What is hyponatremia?
Hyponatremia is defined as a sodium level less than 135 mg/dl. Although it is a sodium level, it is generally interpreted to mean that there is an excess of free water in the serum relative to the usual level of sodium. This is a key concept as we go forward in the work-up.
Hyponatremia And Serum Osmolarity
The first question to ask yourself is: Is this a true hyponatremia? In other words, is there truly an imbalance between the ratio of sodium to free water. Sometimes, such as in diabetic ketoacidosis, there is an influx of another osmolar substance into the blood, namely glucose. The extra osmoles cause a shift of water into the intravascular space, thus artificially depressing the serum sodium concentration. In these situations, the serum osmolarity is high, and the hyponatremia will resolve automatically once the other osmoles are cleared. To calculate what the true sodium level would be in the face of hyperglycemia, take the current level, and add 1.6 times the glucose level minus 100 divided by a hundred. So, if the sodium level is read as 120, but the glucose level 1100, then the true sodium level is 120 + 1.6 * (1100 - 100)/100 = 120 + 16 = 136, or a normal sodium level.
Hyponatremia And Volume Status
If the serum osmolarity is normal, then this is likely a true hyponatremia, so next consider volume status. If the patient is dehydrated, then the low sodium is likely due to the compensatory response of ADH, and the treatment is to gently rehydrate the patient using normal saline. However, if the patient is volume overloaded and edematous, think about causes such as cirrhosis, renal failure, or congestive heart failure and treat those as needed. If the patient has normal volume status, they might have syndrome of inappropriate antidiuretic hormone (SIADH) or something more esoteric like psychogenic polydipsia or beer potomania. In this situation, the treatment is to free water restrict the patient. If the diagnosis is SIADH, you should also try to investigate the cause of the SIADH and treat that as well.
There may be more rare causes of hyponatremia that require more specific work-up, but the general treatment for most causes is described above. When rehydrating with normal saline, remember to rehydrate slowly with no more than 0.5 mg/dl increase in sodium per hour. If you go faster, you risk causing central pontine myelinosis and locked-in syndrome. However, if the patient is having neurologic symptoms due to the hyponatremia, then it is okay to use 3% saline because the goal at that point is simply to get the sodium level up and stop the neurologic problems. At the end of the day, always keep in mind that the sodium level truly represents the body's free water status.
If you want to learn more specifics about hyponatremia and renal function in general, check out Renal Pathophysiology: The Essentials by Rennke/Denker:
If you want a practical guide to handling common problems on the medicine wards, I would recommend Pocket Medicine: The Massachusetts General Hospital Handbook of Internal Medicine by Sabatine: