Hyponatraemia

Differentials

Syndrome of Inappropriate ADH secretion (SIADH)
- Medications: SSRIs, TCAs, carbamazepine, cyclophosphamide, vincristine, antipsychotics
- Malignancy: small cell lung carcinoma, pancreatic, prostate, leukaemia, cervical cancers
Hypothyroidism
Secondary adrenal insufficiency
Water intoxication (e.g., psychogenic polydipsia)
“Beer potomania” (low solute intake)

Severity of symptoms is related to the rapidity and extent of sodium decline.

Serum Na⁺ > 125 mmol/L
- Typically asymptomatic
Serum Na⁺ 115–125 mmol/L
- Lethargy
- Confusion
- Anorexia
- Nausea and vomiting
Serum Na⁺ < 115 mmol/L
- Muscle cramps and weakness
- Seizures
- Coma

Identify potential underlying causes
- History of fluid losses (e.g. vomiting, diarrhoea, burns)
- History of excessive fluid intake (e.g. psychogenic polydipsia)
- Medication history (e.g. diuretics, SSRIs, antiepileptics)
- History of chronic disease (e.g. cirrhosis, heart failure, malignancy)
Assess volume status
- Hypovolaemia: dry mucous membranes, hypotension, tachycardia
- Euvolaemia: normal volume status (e.g. in SIADH)
- Hypervolaemia: peripheral oedema, ascites, raised JVP
Evaluate neurological status
- Confusion, decreased GCS, focal deficits, seizures

Cerebral oedema
- Secondary to abrupt sodium losses and free water shift from vascular to interstitial space
ECG changes
- Cause of non-ischaemic ST elevation on ECG
Pontine demyelinosis (no clear evidence that associated with rapid correction)
- Develops 3-5 days after treatment
- Demyelination of central pons, corticobulbar and corticospinal tracts
- Altered mental state, pseudobulbar palsies
- Dysphasia and spastic quadriparesis
- More likely in chronic hyponatraemia

Depends on rapidity of onset and clinical symptoms

Hypovolaemic hyponatraemia
- may respond to intravenous sodium chloride 0.9%, with potassium supplements if required.
Hypervolaemic hyponatraemia
- who are oedematous due to heart, liver or kidney failure may respond to fluid restriction.
- A loop diuretic may be added as long as the effective intravascular volume is not depleted further
Euvolaemic hyponatraemia

is based on:

the presence of central nervous system symptoms (unconsciousness, seizure, drowsiness, headache)
the severity
the rate of development.

Mild to moderate in euvolaemic or hypervolaemic patients (serum sodium concentration from 120 to 135 mmol/L, no cerebral symptoms)
- fluid restriction
  - (eg restrict to 500 mL to 1 litre per 24 hours, or 500 mL less than daily urine output)
  - Monitor serum electrolytes, creatinine and urine output daily or twice daily
Severe Hyponatremia
- serum sodium concentration lower than 120 mmol/L OR
- with cerebral symptoms
- Treat with
  - IV sodium chloride 3% (513 mmol/L)
- The initial target serum Na should NOT be higher than 120 mmol/L
- Correction should be:
  - not more than 10 mmol/L in the first 24 hours
  - not more than 18 mmol/L in the first 48 hours.

Calculations:

Correction

In acute severe hyponatraemia, aim for 1-2mEq/hour correction
In chronic severe hyponatraemia aim for 0.5-1mEq/hour correction.
Hypertonic saline replacement
- 3% saline (513mEq/L) by giving (deficit/513) to the patient at the rate of 1mEq/hour over 4 hours

More rapid initial correction can be considered in patients with:
- seizures or coma, regardless of whether the hyponatraemia is known to be chronic
- self-induced acute water intoxication (eg psychiatric conditions, endurance exercise)
- known hyponatraemia for less than 24 to 48 hours
- intracranial pathology or increased intracranial pressure