Hyponatremia and psoas abscess

Last week we discussed a patient with chronic liver disease secondary to alcohol presenting with gram-negative bacteremia, hyponatremia, and a psoas abscess. We focused mostly on the diagnosis and management of hyponatremia.

Hyponatremia: there are a few questions you must ask yourself:

1. Is it real?

• Think of other osmoles - glucose, mannitol, paraproteins

2. Is the urine reponse appropriate - ie what is the Urine Osmolality?

• If low, the urine is appropriately dilute
• If > 100, it is inappropriately concentrated - there must be ADH on board

3. What is the patients volume status?

• Determine if hypo-, iso-, or hypervolemic
• Physical exam is helpful in the extremes, but interobserver reliability is low.
• In the absence of diuretics, Urine Na can be helful - less than 20 suggests the kidneys are avidly reabsorbing Na, so the patient is either hypovolemic or has low effective circulating volume but hypervolemic. The physical exam is helpful in differentiating these states.
• If the patient is on a diuretic, the fractional excretion of urea can be used (value >35% is euvolemic). Defined as Fractional Excretion of Urea (FEUrea) = (Serum_Cr * U_Urea ) / (Serum_Urea x U_Cr) %.

Once you have answered these three questions, you can accurately classify their hyponatremia as hypovolemic (renal or extra-renal losses, the urine Na can help determine), euvolemic (SIADH based on your urine Osm, adrenal insufficiency, hypothyroidism, or primary polydipsia/low solute diet if the urine is appropriately dilute), or hypervolemic (ie liver, cardiac, or renal failure).

In managing hyponatremia, it is essential to avoid overly rapid correction which can cause osmotic demyelination syndrome. Treatment modality will depend on the patients volume status. There are very few indications for hypertonic saline - essentially only if the patient is seizing and you think it's related to their sodium. Remember to monitor the sodium level frequently as well as the urine output - once you have volume repleted the patient, the stimulus for ADH gets 'shut off', resulting in sudden outpouring of dilute urine which can cause a sudden (too quick!) rise in your serum sodium.

Here is a brief review of hyponatremia.

If you are interested in learning more about psoas absecesses, take a look at this review.