Hyponatraemia

Causes and features of true hyponatraemia (serum osmolality is < 280 mOsmol/kg) with abnormal water excretion (urine osmolality > 100 mOsmol/kg)

 MRCP  renal

SIADHhypovolaemiarenal salt wasting
clinical cluesmass on CXR, known drug causesigns of oedema / dehydration, heart failure, chronic liver disease, nephrotic syndrome
urine sodium> 20 mmol/L (often > 40 mmol/L)< 40 mmol/L> 40 mmol/L
normal saline challenge< 5 mmol/L change in urine sodium> 5 mmol/L change in urine sodium
uric acidlownormal or slightly increasedlow
fractional excretion of urateraised (normalises with sodium correction)raised (does not normalise with sodium correction)
renin / aldosteronenormal or lowraised *renin (normal or raised), aldosterone raised
extracellular volumehigh normal or raisedreduced *reduced
random card | random quiz

Hypothyroidism and adrenal insufficiency need to be excluded. Also, if water excretion is normal (urine osmolality < 100 mOsmol/kg) then primary polydipsia may be the cause.

Normal saline challenge is 2 L/day of IV normal saline in those cases with indeterminate urinary sodium (20-40 mmol/L).

* Renin / aldosterone levels and extracellular volume (ECV) are not usually measured in hypovolaemia as the cause should be obvious. However, they may help differentiate SIADH from renal salt wasting.

Sources:

Milionis HJ, Liamis GL, Elisaf MS. The hyponatremic patient: a systematic approach to laboratory diagnosis. CMAJ 2002 Apr;166(8):1056–1062. Available from: http://www.ncbi.nlm.nih.gov/pmc/articles/PMC100882/

O'Donoghue D, Trehan A. SIADH and hyponatraemia: foreword. NDT Plus 2009 Nov;2(suppl_3):iii1–iii4. Available from: http://www.ncbi.nlm.nih.gov/pmc/articles/PMC2762825/

Maesaka JK, Miyawaki N, Palaia T, Fishbane S, Durham JHC. Renal salt wasting without cerebral disease: Diagnostic value of urate determinations in hyponatremia. Kidney International 2007;71(8):822–826. Available from: http://www.nature.com/ki/journal/v71/n8/full/5002093a.html



related

Renal tubular acidosis types | Hyperparathyroidism classification | Jaundice | Granulomatous mycobacterial infections of the skin | Endocarditis | Cutaneous infestations | Eponymous cardiac murmurs | Inflammatory bowel disease | Oligomenorrhoea and amenorrhoea | Renin-angiotensin-aldosterone system | Emphysema | Motor defects | Valvular heart disease | Familial hyperlipidaemias |