Hypernatraemia

Question 1

Normal sodium concentration?

Answer 1

135-145mmol/l
Hypernatraemia > 145mmol/l
Symptoms > 160mmol/l

Question 2

What are causes for hyponatraemia?

Answer 2

Hypovolaemic hyponataemia:
Diuretics (loop)
Dehydration/fluid restriction - diarrhoea, vomiting, burns, excessive sweating
Acute tubular necrosis - due to early polyuric stage
Hyperosmolar states - hyperosmolar hyperglycaemic state

Euvolaemic hypernatraemia
Diabetes insipidus

Hypervolaemic hypernatraemia
Excessive hypertonic saline administration
Steroid excess - Conn’s/Cushing’s

Question 3

What is diabetes insipidus? Types? Presentation? Diagnosis?

Answer 3

Excessive (5-20L) excretion of dilute urine and an increased thirst response
Classified as:
Cranial due to impairment of ADH secretion from the posterior pituitary often after pituitary surgery or head trauma
Nephrogenic - imparted response of renal tubules to ADH

Patients present with polyuria and compensatory polydipsia
Confirm diagnosis with water deprivation test - patient deprived of fluids for up to 8 hours following which desmopressin is given:
Normal: urine osmolarity >600mOsm before desmopressive test
Cranial DI : urine osmolarity increases >600mOsm after desmopressive test
Nephrogenic: urine osmolality does not increase after desmopressin test

Question 4

What are clinical features of hypernatraemia?

Answer 4

Asymptomatic
Excessive thirst

Severe: weakness, lethargy, irritability, confusion, coma seizures

Na>200mmol/l neurological deficits - ataxia, tremor, coma, seizures

Question 5

What investigations for hypernatraemia?

Answer 5

metabolic bloods - glucose, potassium, chloride, urea and creatinine, blood gas for acid base

Urine osmolality can aid in diagnosis - any recorded hypertonic urine is seen with extra-renal fluid loss such as vomiting, burns. Isotonic urine can be seen with diuretic use, osmotic diuresis. Hypotonic urine is associated with polyuria from DI
(normal kidney response to hypernatraemia is to excrete minimal amount of urine that is maximally concentrated (>800mmol/kg)

ADH levels/CT head to assess pituitary

Question 6

What values of sodium concentration indicate cause?

Answer 6

150-170 volume depletion
>170 diabetes insipidus
>190 exogenous sodium gain

Question 7

What is management of hypernatraemia?

Answer 7

Replace any fluid deficit and correct serum sodium
Correcting sodium too rapidly can cause cerebral oedema - aim to lower sodium by 10mmol/L./day

When replacing fluid enteral free water replacement is preferred where possible including via NG tube if needed

If enteral intake is not possible - IV fluid includes 5% dextrose, 0.9% saline (if volume depletion) or 0.45% saline or Hartmann’s - adjust rates and fluid composition depending on sodium levels and fluid status

Question 8

What is the target aim to lower serum sodium in hypernatraemia? why?

Answer 8

10mmol/l/day

Rapid lowering can cause cerebral oedema