Hyponatremia

Question 1

What is hyponatremia?

Answer 1

Serum sodium concentration of <135 mmol/L.

It is a disorder of water balance reflected by an excess of total body water relative to electrolytes (total body sodium and potassium) leading to low plasma osmolality (i.e., <275 mmol/kg).

Question 2

Briefly describe what causes hyponatremia

Answer 2

Hyponatraemia is generally caused by an increase in renal water reabsorption due to release of vasopressin (arginine vasopressin also known as antidiuretic hormone) along with water intake, and can occur in situations of volume depletion, volume overload, or normal volume.

Question 3

What are the 3 different types of hyponatremia?

Answer 3

1. Hypovolaemia
2. Euovolemia
3. Hypervolaemia

Question 4

Briefly describe hypovolaemic hyponatremia

Answer 4

Loss of intravascular volume.

Causes:

• Gastrointestinal fluid loss (e.g. severe diarrhoea or vomiting)
• Third spacing of fluids (e.g. pancreatitis, severe hypoalbuminaemia)
• Addison’s Disease
• Salt-wasting nephropathy
• Cerebral salt-wasting syndrome (a rare cause of hyponatraemia resulting from urinary salt wasting)
• Mineralocorticoid deficiency

Question 5

Briefly describe euvolaemic hyponatremia

Answer 5

Normal volume status.

Causes:

• Medications (e.g. vasopressin, diuretics, antidepressants, opioids)
• SIADH
• High fluid intake: can result from intense/prolonged physical activity or surgery
• Hypothyroidism

Question 6

Briefly describe hypervolaemic hyponatremia and what causes this

Answer 6

Loss of effective intravascular volume.

Causes:

• Acute kidney injury/chronic kidney disease
• Congestive heart failure
• Cirrhosis
• Nephrotic syndrome

Question 7

What is the most common form of hyponatremia?

Answer 7

Hypotonic (hypo-osmolar) hyponatraemia.

Question 8

What are the risk factors of hyponatremia?

Answer 8

• Older age
• Hospitalisation
• SSRI use
• Thiazide diuretic use
• Underlying medical conditions
  
  • Congestive heart failure
  • Cirrhosis
  • Nephrotic syndrome
  • Acute kidney injury
  • Chronic kidney disease

Question 9

What are the signs of hyponatremia?

Answer 9

• Signs of cerebral oedema→ altered mental status, seizures and coma
• Orthostatic hypotension
• Abnormal JVP
• Poor skin turgor
• Dry mucous membranes
• Absence of axillary sweat
• Oedema
• Rales or crackles on lung ausculation

Question 10

What are the symptoms of hyponatremia?

Answer 10

• High fluid intake
• Fluid losses
• Low urine output
• Weight changes
• Mild cognitive symptoms→ include confusion, headache and balance difficulties

Question 11

Why can hyponatremia lead to seizures?

Answer 11

Osmolality inside the brain cells remains higher than the serum and water enters the brain cells causing cerebral oedema. This leads to symptoms including nausea, vomiting, altered mental status, and eventually seizures and/or brain herniation and death.

Question 12

What investigations should be ordered for hyponatremia?

Answer 12

• Serum sodium concentration
• Serum electrolytes, urea, creatinine and glucose
• Serum osmolality
• Urine sodium concentration

Question 13

Why investigate serum sodium concentration?

Answer 13

Should be ordered in all patients with suspected hyponatraemia.

A serum sodium concentration <135 mmol/L (corrected for hyperglycaemia) confirms the presence of hyponatraemia.

Question 14

Why investigate serum electrolytes, urea, creatinine and glucose?

Answer 14

Should be ordered in all patients with suspected hyponatraemia. May reveal other electrolyte abnormalities or renal impairment.

May show mixed electrolyte abnormalities.

Question 15

Why investigate serum osmolality?

Answer 15

Serum osmolality can be used to differentiate between hypotonic and hypertonic hyponatraemia.

Variable:

• Serum osmolality <275 mmol/kg: indicates hypotonic hyponatraemia
• Serum osmolality >295 mmol/kg: indicates hypertonic hyponatraemia
• Serum osmolality normal: indicates isotonic hyponatraemia (pseudohyponatraemia)

Question 16

Why investigate urine sodium concentration?

Answer 16

Ordered to confirm the presence of hypovolaemia or euvolaemia. A spot urine test allows urinary sodium concentration to be measured quickly and conveniently

Question 17

Briefly describe the treatment principles of treating hyponatremia

Answer 17

The management of hyponatraemia depends primarily on whether the onset is acute (i.e., <48 hours) or chronic (≥48 hours).

This is because acute onset indicates the likelihood of cerebral oedema, which requires prompt treatment in a hospital. Chronic hyponatraemia should be managed more conservatively to avoid the consequences of rapid correction (e.g., myelinolysis); specific treatment depends on the volume status of the patient.

Question 18

Briefly describe the treatment of acute onset (<48 hours) or symptomatic hyponatremia

Answer 18

1. Hypertonic (3%) saline infusion
2. Supportive care (e.g. establishing intravenous access, giving supplemental oxygen, seizure control, intubation) should be initiated depending on the presentation
3. Treat underlying cause

Question 19

Briefly describe the treatment for hypovolaemic hyponatremia

Answer 19

1. Isotonic intravenous fluids (e.g. normal saline 0.9% or a balanced solution such as lactated Ringer’s solution)
2. Treat underlying cause

Question 20

Briefly describe the treatment for hypervolemic hyponatremia

Answer 20

1. Fluid restriction
2. Treat underlying cause
3. A diuretic can be added if hypervolaemia is present and the underlying condition warrants it
4. If fluid restriction fails, a ADH receptor antagonist should be considered

Question 21

Briefly describe the treatment for euvolemic hyponatremia

Answer 21

1. Fluid restriction
2. Treat underlying cause
3. If fluid restriction fails, a ADH receptor antagonist should be considered (e.g. tolvaptan, deomeclocycline)

Question 22

What are the complications of hyponatremia?

Answer 22

• Cerebral oedema
• Myelinolysis
• Osteoporosis

Question 23

How should over correction of serum sodium (via treatment) be treated?

Answer 23

All active treatment should be stopped and treatment with free water intake and/or desmopressin should be initiated.

Question 24

Cerebral oedema and myelinolysis can occur due to rapid sodium correction. How can this be prevented?

Answer 24

Slow rate of serum sodium correction (i.e., <8 mmol/L/day).

Question 25

What differentials should be considered for hyponatremia?

Answer 25

• Hypotonic replacement of excess fluid loss
• Drug induced
• Renal failure

Question 26

How does hyponatremia and hypotonic replacement of excess fluid loss differ?

Answer 26

Differentiating signs and symptoms:

• History of excessive sweating, vomiting, diarrhoea, GI fistulas or drainage tubes, or third spacing of fluids (peritonitis, pancreatitis, burns, small bowel obstruction) and fluid replacement by tap water or hypotonic intravenous fluids.
• Clinical signs of volume depletion: decreased skin turgor, reduced jugular venous pressure, decreased blood pressure; small bowel obstruction: abdominal distension; peritonitis: rebound abdominal tenderness; cutaneous burns.

Differentiating investigations:

• Urine sodium level: ≤20 mmol/L
• Serum osmolality: <280 mmol/kg H₂O

Question 27

How does hyponatremia and drug-induced differ?

Answer 27

Differentiating signs and symptoms:

• History of use of thiazide diuretics, vasopressin, non-steroidal anti-inflammatory drugs, nicotine, chlorpropamide, carbamazepine, tricyclic antidepressants, SSRIs, vincristine, thioridazine, cyclophosphamide, clofibrate, mannitol.
  
  • Usually normal.

Differentiating investigations:

• Trial of discontinuation of causative medication→ hyponatraemia resolves.

Question 28

How does hyponatremia and renal failure differ?

Answer 28

Differentiating signs and symptoms:

• Patient with chronic renal failure
• Hypervolaemic with elevated jugular venous pressure, peripheral oedema; signs of renal failure: jaundice, skin bruising, lung rales, pericardial rub, oedema, poor concentration/memory, myoclonus

Differentiating investigations:

• Urine sodium level: ≤20 mmol/L
• Serum osmolality: <280 mmol/kg H₂O
• Serum creatinine: elevated
• Urinalysis: haematuria and/or proteinuria

Question 29

How should an unwell patient (e.g. seizures or coma) with hyponatremia be treated?

Answer 29

If the patient is unwell due to their hyponatraemia (e.g. having seizures, or is comatose) then hypertonic (3%) saline can be given.

Question 30

Why is correcting sodium very quickly dangerous?

Answer 30

Correcting sodium faster than 12mmol/L/day leads to a significant risk of central pontine myelinosis because of fluid shifts.