Disorders of sodium + water balance

Question 1

What is the most common electrolyte abnormality?

Answer 1

hyponatraemia

Question 2

What is the classification of hyponatraemia?

Answer 2

according to severity
130-135 = mild
125-129 = moderate
<125 = severe

Question 3

Clinical features of acute hyponatraemia (<48h)

Answer 3

seizures
coma
respiratory distress

Question 4

Clinical features of chronic hyponatraemia (>48h)

Answer 4

frequently mild or no symptoms
headache
restlessness
muscle cramps
nausea
vomiting
lethargy
confusion
disorientation

Question 5

What can acute hyponatraemia do to the brain?

Answer 5

brain oedema

Question 6

What can happen if you rapidly correct chronic hyponatraemia?

Answer 6

osmotic demyelination syndrome
(central pontine myelinolysis)

Question 7

What can cause isoosmolar or hyperosmolar hyponatraemia?

Answer 7

TURP and hysteroscopy (associated with absorption of irrigation solutions)

Question 8

What is pseudohyponatraemia?

Answer 8

an uncommonly encountered laboratory abnormality defined by a serum sodium concentration of less than 135 mEq/L in the setting of a normal serum osmolality (280 to 300 mOsm/kg)

Question 9

What is TURP syndrome?

Answer 9

This occurs when too much of the fluid used to wash the area around the prostate during the procedure is absorbed into the bloodstream
intravascular absorption of hypotonic irrigating fluids
causes acute volume overload –>pulmonary oedema

Question 10

How is TURP syndrome treated?

Answer 10

supportive care of cardiac and pulmonary functions
airway protection if indicated
fluid restriction
furosemide
hypertonic saline for severe hypernatraemia (<120)

Question 11

What test should be done if you suspect pseudohyponatraemia?

Answer 11

measure serum sodium concentration using direct potentiometry

Question 12

What drugs commonly cause hyponatraemia?

Answer 12

thiazide and thiazide-like diuretics
amiloride
carbamazepine
sulphonylureas (but not gliclazide)
PPIs
antidepressants, particularly SSRIs
ACE-is and ARBs
opiates

Question 13

In a hyponatraemic patient, a high urine osmolality is indicative of what?

Answer 13

SIADH

Question 14

Diagnostic criteria for SIADH

Answer 14

effective serum osmolality <275mOsm/kg
urine osmolality >100mOsm/kg
urine Na+ >30mmol/L with normal dietary salt and water intake
absence of adrenal, thyroid, pituitary or renal insufficiency
no recent use of diuretic agents

Question 15

What defines over correction of sodium?

Answer 15

> 8mEq/L in first 24h
18mEq/L in first 48h

Question 16

How do you treat excessive sodium correction?

Answer 16

IV desmopressin with dextrose

Question 17

Why do you assess hydration status in a hyponatraemic patient?

Answer 17

hyponatraemia is:
- hypovolaemic (dehydrated)
- hypervolaemic (overloaded)
- euvolaemic

Question 18

What are the 2 main overarching causes of hypovolaemic hyponatraemia, and how can you differentiate between them?

Answer 18

due to renal sodium loss
due to sodium loss elsewhere (extra-renal)

urine sodium >20mmol/L suggests renal cause - kidneys are not conserving sodium

urine sodium <20mmol/L suggests non-renal cause - kidney is conserving sodium but sodium is being lost from elsewhere

Question 19

Renal causes of hypovolaemic hyponatraemia

Answer 19

Renal failure (high creatinine, low eGFR)
Addison’s disease - associated hyperkalaemia
Excess diuretic medications
Osmotic diuresis - severe hyperglycaemia

Question 20

Non-renal causes of hypovolaemic hyponatraemia

Answer 20

GI losses - vomiting, diarrhoea, small bowel obstruction, fistulae

skin loss - excess sweating, burns

Question 21

Describe what hypervolaemic hyponatraemia is

Answer 21

in a fluid overloaded patient, fluid accumulates in the extracellular (3rd) space
extra fluid has dilutional effect on serum sodium
more water than sodium, so relative sodium deficiency

Question 22

Causes of hypervolaemic hyponatraemia

Answer 22

anything causing fluid overload

cardiac failure
liver cirrhosis (hypoalbuminaemia reduces oncotic pressure in bloodstream, causing fluid loss into 3rd space, portal hypertension also causes ascites)
end-stage renal failure
nephrotic syndrome - inappropriate loss of albumin through the glomerulus leads to hypoalbuminaemia, which decreases plasma oncotic pressure, and fluid accumulates in the extracellular space)

Question 23

Euvolaemic hyponatraemia causes

Answer 23

SIADH
primary polydipsia
severe endocrine disturbances (severe hypothyroidism/cortisol deficiency)

Question 24

What serum and urine osmolalities would be present in SIADH?

Answer 24

raised urine osmolality
low serum osmolality

kidney inappropriately producing concentrated urine despite low serum osmolality

Question 25

What should you test in euvolaemic hyponatraemia?

Answer 25

urine osmolality

Question 26

If urine osmolality is low in euvolaemic hyponatraemia, what could the causes be?

Answer 26

primary polydipsia (water intoxication) severe hypothyroidism glucocorticoid deficiency

Question 27

Signs of hypovolaemia

Answer 27

tachycardia hypotension dry mucous membranes reduced skin turgor

Question 28

Signs of hypervolaemia

Answer 28

peripheral oedema raised JVP bibasal lung field crepitations

Question 29

Neurological signs of severe hyponatraemia

Answer 29

cognitive impairment drowsiness signs of seizure activity/brainstem herniation (indicate cerebral oedema)

Question 30

Serum osmolality calculation

Answer 30

(2 x Na) + Glucose + Urea (all in mmol/L)

Question 31

What is pseudohyponatraemia and what can cause it?

Answer 31

Hyperlipidaemia, hyperproteinaemia or hyperbilirubinaemia (secondary to obstructive jaundice) can interrupt some laboratory analysis methods of measuring serum sodium, leading to falsely low readings

Question 32

What can you test to show it could be pseudohyponatraemia?

Answer 32

Serum osmolarity is classically normal if measured. Pseudohyponatraemia is rare but important to exclude before initiating treatment for hyponatraemia

Question 33

Describe hyperglycaemia associated hyponatraemia

Answer 33

In patients with significant hyperglycaemia (e.g. DKA / HHS), the increase in serum glucose raises serum tonicity, pulling water out of cells and expanding the extracellular space, causing a dilutional effect on serum sodium concentrations. In this case, total body sodium remains unchanged, and sodium concentration will generally correct with the correction of the hyperglycaemia. Calculated serum osmolality is classically raised.

Question 34

Acute severe hyponatraemia treatment

Answer 34

IV hypertonic saline bolus (100ml 3% NaCl) with close monitoring of serum sodium

Question 35

How is hyponatraemia without severe neurological symptoms treated?

Answer 35

In the absence of acute severe hyponatraemia, a cautious approach to management is taken. If the sodium is corrected too quickly, patients are at risk of severe complications (osmotic demyelination syndrome) correct by no more than 6mmol/L in the first 6 hours and no more than 10mmol/L in the first 24 hours Hypovolaemic hyponatraemia Rehydration with intravenous 0.9% normal saline, with regular monitoring of serum sodium. Hypervolaemic hyponatraemia Fluid restriction (<1.5L/24h), with regular monitoring of serum sodium. Euvolaemic hyponatraemia Fluid restriction (1.5L/24h), with regular monitoring of serum sodium. Oral salt tablets may be required if fluid restriction alone is ineffective.

Question 36

Hypovolaemic hyponatraemia with no neurological symptoms treatment

Answer 36

Rehydration with intravenous 0.9% normal saline, with regular monitoring of serum sodium.

Question 37

Hypervolaemic hyponatraemia with no neurological symptoms treatment

Answer 37

Fluid restriction (<1.5L/24h), with regular monitoring of serum sodium

Question 38

Euvolaemic hyponatraemia with no neurological symptoms treatment

Answer 38

Fluid restriction (1.5L/24h), with regular monitoring of serum sodium. Oral salt tablets may be required if fluid restriction alone is ineffective

Question 39

Complications of hyponatraemia

Answer 39

gait disturbance and falls cerebral oedema - can lead to brainstem herniation and death osmotic demyelination syndrome - typically presents with quadriplegia and pseudobulbar palsy

Question 40

How does osmotic demyelination syndrome typically present?

Answer 40

2-4 days after hyponatraemia treatment quadriplegia and pseudobulbar palsy