A Quick rundown of AFP's " Management of Hyponatremia "

A synopsis of: Management of Hyponatremia 
KIAN PENG GOH, M.R.C.P., Alexandra Hospital, Singapore
http://www.aafp.org/afp/2004/0515/p2387.pdf
Am Fam Physician. 2004 May 15;69(10):2387-2394.

     I came across a bunch of questions about hyponatremia while studying, so I figured I would see what AFP has to offer on the subject. So in the immortal words or Eddie Vedder, "and away we go!"
     Hyponatremia is diagnosed when the plasma Na drops below 135 mEq/L. Neurologic and gastrointestinal symptoms (such as headache and nausea) occur when it drops below 120 mEq/L.  There are three classes of hyponatremia: hypervolemic, hypovolemic, and euvolemic. 
     Hypervolemic hyponatremia is an excess of total body water, causing edema. The common causes are CHF, liver cirrhosis, and renal disease. These causes should be apparent from the history and physical.  The BUN/creatinine will be prerenal (>20). Hypovolemic hyponatremia means that the patient is volume depleted and sodium depleted. Euvolemic hyponatremia bus means that the total body volume is normal and the sodium is low. To differentiate between the hypo- and euvo- you need to figure out the plasma osmolarity and the urine sodium concentration.
     Patients with hyponatremia and normal osmolarity (280-300) may have pseudohyponatremia or post-TURP syndrome.  Pseudohyponatremia occurs when the relative concentration or sodium does not change, but  "other" particles (protein, fats, etc) skew the percentages. The post-TURP syndrome cause of hyponatremia is still uncertain. 
     Hyponatremic patients with increased osmolarity can be from hypoglycemia. An increase in blood glucose draws water from the intracellular space, thus diluting the sodium concentration. When the glucose gets to the kidney tubules, it creates an osmotic diuresis and hypovolemia. 
    If the patient is hyponatremic with a decreased plasma osmolarity (<280), then the urine sodium concentration is needed to determine the cause. A high urine sodium here is due to renal, endocrine, SIADH, drugs, or medications. Common renal disorders here are polycystic kidney disease, pyelonephritis, ischemia, and renal artery stenosis. Endocrine disorders include addison's, hypothyroidism, and reset osmostat syndrome. When the urine concentration of sodium is low, consider vomiting (as the cause, don't actually consider vomiting), diarrhea, burns or water overload (but I think some of these causes,  like burns, should be pretty obvious...)
     The two most common medications that cause hyponatremia are diuretics and SSRIs. Most medications will cause euvolemic hyponatremia, like in SIADH. 
     Treatment can be broken down into urgent or non-urgent  Patients with neurologic symptoms need to be treated urgently to avoid cerebral edema or encephalopathy. They would be given hypertonic saline at 1-2 mmol/l/hr with a return to baseline to take 48 hours (slowly!!). Non-urgent treatment focuses on correcting the underlying cause. Fluid restriction (1-1.5L/day) should help as well. Demeclocycline can be used in some situations of severe, persistent hyponatremia. 
    So in summary, when you have a hyponatremic patient, check the BUN/cr, if it is less than 20, then check the plasma osmolarity, If it is low, then check urine Na concentration.