Hypernatraemia
Description
- Na+ >145 mmol/L - always reflects a deficit of water relative to Na+
- Implies impaired thirst or restricted access to water (rarely pure Na+ excess)
- Classified by volume status: hypovolaemic / euvolaemic / hypervolaemic hypernatraemia
Epidemiology
- Uncommon in ambulant adults with intact thirst and free water access
- Highest risk: elderly, infants, ICU/ventilated patients, impaired consciousness - anyone who cannot access water independently
Aetiopathogenesis
Hypovolaemic (water loss > Na+ loss)
- GI losses (diarrhoea, vomiting), excess sweating, osmotic diuresis (glucose, mannitol, urea)
Euvolaemic (pure water loss)
- Diabetes insipidus
- Central - inadequate ADH (pituitary/hypothalamic injury, tumour, idiopathic)
- Nephrogenic - renal ADH resistance (lithium, hypercalcaemia, hypokalaemia, genetic)
- Insensible losses (fever, mechanical ventilation) without replacement
Hypervolaemic (Na+ gain > water gain)
- Hypertonic saline or sodium bicarbonate administration, mineralocorticoid excess (rare cause)
- Impaired thirst mechanism (elderly, hypothalamic disease) is usually the permissive factor - hypernatraemia rarely develops if thirst and access are intact
Diagnosis
- Serum Na+ >145 mmol/L on U&E
- Assess volume status clinically (mucous membranes, skin turgor, JVP, postural BP, weight change)
- Urine osmolality distinguishes DI from other causes:
- Urine osm <300 mOsm/kg despite hypernatraemia = DI (inappropriately dilute urine)
- Urine osm >600-800 = appropriate renal response, look for extrarenal water loss
- Water deprivation test + desmopressin trial distinguishes central (urine concentrates with desmopressin) from nephrogenic DI (no response)
Management
Principles
- Treat underlying cause (stop lithium, treat hypercalcaemia, desmopressin for central DI)
- Correct slowly - max fall of ~8-10 mmol/L per 24h (chronic hypernatraemia)
- Brain has adapted with idiogenic osmoles - rapid correction -> cerebral oedema
- Acute hypernatraemia (<48h, e.g. iatrogenic hypertonic saline) can be corrected faster given less cerebral adaptation
Fluid choice by volume status
- Hypovolaemic - isotonic (0.9% saline) initially to restore volume, then hypotonic fluid (5% dextrose or 0.45% saline) to replace free water deficit
- Euvolaemic (DI) - free water replacement (oral water preferred if able) + desmopressin if central DI
- Hypervolaemic - stop the offending Na+ source, diuretics + free water
Free water deficit (estimate)
- Deficit = Total body water x [(Na+/140) - 1]
- Replace over 48-72h, reassessing Na+ every 4-6h during active correction
Associations
- Central DI - pituitary surgery/trauma, craniopharyngioma, sarcoidosis, Langerhans cell histiocytosis
- Nephrogenic DI - lithium, chronic hypercalcaemia, chronic hypokalaemia, congenital (AVPR2, AQP2 mutations)
- Hypertonic states in ICU - tube feeds with high solute load and inadequate free water
Natural history & complications
- Untreated severe/acute hypernatraemia - seizures, coma, intracranial haemorrhage (from brain shrinkage tearing bridging veins)
- Over-rapid correction - cerebral oedema, herniation
- Chronic well-managed DI - normal life expectancy with desmopressin/thiazide (nephrogenic) therapy
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