DB’s Medical Rants

This patient has water intoxication, likely beer potomania.

This is how I would work this patient’s hyponatremia up.

Step 1: check measureed serum osmolarity ( not calculated). If it is low, as in this case, then the patient has hypoosmolar hyponatremia ( by far the most common cause). If the measured serum osmolarity is either elevated or normal then the patient either has hyperosmolar hyponatremia ( e.g. hyperglycemia , mannitol) or pseudohyponatremia (markedly elevated serum total cholesterol, triglycerides or total protein- is a lab measurement error). So this patient has hypoosmolar hyponatremia and therefore would move to step 2

Step 2- check the urine osmolarity. If it less than 100 mosm/Kg then the urine is maximally dilute. A dilute urine implies that there is no serum ADH present, which is appropriate in the face of hyponatremia. In this case the urine osmolarity is dilute and the work up is complete. The DDX for hypoosmolar hyponatremia and a dilute ( urine osmolarity less than 100) is short and includes psychogenic polydipsia, beer potomania and tea and toast diet. So what is the difference between these? They are all essesntially the same. Will explain:

How to estimate the maximum amount of fluid that can be consumed in one day before becoming hyponatremic? What are the osmoles made of in the urine osmolarity measurement? Answer: urea ( derived from dietary protein) and sodium. A normal diet consists of enough protein and salt to produce 750 mosm per day of solute. If a patient has normal renal function and can dilute the urine to a minimum of 50 mosm/Kg then the patient can drink 15 liters of water and not become hyponatremic. If the patient drinks a 16th liter then there are no longer any osms left to excrete that 16th liter and the patient becomes hyponatremic. ( each liter of urine contains 50 mosm and after 15 liters of urine output all 750 mosms have been used up).

Take this patient. If she drinks mostly beer then her diet consists of mostly carbohydrates and little sodium and protein. This patient’s diet may only contain 200 mosm solute per day. If she can dilute her urine to 50 mosm/L then she can drink 4 liters of fluid per day and not become hyponatremic. If she drinks a 5th liter then she retains that liter and becomes hyponatremic. Note, this is a simplification. I did not account for insensible losses.

If the urine osmolarity was greater than 100 than the patient’s urine is concentrated and ADH is present- either appropriately ( e.g.,hypovolemia) or inappropriately ( e.g.SIADH)

Step 3- if in step 2 urine is greater than 100 then patient must be categorized into hypoosmolar hypervolemic hyponatremia, hypoosmolar euvolemic hyponatremia or hypoosmolar hypovolemic hyponatrermia

Use H&P, blood tests and urine tests to sort it out. Also stop HCTZ which does not fit into one category easily.

Hypervolemic- CHF, cirrhosos, esrd, nephrosis- all should be obvious on exam and by history. Beware the urine Na may be low here due to effective volume depletion ( don’t be fooled into thinking the patient has hypovolemia)

Hypovolemic vs euvolemic- by exam and also labs

hypovolemic
urine Na 40 textbook answer ( my experience usually greater than 100)
BUN/Cr 10:1
serum uric acid is low

Note, in the presense of metabolic alkalosis , use the urine Cl to assess volume status, not the urine Na. Check the urine pH. If it is greater than 7.0 then the urine Na may be unreliable in determining volume status and need to rely on urine Cl-

it is not posting correctly

hypovolemic should read as euvolemic

hypovolemic should read:
urina Na <20
BUN/CR ratio elevated