Introduction
Hyponatraemia is a serum sodium level of less than 135 millimoles per litre. It is condition that occurs when the level of sodium in the blood is too low.
Plasma Na+ < 135mmol/L = Hyponatraemia
Aetiology
There are different types of hyponatraemia with varied aetiologies
Pseudo-hyponatraemia:
- With normal plasma osmolality as seen in hyperlipidaemia or hyper proteinaemia
- With increased plasma osmolality as seen in hyperglycaemia, infusion of mannitol
Hypo-osmolar hyponatraemia:
- Due to a primary water gain and secondarybsodium loss, or a primary sodium loss and secondary water gain
- Integumentary loss: sweating, burns
- Loss from the GIT: vomiting, tube drainage, fistula
- Renal loss: diuretics, hypoaldosteronism, salt wasting neuropathy, obstructive diuresis
- Primary polydypsia
- Cardiac failure
- Hepatic cirrhosis
- Nephritic syndrome
- Decreased solute intake: SIADH (Syndrome of inappropriate antidiuretic hormone secretion)
- Glucocorticoid deficiency
- Hypothyroidism
- Chronic renal insufficiency
Clinical features
- Cerebral oedema.
- May be asymptomatic
- Otherwise nausea, malaise, headache, lethargy, confusion, and altered consciousness
- Coma when plasma sodium is less than 120 millimoles per litre
Differential diagnoses
- Congestive cardiac failure
- Hepatic cirrhosis
- Nephritic syndrome
Investigations
Directed at establishing the cause and severity of hyponatraemia
Treatment objectives
- To correct plasma sodium concentration by
restricting water intake and promoting water loss - To correct the underlying disorder
Management
- Mild asymptomatic hyponatraemia requires no treatment
- Mild hyponatraemia with ECF volume contraction: Sodium releption with isotonic saline infusion
- Hyponatraemia associated oedematous states:
- Restriction of both sodium and water intake
- Promotion of water loss in excess of sodium by use of a loop diuretic
- For severe cases which are symptomatic (plasma sodium concentration <115 mmoles/L):
- Hypertonic saline to raise sodium concentration by 1-2 mmol/L/hour for the first 3 hours, but not more than 12 mmoles/L during the first 24 hours
Calculation of the total amount of sodium to administer
Amount of sodium = (desired concentration – actual concentration) × body weight X 0.6