Sign: The classic presentation is the elderly nursing home resident admitted with "confusion". They have a mild infection (UTI), stop drinking, and present with Na 160+. They are simply dehydrated.
Tube Feed Dangers: Patients on NG feed are at high risk. Feed is high in protein (urea load) and electrolytes but low in free water. If "water flushes" are missed, the patient develops "Tube Feed Hypernatraemia".
Rapid Correction Risk:
- Hyponatraemia correction too fast -> Demyelination (Brain shrinks).
- Hypernatraemia correction too fast -> Cerebral Oedema (Brain swells).
- Mnemonic: "High to Low, the Brain will Blow" (Cerebral Oedema).
Demographics
- Hospitalised Patients: Occurs in 1-3%. Iatrogenic (poor fluid prescription) is a major cause.
- Mortality: 40-60% in severe cases (Na >160), though this often reflects underlying morbidity rather than the sodium itself.
Mechanism of Brain Injury
- Hypertonicity: ECF becomes hypertonic. Water moves out of brain cells by osmosis -> Brain Shrinkage.
- Acute (less than 48h): Physical shrinkage tears bridging veins -> Subdural Haemorrhage.
- Chronic (>48h): Brain cells adapt by creating "Idiogenic Osmoles" (Inositol, Taurine) to pull water back in and restore cell volume.
- Rehydration Danger: If you add water too fast to a "chronic" brain (loaded with osmoles), it sucks up water uncontrollably -> Cerebral Oedema.
- Dehydration (Unreplaced skin/gi losses).
- Diuretics (Loop/Osmotic - lose water > salt).
- Diarrhoea / Vomiting.
- Diabetes Insipidus (Central or Nephrogenic).
- Diabetes Mellitus (HHS - Osmotic diuresis).
- Doctors (Iatrogenic - Saline overload, Bicarbonate, missed water flushes).
Classification by Volume Status
- Hypovolaemic (Commonest): Water loss > Na loss (Diuretics, GI loss).
- Euvolaemic: Pure water loss (Diabetes Insipidus).
- Hypervolaemic (Rare): Sodium gain (Salt poisoning, Hypertonic saline, Conn's).
Symptoms
Signs of Dehydration
Urine Osmolality (The Key Test)
Differentiates Renal vs Extra-Renal loss.
-
Urine Osm > 700 (Concentrated):
- Kidneys are working properly (retaining water).
- Cause: Extra-renal loss (Sweat/GI) or poor intake.
-
Urine Osm < Urine Plasma (Dilute):
- Kidneys are FAILING to concentrate despite hypernatraemia.
- Cause: Diabetes Insipidus or Diuretics.
Polyuria Workup (If Urine Dilute)
- Water Deprivation Test:
- Restrict fluid. If Urine Osm stays low -> Confirm DI.
- Give Desmopressin (DDAVP).
- Urine Concentrates = Cranial DI.
- No Change = Nephrogenic DI.
Management Algorithm
HYPERNATRAEMIA (>145)
↓
ASSESS VOLUME / SHOCK (BP/HR)
┌─────────┴─────────┐
SHOCK STABLE
(Hypovolaemic) (Any Volume)
↓ ↓
RESUSCITATE CALCULATE WATER
WITH 0.9% DEFICIT
SALINE (Use Formula)
(Fix perfusion ↓
first) SELECT FLUID
↓
┌───────────┴───────────┐
ABLE TO DRINK? UNABLE TO DRINK
↓ ↓
ORAL WATER IV 5% DEXTROSE
(Preferred) (Or 0.45% Saline)
↓ ↓
MONITOR & ADJUST RATE
Target fall: less than 10 mmol/L/day
1. Calculate Water Deficit
Formula: Free Water Deficit (L) = Total Body Water x ((Serum Na / 140) - 1)
- TBW = 0.6 x Wt (Men) or 0.5 x Wt (Women/Elderly).
- Example: 70kg Man with Na 160.
- TBW = 42L.
- Formula: 42 x ((160/140) - 1) = 42 x 0.14 = ~6 Litres.
2. Rate of Correction
- Chronic (>48h): Max drop 0.5 mmol/L per hour (10-12 mmol in 24h).
- Acute (less than 48h): Can correct fast (1 mmol/L per hour).
3. Special Scenarios
- Diabetes Insipidus:
- Cranial: Desmopressin (Oral/Nasal/IV).
- Nephrogenic: Stop Lithium. Thiazides/NSAIDs (Paradoxical effect).
- Hypervolaemic (Salt Load):
- Furosemide (to lose salt) + D5W (to replace Volume).
- Dialysis if renal failure.
- Cerebral Oedema: Iatrogenic (too fast).
- Subdural/Intracerebral Haemorrhage: Mechanics of brain shrinking.
- Rhabdomyolysis: Severe hypertonicity damages muscle.
- Venous Thrombosis: Dehydration leading to stasis.
- High mortality (>60% in acute settings) largely due to the severity of the underlying illness causing the inability to drink.
- Neurological sequelae prominent in infants.
Key Guidelines
| Guideline | Organisation | Key Recommendations |
|---|---|---|
| Best Practice | BMJ / Adrogué | Formulas for deficit correction. |
| Intravenous Fluids | NICE [CG174] | Use 5% Dextrose for pure water deficit. Monitor 4-hourly. |
Landmark Evidence
1. Adrogué & Madias (NEJM 2000)
- The seminal review establishing the physiological basis for slow correction of chronic hypernatraemia to prevent cerebral oedema.
Why is my Sodium high?
Sodium levels are a balance between salt and water. High sodium usually means you are "dried out" (dehydrated) - you have lost too much water, leaving the blood concentrated and salty. This happens often in elderly people who lose their thirst drive or get an infection.
Why not just drink lots of water quickly?
Your brain acts like a sponge. Because your blood has been salty/concentrated for a few days, your brain cells have shrunk slightly to cope. If we flood your system with fresh water too fast, the brain will soak it up like a dry sponge and swell dangerously. We have to "drip feed" the water back in over 2-3 days to let the brain expand slowly and safely.
Is it dangerous?
Severe dehydration can cause confusion and kidney strain. However, once we start the fluids, the outlook is usually good, provided we treat the underlying cause (like the urine infection).
Primary Sources
- Adrogué HJ, Madias NE. Hypernatremia. N Engl J Med. 2000.
- NICE Guideline [CG174]. Intravenous fluid therapy in adults in hospital. 2013.
- Garrahy A, et al. Diagnosis and management of central diabetes insipidus in adults. Clin Endocrinol. 2019.
Common Exam Questions
- Differentiator: "Hypernatraemia + Low Urine Osmolality?"
- Answer: Diabetes Insipidus.
- Treatment: "Best fluid for correction?"
- Answer: Oral water (if conscious) or 5% Dextrose.
- Complication: "Rapid correction causes?"
- Answer: Cerebral Oedema.
- Calculation: "70kg man, Na 154?"
- Answer: Deficit ~4 Litres.
Viva Points
- Why Dextrose?: Dextrose 5% is isotonic in the bag (so it doesn't lyse red cells). Once infused, the liver metabolizes the sugar instantly, leaving pure free water.
- Nephrogenic DI Causes: Lithium, Hypercalcaemia, Hypokalaemia.
Medical Disclaimer: MedVellum content is for educational purposes and clinical reference. Clinical decisions should account for individual patient circumstances. Always consult appropriate specialists.