Fluids and Electrolytes

Question 1

What are the two commonest causes of hyponatraemia in neurosurgical patients?

Answer 1

1. SIADH;

2. CSW.

Question 2

What are the features of SIADH?

Answer 2

1. Dilutional hyponatraemia with normal or elevated intravascular volume;
2. Treated with fluid restriction.

Question 3

What are the features of CSW?

Answer 3

1. Inappropriate natriuresis with volume depletion;

2. Treated with volume and sodium replacement.

Question 4

What feature differentiates SIADH from CSW?

Answer 4

Volume status:

1. Clinical assessment of hydration;
2. Normal saline infusion test, if hyponatraemia corrects then dehydration was the cause;
3. CVP <5cm H2O suggests hypovolaemia;
4. Serum potassium decreased or normal in SIADH, and increased or normal in CSW.

Question 5

What are the symptoms of mild hyponatraemia?

Answer 5

1. Headache;
2. Anorexia;
3. Difficulty concentrating;
4. Irritability;
5. Muscle weakness;
6. Dysgeusia (disturbed taste).

Question 6

What are the symptoms of severe hyponatraemia?

Answer 6

1. Confusion;
2. Muscle twitching and cramps;
3. Nausea and vomiting;
4. Seizures;
5. Respiratory arrest;
6. Permanent neurological deficit;
7. Coma;
8. Death.

Question 7

What three diagnostic criteria are required for SIADH?

Answer 7

1. Hyponatraemia;
2. Inappropriately concentrated urine;
3. Normal renal and adrenal function.

Question 8

What is the risk of rapid correction of hyponatraemia?

Answer 8

Central pontine myelinolysis.

Question 9

What is the pathophysiological mechanism of CSW?

Answer 9

Renal loss of sodium as a result of intracranial disease producing hyponatraemia and decreased extracellular fluid volume from an unknown mechanism in which the kidneys fail to conserve sodium.

Question 10

What are the treatment goals of CSW?

Answer 10

1. Volume replacement;

2. Positive salt balance.

Question 11

What is the commonest cause of hypernatraemia in neurosurgical patients?

Answer 11

Diabetes insipidus.

Question 12

What is the mechanism of DI?

Answer 12

Low levels of ADH (or rarely due to renal insensitivity to ADH).

Question 13

What is the urine output in patients with DI? Serum osmolality and sodium?

Answer 13

High urine output of dilute urine (SG <200 mOsmol/L). Serum osmolality is normal or high, serum sodium is high.

Question 14

What percentage of ADH secretory capacity must be lost before DI ensues?

Answer 14

85%.

Question 15

What is the differential diagnosis of DI?

Answer 15

1. Neurogenic (true DI);
2. Nephrogenic;
3. Psychogenic (polydipsia);
4. Osmotic diuresis;
5. Diuretic use.