Salt and water balance

Question 1

What are the potential causes of hypernatraemia?

Answer 1

1. Dehydration:
   - inadequate water intake
   - diabetes insipidus - either cranial or nephrogenic in origin
   - thirst impairment (dementia, hypothalamic lesions)
2. Hypotonic fluid loss (dehydration + hypovolaemia):
   - dermal losses (burns, excessive sweating)
   - GI losses (diarrhoea, vomiting, NG drains, fistulas)
   - Urinary losses (loop diuretics, osmotic diuretics, acute tubular necrosis)
3. Hypertonic sodium gain
   - Iatrogenic:
   (i) Use of hypertonic saline
   (ii) Tube feeding
   (iii) IV abx with sodium
   (iv) IV sodium bicarbonate
   (v) Hypertonic dialysis
   (vi) Use of isotonic saline to replace losses
   - Excess salt ingestion:
   (i) Inadvertent
   (ii) Poisoning (rare)- Hyperaldosteronism
4. Intracellular shift of water (rare)

Question 2

What are the different categories of hyponatraemia?

Answer 2

1. Hypovolaemic
2. Euvolaemic
3. Hypervolaemic

Question 3

What are the causes of hypovolaemic hyponatraemia?

Answer 3

1. Vomiting
2. Diarrhoea
3. Skin losses (sweat, burns)
4. Adrenocortical deficiency
5. Renal failure/disease
6. Diuretics
7. Cerebral salt wasting

Question 4

What are the causes of euvolaemic hyponatraemia?

Answer 4

1. Acute water load
2. Psychogenic polydipsia
3. Poor diet
4. SIADH
5. Glucocorticoid deficiency
6. Severe hypothyroidism
7. Chronic water load

Question 5

What are the causes of hypervolaemic hyponatraemia?

Answer 5

1. CCF
2. Cirrhosis
3. Nephrotic syndrome
4. Primary polydipsia
5. Renal failure or disease

Question 6

What are the potential causes of SIADH?

Answer 6

1. Neurological:
   - Tumour
   - Trauma
   - Infection
   - SLE
2. Pulmonary:
   - Small-cell lung cancer
   - Pneumonia
   - TB
   - Asthma
3. Malignancy:
   - Oropharyngeal
   - Stomach
   - Pancreas
   - Lymphoma
4. Drugs:
   - SSRIs
   - Thiazide diuretics
   - ACE-Is
   - PPIs
5. Miscellaneous:
   - Idiopathic
   - Hereditary
   - Post-op

Question 7

Describe the classification of causes of DI on the basis of water deprivation and DDAVP response.

Answer 7

• Urine osmolality after fluid deprivation <300
• Urine osmolality after DDAVP >800
  → likely diagnosis = Cranial DI
• Urine osmolality after fluid deprivation <300
• Urine osmolality after DDAVP <300
  → likely diagnosis = Nephrogenic DI
• Urine osmolality after fluid deprivation >800
• Urine osmolality after DDAVP >800
  → likely diagnosis = Primary/psychogenic polydipsia
• Urine osmolality after fluid deprivation <300
• Urine osmolality after DDAVP >800
  → likely diagnosis = Partially cranial DI or nephrogenic or PP or diuretic abuse

Question 8

What is diabetes insipidus?

Answer 8

• condition cause by hyposecretion or, or insensitivity to the effects of, ADH
• its deficiency or failure to act causes an inability to concentrate urine in the distal renal tubules, leading to the passage of copious volumes of dilute urine
• usually the person with DI passes >3L/24h of low osmolality (<300 mOsmol/kg) urine

Question 9

What are the 2 major forms of diabetes insipidus?

Answer 9

1. Cranial DI: decreased secretion of ADH
   - decreased secretion of ADH reduces the ability to concentratre urine and so causes polyuria and polydipsia
2. Nephrogenic DI: decreases ability to concentrate urine because of resistance of ADH in the kidney

Question 10

What is psychogenic polydipsia?

Answer 10

• compulsive water drinking, irrespective of osmotic status of plasma and urine
• malfunction of the hypothalamic thirst centre
• more common in people with mental illnesses (particularly schizophrenia)

Question 11

How should hypernatraemia be managed?

Answer 11

1. treatment aims:
   - treat any underlying disorder
   - correct dehydration by replacing free water losses
   - correct hypovolaemia (if present) by giving electrolytes
2. management:
   - hypovolaemic: use normal saline to restore circulating volume
   - hypervolaemic: diuretics and 5% dextrose to offload fluid and provide free water
   - concurrent loss of renal function ± serum sodium is elevated: haemodialysis or filtration
   - otherwise: hypotonic fluids (0.45% saline, 5% dextrose, oral water)

Give fluids IV as a last resort

Question 12

How should hyponatraemia be managed?

Answer 12

1. Correct underlying cause: stop diuretic
2. Hypovolaemic:
   - IV saline replacement
   - as euvolaemic is regained, ADH is suppressed and the resulting diuresis may elevate Na+ levels too quickly → if thise happens, desmopressin may be used
3. Euvolaemic:
   - Fluid restrict (500-1000mL/day for adults)
   - If inadequate: demeclocycline → blocks ADH and induces partial nephrogenic DI
   - If above is inadequate: vasopressin receptor antagonists → expensive and rarely used (unless absolutely necessary in an emergency)
4. Hypervolaemic:
   - Underlying cause should be treated - heart failure/renal injury/liver failure
   - Loop diuretics
   - Fluid restrict