Sunday, November 15, 2009

Diagnosing Reset Osmostat

The "reset osmostat" is a cause of hyponatremia, sometimes considered a variant of SIADH, in which the kidney retains its ability to appropriately concentrate and dilute the urine; however, the threshold for ADH secretion is reset downward. That is, instead of ADH being secreted with the serum osmolality increases beyond 280-285 mOsm/kg as in most individuals, it is secreted at a lower value.

Diagnosing reset osmostat is a diagnosis of exclusion. Individuals must be euvolemic, and a thorough exclusion of other causes of euvolemic hyponatremia (e.g., hypothyroidism, cortisol deficiency, medications, etc) must take place. A key feature of reset osmostat is that individuals should be able to concentrate and dilute the urine appropriately. Thus, a water challenge should result in a dilute urine (e.g., less than 100 mOsm/kg) and a water deprivation test should result in a concentrated urine. Sometimes, a patient given a diagnosis of SIADH will be proven to be reset osmostat when it becomes apparent that fluid restriction does not successfully raise the serum sodium level.

Reset osmostat classically occurs in neurologic conditions such as epilepsy and paraplegia, in addition to pregnancy, malignancy, and malnutrition. It has also been observed in healthy individuals, such as this 60 year-old man with a chronic sodium level between 125-130 mmol/L; the authors suggest that a 1951 grenade explosion the patient experienced may have caused the osmostat to reset!

2 comments:

Anonymous said...

question: in hyponatremia: how can you determine if the Uosm are appropriate to the serum osm?

Anonymous said...

Is the reset permanent? Or can it again be reset, and if so how? Mine seems to have reset at a value between 120 and 125. When it gets higher I have excessive thirst and major edema! It was recently discovered to be critically low, 115, at a time when I was having bowel trouble and unable to eat for a week. All major causes have been ruled out. So it I usually function fine at this reset number, does it have to be corrected? Do I only need to take caution when I am I'll?
Thank you!
Cm