Sodium - Hypernatraemia
Hypernatraemia is defined as serum Na>150mmol l-1. There is essentially a relative deficiency of Total Body Water (TBW) compared to Na+. There is a high mortality (up to 50%). When managing these patients volume status and whether it is acute or chronic are key aspects.
Patients developing hypernatremia in the community are generally elderly and debilitated, and often present with an intercurrent acute (febrile) illness. The root cause is then inadequate oral intake which can be from many causes ranging from decreased mobility of stroke or injury to altered level of consciousness. Increased fluid losses (e.g diuretics, diarrhoea, fever and vomiting). Diabetes insipidus and lithium toxicity are other causes.
Causes of hypernatraemia
Characteristics and symptoms of hypernatraemia
|Characteristics of hypernatremia||Symptoms related to the characteristics of hypernatremia|
|Cognitive dysfunction and symptoms associated with neuronal cell shrinkage||Lethargy, obtundation, confusion, abnormal speech, irritability, seizures, nystagmus, myoclonic jerks|
|Dehydration or clinical signs of volume depletion||Orthostatic blood pressure changes, tachycardia, oliguria, dry oral mucosa, abnormal skin turgor, dry axillae|
|Other clinical findings||Weight loss, generalised weakness|
Table Source: Lukitsch, I et al. (2014) 'Hypernatraemia Clinical Presentation' Medscape.
Signs manifest changes in serum osmolality. Brain shrinkage secondary to free water loss.
>350 Excessive thirst
>375 Weakness and lethargy. Irritability
>400 Ataxia, tremor
>420 Focal neurological deficit; Hyperreflexia and Spasticity
>430 Coma and seizures
Source: Life In The Fast Lane - Hypernatraemia
Your investigation strategy is aimed at identifying the pathophysiological process and any underlying pathology. Use these with your clinical estimation of volume status.
Serum electrolytes (Na+, K+, Ca2+).
Urine electrolytes (Na+, K+).
Urine and plasma osmolality.
Urine Na should be
Urine Na >20 mEq/L and isotonic or hypoyeonic urine indicates renal fluid losses such as diuretics, osmotic losses (DM) and renal disease.
Nonrenal causes with appropriately high urine osmolality - Isolated hypodipsia, increased insensible losses.
Renal water loss indicated by inappropriately low urine osmolality - Diabetes insipidus (often Uosm 2 O (central, nephrogenic, partial, gestational diabetes insipidus).
Note: Maximum Uosm in an elderly patient may be only 500-700mOsm/kg.
DDAVP test is not generally an ED test. Get an increase in urine osmolality of greater than 50% reliably indicates central diabetes insipidus, while an increase of less than 10% indicates nephrogenic diabetes insipidus; responses between 10% and 50% are indeterminate.
The management principles are the same as for all ED patients in terms of immediate resuscitation and the ongoing management often being dependent on many other factors.
Hypovolaemic patients: Water deficit is 0.5 times weight (serum Na- 140/140).
Resuscitate - Hartmanns or NS small boluses
Correct water deficit - ½ NS or Dextrose, oral fluids (over 48 hours)
Stop ongoing losses - Treat as per cause
Treat the underlying cause - Treat as per cause
To avoid complications of cerebral oedema lower the Na no faster than 0.5mmol/L per hour. Note this hard to do without serial sampling as there is great variation when management is based on calculations.
If sugars high then follow management of HHS.
In euvolaemic and hypervolaemic patients the management is variable depending on the cause. Initially manage on resuscitation principles and seek advice from senior ED and referral partners.
Further References and Resources
Lukitsch, I et al. (2014) 'Hypernatraemia Clinical Presentation' Medscape.
Life In The Fast Lane - Hypernatraemia
NSW Health Guideline - Infants and Children: Management of Acute Gastroenteritis (4th Edition, December 2014)
Tintinalli's Emergency Medicine: A Comprehensive Study Guide, Seventh Edition (Book and DVD).