Sodium - Hypernatraemia
- Hypernatraemia can be classified as:
- Acute (<48hrs) or more commonly Chronic (>48hrs)
- mild (146 – 149 mmol/L)
- moderate (150-169 mmol/L)
- severe (>170 mmol/L)
- Sodium makes up most of the effective osmoles in the extracellularly fluid, therefore hypernatraemia (serum sodium level > 145mmol/L) indicates hypertonicity and a decrease in cell volume.
- The commonest cause of hypernatraemia is water depletion
The commonest cause of hypernatraemia is water depletion due to water being unavailable (the patient unable to communicate the need such as paediatric patients or dementia patients), or the urge to drink is impaired (hypothalamic dysfunction). Other causes include abnormally large unreplaced water losses, such as diabetes insipidus (DI), a large sodium gain such as iatrogenic use of hypertonic sodium solutions or hypotonic fluid loss (dehydration and hypovolaemia) such as with burns, excessive sweating, or GI and urinary losses.
Diuretics (ie thiazides, indapamide)
(a) central – lack ADH
(b) nephrogenic - resistance to vasopressin (AVP)
Sodium load (oral)
History and examination
The signs and symptoms usually relate to CNS dysfunction:
- Lethargy, weakness, confusion, irritability, abnormal speech, myoclonic jerks, hyperreflexia, spasticity, seizures, and coma.
Dehydration and hypovolemia:
- Dry mouth, abnormal skin turgor, oliguria, tachycardia, orthostatic hypotension.
- Thirst, polydipsia, and polyuria.
- Weight loss, generalised weakness, fever.
- Serum osmolality (along with urine osmolality in suspected DI)
- Serum electrolytes (Na+, K+, Ca2+).
- Glucose level.
- Lithium levels (hypernatraemia reduces lithium elimination and increases the risk of toxicity).
- Urine electrolytes (Na+, K+).
- Urine osmolality.
- Urinary Na.
- Assess the severity of hypernatraemia,
- Confirm the true result (exclude pseudohyponatraemia from hypoproteinaemic states)
- Assess the patient's clinical condition and treat underlying disorders
- Correct dehydration by replacing free water losses.
- Correct hypovolemia, if present, by administering electrolytes in addition to free water.
** most cases hypernatraemia are chronic (>48hrs), in specific cases of acute (<48hrs) hypernatraemia (i.e. diabetes insipidus may develop hypernatraemia rapidly if access to water is restricted and desmopressin therapy is withheld) you may need to speed up rate of correction fluid replacement. See up to date hypernatraemia.
How to determine fluid requirements:
Calculate water deficit - Water deficit = 0.6 x premorbid weight x [1 – 140/serum Na+]
NB: the above formula assumes total body water of 60%. Mdcalc will more accurately account for the varieties that exist with fat content of the body, age, and sex.
- Resuscitation fluids - Hartmann's or 0.9% normal saline (NS) boluses.
- Correct water deficit - ½ NS or Dextrose, oral fluids (over 48 hours)
- Stop ongoing losses.
- Treat the underlying cause.
Rate of correction:
To avoid complications of cerebral oedema, lower the Na:
- no faster than 0.5mmol/L per hour
- not more than 10mmol/L in 24hrs
Consider consultation with critical care services if
Neurological symptoms present (Lethargy, weakness, confusion, irritability, abnormal speech, myoclonic jerks, hyperreflexia, spasticity, seizures, and coma)