Sodium Flashcards
What is the normal range of sodium in the blood?
135-145mmol/l
What percentage of body sodium is freely exchangeable? Where is the rest?
70%
Rest is complexed in bone
Below what value should hyponatraemia be treated
125mmol/l and symptomatic
What are the symptoms of hyponatraemia?
Nausea, vomiting, confusion
< 125mmol/l seizures, non-cardiogenic pulmonary oedema
<117mmol/l coma and eventual death
What causes apparent hyponatraemia with high osmolality?
High glucose, mannitol infusion
What causes apparent hyponatraemia with normal osmolality?
Hyperlipidaemia, paraproteinaemia, sample taken from arm receiving IV fluids
What is a true hyponatraemia?
Hyponatraemia with low osmolality
Name three causes of hypervolaemic hyponatraemia
Heart failure, liver failure, kidney failure
How is hypervolaemic hyponatraemia treated?
Fluid restriction, correct cause
Name three causes of euvolaemic hyponatraemia
Hypothyroidism, glucocorticoid insufficiency, SIADH, primary polydipsia
What investigations are required with euvolaemic hyponatraemia?
TFTs, short synACTHen test, paired urine and serum osmolality
How is hypovolaemic hyponatraemia managed?
Fluid restoration with 5% dextrose.
Name three causes of hypovolaemic hyponatraemia with >20mmol/l urinary sodium
Diuretics, Addison’s disease, salt-wasting nephropathies
Name three causes of hypovolaemic hyponatraemia with <20mmol/l urinary sodium
Vomiting, diarrhoea, excess sweating, ascites, burns
How does cirrhosis cause hyponatraemia?
Poor breakdown of vasodilators (e.g. nitric oxide) leads to hypotension, stimulating ADH release and causing water retention and dilution of serum sodium
What is the danger of correcting hyponatraemia too fast?
Central pontine myelinolysis
What is the biggest risk factor for central pontine myelinolysis?
Alcoholism
What are the features of central pontine myelinolysis?
Pseudobulbar palsy, paraparesis, locked-in syndrome
State the laboratory criteria for diagnosing SIADH
True hyponatraemia (low serum osmolality), euvolaemia, inappropriately high urine osmolality, increased renal sodium excretion >20mmol/l, normal 9am cortisol and TFTs
State at least 5 causes of SIADH
Malignancy, meningoencephalitis, CNS abscess, CNS haemorrhage, TB, pneumonia, lung abscess, drugs
State at least 3 drug classes which can cause SIADH
Opiates, SSRIs, carbamazepine, proton pump inhibitors
Which malignancy most commonly causes SIADH?
Small cell lung cancer
State at least 3 causes of hypovolaemic hypernatraemia
Diarrhoea, vomiting, excessive sweating, burns, loop diuretics, osmotic diuresis following initial hyponatraemia
State at least 3 causes of euvolaemic hypernatraemia
Tachypnoea, sweating, fever, diabetes insipidus
State 2 causes of hypervolaemic hypernatraemia
Mineralocorticoid excess (Conn’s syndrome), inappropriate IV saline
How is hypernatraemia managed?
Slow fluids - oral if possible, if not IV dextrose or Hartmann’s
What are the clinical features of diabetes insipidus?
Polyuria, polydipsia, lethargy, thirst, irritability, coma, seizures
What is the urine: plasma osmolality in diabetes insipidus?
<2
State 3 causes of cranial diabetes insipidus
Brain surgery, head trauma, brain tumours
State 3 causes of nephrogenic diabetes insipidus
Inherited channelopathies, lithium, demeclocycline, hypercalcaemia
What is a normal response in an 8h fluid deprivation test?
Urine concentration increases to >600mOsmol/kg
What result in an 8h fluid deprivation test suggests primary polydipsia?
Urine concentration increases to <400-600mOsmol/kg
What result in an 8h fluid deprivation test suggests cranial diabetes insipidus?
Urine concentrates only after giving desmopressin
What result in an 8h fluid deprivation test suggests nephrogenic diabetes insipidus?
Low concentration urine even after desmopressin
