SODIUM DISORDERS

Question 1

Imbalances in sodium affect which organ primarily?

Answer 1

The brain

Question 2

What are the three diagnostic tests in hyponatraemia?

Answer 2

• Plasma osmolality
• Urinary sodium
• Urinary osmolality

Question 3

Equation for plasma osmolality

Answer 3

Calculated osmolarity = 2 Na + Glucose + Urea (all in mmol/L)

To calculate plasma osmolality use the following equation (typical in US):

= 2[Na+] + [Glucose]/18 + [BUN]/2.8[7] where [Glucose] and [BUN] are measured in mg/dL

Question 4

What would you expect plasma osmolality to be in low sodium states? What are 2 key exceptions?

Answer 4

Low

​Hyponatraemia with high osmolality can occur in hyperglycaemia and mannitol use as substances both osmoles

Question 5

First step of evaluating hyponatraemia of unknown cause?

Answer 5

Checking plasma osmolality

Question 6

If ADH level is always high what is controlling sodium balance?

Answer 6

Sodium level will be determined by the amount of free water that is taken in. Increased intake of free water will easily result in hyponatraemia in these people

Question 7

In the setting of hyponatraemia how do you expect normally function kidneys to respond?

Answer 7

Urine should be dilute: Low osmolality, low urine sodium

Indicates that cause of low sodium lies outside kidneys (ADH). Conversely, if concentrated this indicates problem with kidneys

Question 8

What are the four main categories of causes of hyponatraemia?

Answer 8

1. Heart failure, cirrhosis
2. Ineffective kidneys
3. High ADH
4. Psychogenic polydipsia/diet

Question 9

Perceived hypovolaemia causes hyponatraemia in which two conditions? What causes the perceived hypovolaemia in each condition?

Answer 9

• Heart failure: reduced cardiac output causes ADH release
• Cirrhosis: vasodilation + reduced systemic vascular resistance causes ADH release

BOTH CONDITIONS CAUSE NON-OSMOTIC ADH RELEASE

Question 10

In someone with a hyponatraemia who is on a loop or thiazide diuretic, how do you manage?

Answer 10

Hold and see if sodium normalises

Question 11

Why is the tendency for hyponatraemia much higher in patients taking thiazide diuretics versus loop?

Answer 11

• Loops diminish medullary osmolality and thus the drive to re-absorb water in collecting duct
• Thiazides do not diminish medullary osmolality therefore water is more likely to be reabsorbed from collecting duct → causing hyponatraemia

Question 12

How does cortisol affect ADH levels?

Answer 12

Suppresses ADH release, therefore adrenal insufficiency will cause high ADH → hyponatraemia

Question 13

How does hypothyroidism affect ADH levels?

Answer 13

Increases ADH → low-sodium

Question 14

In a psychiatric patient with hyponatraemia and low urine osmolality what condition should you consider? How would you treat?

Answer 14

Psychogenic polydipsia: need to drink more than 18L per day

fluid restriction

Question 15

Why does a low intake of sodium cause hyponatraemia? What type of patients does this affect?

Answer 15

Urine has minimum osmolality needed to allow for excretion of free water, therefore some sodium must be excreted to allow water excretion.

Alcoholics: bear potomania, tea and toast diet

Question 16

What can overly rapid correction of sodium cause? How slowly should you correct sodium?

Answer 16

Central pontine myelinolysis

Demyelination of central pontine axons causing loss of corticospinal and corticobulbar tracts → quadriplegia, similar to locked in syndrome

No more than 10 mmol/day

Question 17

What are the two broad causes of hypernatraemia?

Answer 17

1. Water loss from skin, lungs: more water loss than sodium
2. Diabetes insipidus: loss of ADH activity

Question 18

What are the two main symptoms of diabetes insipidus?

Answer 18

Polydipsia, polyuria

Question 19

How do you diagnose diabetes insipidus? How can you differentiate between the two types of diabetes insipidus?

Answer 19

Fluid restrict for 8 hours: urine should be concentrated. If it remains dilute this indicates absent or ineffective ADH

• Administer vasopressin or desmopressin
• Should concentrate urine if kidneys work
• If no concentration → nephrogenic DI, Kidneys cannot respond to ADH
• If concentration → central DI, Loss of ADH release from pituitary

Question 20

What are two treatments of nephrogenic diabetes insipidus? How does each work?

Answer 20

• Thiazide diuretics: cause mild volume depletion resulting in increase in proximal sodium and water resumption → less water to collecting duct resulting in paradoxical antidiuretic affect
• NSAIDs: inhibit renal synthesis of prostaglandins, which are ADH antagonists

Question 21

Rapid correction of high sodium results in which condition?

Answer 21

Cerebral oedema, correct no more than 12 mmol per litre per day

Question 22

What condition should you consider in a patient who is euvolaemic and has low sodium levels with concentrated urine?

Answer 22

SIADH