Sodium disorders

Question 1

What is hyponatraemia?

Answer 1

Low sodium serum concentration

Question 2

What are the two ways that hyponatraemia can occur?

Answer 2

• Depletional hyponatraemia - salt loss
• Dilutional hyponatraemia - fluid overload

Question 3

What is osmolality?

Answer 3

A measure of the overall number of solute particles in a fluid.

Question 4

How can osmolality be estimated?

Answer 4

Serum osmolality (mmol/kg) =

2x serum [sodium] + [urea] + [glucose] (mmol/l)

Question 5

What level of serum osmolality would normally reflect hyponatraemia?

Answer 5

Low serum osmolality

Question 6

What causes hyponatraemia with high serum osmolality?

Answer 6

• Hyperglycaemia
• Alcohol
• Mannitol

Question 7

What causes a test result of hyponataemia with normal serum osmolality?

Answer 7

Pseudohyponatraemia - serum sodium concentration actually normal

(abnormal test result can be a result of lipaemia or hyperproteinaemia)

Question 8

How would you assess whether hyponatraemia is due to salt loss or fluid overload?

Answer 8

Assess ECF volume clinically

• High - oedema, elevated JVP, lung crakles
  
  • fluid overload
• Low - significant postural BP drop, mucus membranes dry
  
  • salt loss

Question 9

What causes hyponatraemia with high ECF volume (fluid overload)?

Answer 9

• Congestive cardiac failure
• Liver failure
• Nephrotic syndrome

Question 10

What test would you do if a patient is hyponatraemic with low ECF volume?

Answer 10

Urine sodium

• Urine sodium low
  
  • D&V
• Urine sodium high
  
  • Diuretics
  • Adrenal insufficiency
  • CSWS (cerebral salt wasting syndrome)
  • Salt wasting nephropathy

Question 11

What test would you do if a patient was hyponatraemic with normal ECF volume?

Answer 11

Urine osmolality

• Urine osmolality high
  
  • SIADH (syndrome of inappropriate anti-diuretic hormone)
• Urine osmolality low
  
  • water intoxication

Question 12

How does water intoxiacation occur?

Answer 12

• rare
• water intake overcomes bodys ability to excrete free water
• for a healthy adult with normal renal function it is very difficult to drink yourself hyponatraemic
• psychogenic polydipsia? - patient compulsively drinks

Question 13

Pathophysiology of SIADH

Answer 13

• often caused by illness
• ↑ ADH
• ↑ water absorption
• dilutional hyponatraemia

Question 14

Criteria for diagnosis of SIADH

Answer 14

• Hyponatramia with hypo-osmolarity (<275 mmol/kg)
• Inappropriate urinary concentration (urine osmolality >100 mmol/kg)
• Elevated urinary sodium (>20mmol/L) (except during sodium and water secretion)
• Absence of clinical evidence of volume depletion or overload
• Normal renal function
• Absence of hypothyroidism, glucocorticoid deficiency and recent diuretic therapy

Question 15

Causes of SIADH

Answer 15

• Neoplasia
  
  • bronchial carcinoma
  • lymphoma
  • pancreatic cancer
  • mesothelioma
• Respiratory
  
  • pneumonia
  • tuberculosis
  • lung abscess
• Neurological
  
  • Head injury
  • Meningitis
  • Subdural haematoma
  • Subarachnoid haemorrhage
  • Neurosurgery
• Drugs
  
  • Carbamazepine
  • Cyclophosphamide
  • Ecstasy
  • NSAIDs
  • TCAs
  • Phenothiazines
  • SSRIs

Question 16

Management fo SIADH

Answer 16

• Treat underlying cause if possible
• If asymptomatic
  
  • fluid restriction to 1L/day
  • can consider tolvaptan in certain cases
• If symptomatic and Na <115
  
  • can consider hypertonic saline with furosemide to prevent circulatory overload, but need to avoid raising sodium too rapidly in view of risk of central pontine myelinolysis