ChemPath - Sodium and Fluid Balance

Question 1

What is the most common electrolyte abnormality in hospitalised patients

Answer 1

Hyponatraemia (25%)

Question 2

How is water regulated

Answer 2

ADH
1. ADH synthesised in the hypothalamus
2. Secreted from the posterior pituitary
3. Acts on the V1 receptors in the collecting duct in the kidney
4. Insertion of aquaporin 2 (AQA2) into the collecting duct
5. Increased water resorption (NOT sodium → hyponatraemia)

+ ADH acts on V1 receptors in vascular smooth muscle → vasoconstriction

Question 3

What are the stimuli for ADH secretion and what is it mediated by

Answer 3

Serum osmolality (high) – mediated by hypothalamic osmoreceptors
Blood volume/pressure (low) – mediated by baroreceptors in carotids, atria, aorta

Question 4

What is the initial assessment for a patient who is hyponatraemic

Answer 4

Assessment of serum osmolality - is it true hyponatraemia?

Clinical assessment of volume status (hypo-, euvo-, or hypervolaemic)

• Check pulse
• JVP
• BP
• Skin turgor
• Signs of oedema
• Mental state
• Urine output

+ Urine electrolytes

Assessment of serum osmolality - is it true hyponatraemia?

Question 5

Why does hypovolaemic hyponatramia occur (mechanism)

Answer 5

There is a loss of water AND sodium

hypovolaemia → baroreceptors detect the loss in volume → ADH secretion → reabsorption of water → dilution → hyponatraemia (more sodium than water loss)

Question 6

What are the causes of hypovolaemic hyponatraemia

Answer 6

Anything that causes loss of both water and sodium
urine Na >20: renal causes
Diuretics
Salt-losing nephropathy
Addison’s

urine Na <20: non-renal
Diarrhoea and vomiting
Excess sweating
Third space loss (ascites, burns)

Question 7

What are the clinical signs of hypovolaemia

Answer 7

Tachycardia
Postural hypotension
Dry mucous membranes
Reduced skin turgor
Confusion/drowsiness
Reduced urine output

Question 8

What is the best investigation to detect hyponatraemia in hypovolaemia

Answer 8

Urine sodium (<20) (due to larger loss of water which dilutes the urine)

Question 9

What are the causes of euvolaemic hyponatraemia

Answer 9

SIADH (AQA2 insertion → water retention → increased volume → RAAS suppression → less aldosterone → reduced Na absorption
Hypothyroidism (→ reduced contractility → reduced BP → ADH release)
Adrenal insufficiency (→ less aldosterone → less Na+ reabsorption)

Urine sodium always >20

Question 10

What are the causes of SIADH

Answer 10

CNS pathology – stroke, haemorrhage, tumour
Lung pathology – small cell lung cancer, pneumonia (Legionella), pneumothorax
Drugs – SSRI, TCA, PPI, carbamazepine, opiates
Tumours - small cell, pancreas, prostate, lymphoma
Surgery

Question 11

What investigations should be ordered in someone with euvolaemic hyponatraemia

Answer 11

Hypothyroidism → thyroid function tests
Adrenal insufficiency → short SynACTHen test
SIADH → plasma and urine osmolality → low plasma and high urine osmolality

Question 12

How is SIADH diagnosed

Answer 12

True hyponatraemia <135
Reduced plasma osmolality (resorbing lots of water) <270
Increased urine osmolality >100
High urine sodium >20
No hypovolaemia (euvolaemia)
No hypothyroidism
No adrenal insufficiency

Question 13

What are the clinical signs of hypervolaemia hyponatraemia

Answer 13

Raised JVP
Bilateral crackles
Peripheral oedema

Question 14

What are the causes of hypervolaemic hyponatraemia

Answer 14

Urine Na >20: renal
AKI
CKD
Renal failure → not excreting enough water, not retaining sodium

Urine Na <20: non-renal
Cardiac failure → low pressure → detected by baroreceptors → ADH release
Cirrhosis → vasodilated due to excess NO → low BP → baroreceptors → ADH releasea

Question 15

What is the management for hypovolaemic hyponatraemia

Answer 15

Volume replacement with 0.9% saline - SLOWLY and check Na regularly
Treat the underlying cause

Question 16

What is the management for euvolaemic hyponatraemia

Answer 16

Fluid restriction (<750ml/day)
Treat underlying cause

Question 17

What is the management for hypervolaemic hyponatraemia

Answer 17

Fluid restriction (<750ml/day)
Treat underlying cause

Question 18

What medications can be used to treat SIADH when fluid restriction is insufficient

Answer 18

Demeclocycline → induce nephrogenic diabetes insipidus
- Reduces responsiveness of collecting tubule cells to ADH
- Monitor U&Es as risk of nephrotoxicity

Tolvaptan – V2 receptor antagonist

Question 19

What are the signs of severe hyponatraemia and how is it treated (+ the consideration for this treatment)

Answer 19

Reduced GCS
Seizures

Management: hypertonic 3% saline

If sodium is corrected too quickly → central pontine myelinolysis (osmotic demyelination)
Signs/symptoms: Quadriplegia, dysarthria, dysphagia, seizures, coma, death

Question 20

What are the hypovolaemic causes of hypernatraemia

Answer 20

Urine sodium <20
GI: D&V
Skin loss: excessive sweating, burns

Urine sodium >20
Loop diuretics
Osmotic diuresis e.g. uncontrolled DM, glucose, mannitol
Diabetes insipidus
Renal disease e.g. renal artery stenosis

Question 21

Why does diabetes insipidus cause hypernatraemia

Answer 21

insensitivity to/lack of ADH → water loss → hypovolaemia → Na resorption to compensate → hypovolaemia hypernatraemia

Question 22

What is nephrogenic DI and what are the causes

Answer 22

Receptor defector → insensitivity to ADH

Hypercalcaemia
Hypokalaemia
Lithium
Sickle cell

Question 23

What investigations should be done for suspected diabetes insipidus

Answer 23

Serum glucose – exclude diabetes mellitus → osmotic diuresis
Serum potassium – exclude hypokalaemia → nephrogenic DI
Serum calcium – exclude hypercalcaemia → nephrogenic DI
Plasma and urine osmolality – exclude hyperaldosteronism (high plasma osmolality, low urine osmolality, U:P ratio <2)
Water deprivation test (normal = concentrated urine, no ADH = carry on passing water – dilute urine)

Question 24

What is the management for hypernatraemia

Answer 24

Fluid replacement → 5% dextrose (if the patient is also hypovolemic, then 0.9% saline and 5% dextrose water)
Treat underlying cause

Question 25

What is the normal range for sodium and at what level is it dangerous

Answer 25

135-145

<125 AND Symptomatic is dangerous (asymptomatic - rarely an emergency)

Question 26

What are the Signs and symptoms of hyponatraemia

Answer 26

Nausea and vomiting
Confusion
Seizures, non-cardiogenic pulmonary oedema
Coma

Question 27

What are the causes of an untrue hyponatraemia

Answer 27

High osmolality: glucose (HHS)/mannitol
Normal: spurious, drip arm sample, pseudohyponatraemia (hyperlipidaemia, paraproteinaemia)

Question 28

What is TURP syndrome

Answer 28

Hyponatraemia from irrigation absorbed through damaged prostate
Glycine 1.5% used to irrigate during TURP
Clinical presentation due to metabolism of glycine and hyponatraemia caused by dilution

Question 29

What are the causes of euvolaemic hypernatraemia

Answer 29

Respiratory - tachypnoea
Skin - sweating, fever
Diabetes insipidus

Question 30

What are the causes of hypervolaemic hypernatraemia

Answer 30

Conn’s syndrome
Cushing’s syndrome
Innappropriate saline

Question 31

What is the management for hypernatraemia

Answer 31

Slow fluids
Encourage PO fluids

Question 32

What are the signs and symptoms of diabetes insipidus

Answer 32

Hypernatraemia: lethargy, thirst, irritability, confusion, coma, fits
Clinically euvolaemic
Polyuria and polydipsia
Plasma osmolality <2, urine osmolality low

Question 33

What is cranial diabetes insipidus and what are the causes

Answer 33

Cranial: lack of ADH production
Causes: surgery, trauma, craniopharyngioma
Management: desmopressin

Question 34

What is the management for diabetes insipidus

Answer 34

cranial: desmopressin
Nephrogenic: thiazide diuretics