Diagnosing Hyponatremia!

Report this morning at ZSFG was inspiring. The case was super interesting – a woman who presented with hyponatremia which was found to be 2/2 LONG undiagnosed (like 30 year) hypo pit from postpartum pituitary hemorrhage.
Chief confession time
I think hyponatremia is really hard. In report today, a room full of super wise residents, medical students and Lisa Murphy helped develop a clear, comprehensible hyponatremia algorithm. Here is my deep, philosophical conclusion, that will also help you use this algorithm
  • Most diagnoses are simple with history, exam, and 3 lab tests. A small portion are wicked hard to diagnose.
    • Simple: this patient has been vomiting for a week is on a thiazide. The diagnosis is diuretics + hypovolemia. The patient has pneumonia and has been drinking plenty of water. The diagnosis is SIADH. In these situations, you can use the algorithm below to make confirm your diagnosis and ensure nothing else is going on.
    • Wicked hard: The numbers don’t fall into any of the above buckets, are in a grey zone, the patient is obtunded or you thought you knew the answer but the patient isn’t getting better.
    • GI bleeds are this way too! Most of them are clear from history and basic physical exam. Then there’s the occasional small bowel angioectasia that has you banging your head against the wall
  • Volume status
    • While basing your hyponatremia algorithm on volume status is laudable, in reality, assessing volume status is hard! So this trips people up a lot.
    • See below for a volume status-free algorithm.
Simplified hyponatremia algorithm
  • adapted from Miliones HJ, Liamis GL, Elisa MS. The hyponatremic patient: a systematic approach to laboratory diagnosis. CMAJ 2002; 166 (8): 1056-62. Chart made with Lucidchart.
  • A caveat: none of these branch points are true 100% of the time. The lower down you get on the algorithm, the less accurate it is, like the Brugada criteria. Uric acid levels are particularly iffy – they are most sensitive for SIADH when they are <4
  • The uric acid pearl, is one of VA LT’s favorites. ADH promotes Urea/uric acid excretion along with water retention.
hyponatremia! - Page 1-6
Tips for wicked hard hyponatremia
  • The algorithm in the Miliones article above which has more detail, explains what to do with intermediate values, and gives some specific suggestions for testing
  • Make a hypothesis, try a treatment, and recheck the big 4 – serum sodium, urine sodium, serum osms, urine osms.
  • Renal and endocrine can help you if you’re lost!
Managing hyponatremia is super important and pearls for another time. Search the archives for tons and tons of other hyponatremia pearls.
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