- Either due to water deficit (very common) or rarely Na gain
- Poor prognostic sign (worse than hyponatraemia)
Causes
- Dehydration especially in elderly and cognitively impaired patients (can be a marker of poor care if occurs in a care home resident)
- Osmotic diuresis
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- Hyperglycaemia/HHS
- Diuretics
- Post relief of urinary obstruction
- Other – mannitol, high protein diet
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- Non-renal water losses – burns, exercise, heat, severe diarrhoea
- AVP deficiency or resistance (More here)
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- Central, nephrogenic, lithium. Often euvolaemic.
- Often the cause if Na > 170
- High serum osmolality, low urine osmolality, polyuric
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- High salt intake
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- Will be hypervolaemic
- Usually the cause if Na > 190
- Can occur if you drink copious soy sauce
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- Mineralocorticoid excess
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- Cushings
- Primary hyperaldosteronism (mildly hypernatraemic)
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Clinical features of severely high Na (>160)
- Hyperthermia
- Delirium
- Seizures
- Reduced GCS
- Restlessness
- Neurological signs – muscle twitching, spasticity, hyper-reflexia
Investigations
- Bloods including CBG, bone profile, serum osmolality
- Consider checking renin, aldosterone, midnight cortisol
- Urine electrolytes, urea, osmolality
- Assess urine output, postural BP and volume status
- Assess medications (some cause AVP resistance or deficiency)
- Urine osmolality
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- Should be > 800
- If < 800 and isotonic consider diuretics and osmotic diuresis
- If very low (hypotonic) consider AVP deficiency
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- Calculate the free water deficit. This suggests how much fluid to give to reach a certain target Na.
- Can also calculate the rate of IV fluids you need to achieve ideal Na correction rate – this might be better
- CT and/or MRI head – if no clear cause and severely hypernatraemic
- If unclear on the cause, can calculate the osmolar gap of the urine
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- If the true osmolality = measured osmolality, then no gap, therefore no unmeasured osmole in the urine
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Management
- Fluid management
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- Chronic hypernatremia best treated with 5% dextrose
- Use a higher rate for the bigger the person, aiming about 100ml/hr in a 70kg person
- The osmolality of 5% dextrose is 275 but glucose gets metabolised quickly so effectively its lower (ie hypotonic)
- IV normal saline does not correct hypernatremia much but can be used if extracellular volume depletion with haemodynamic compromise
- Saline is useful in the short term if sodium is very high and you don’t want it to crash
- Can give 5% dextrose with diuretics if hypervolaemic
- In 1L of 0.45% saline, there is 500ml of free water
- In AVP deficiency – use 5% dextrose
- Any strategy probably ok as long as you are closely monitoring and adjusting plan as you go
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- Aim not to correct the sodium any faster than 0.5 / hour (unless HHS, then can correct more quickly) – risk of cerebral oedema.
- If dropping too fast – hold the fluids
- In HHS (more here) – normal saline can be given, can also use half strength 0.45% saline, and change to 5% dextrose once BM < 17