Hyponatremia is defined by the presence of a serum sodium <135 mEq/L. When evaluating low sodium levels, total body water (TBW) must be taken into account as low sodium is almost always due to either increased TBW or loss of sodium in excess of TBW. Measuring the plasma osmolality allows one to divide the possible causes into those with low (<285 mOsm/kg), normal (285-295 mOsm/kg) and high (>295 mOsm/kg) plasma osmolality. Those found to be in the normal and high categories represent a minority of the cases. These include cases of hyperlipidemia, hyperproteinemia and excess glycine or other isotonic solutions [normal osmolality], as well as hyperglycemia and mannitol administration [high osmolality].
Further breaking down those with a low plasma osmolality can be done by assessing the extracellular volume status and urine sodium levels . Those with decreased extracellular volume are broken down into renal [urine Na>20 mEq/L] or extrarenal losses [urine Na<10 mEq/L] (primarily GI). Those with normal extracellular volume (lack of edema or hypovolemia) are primarily due to adrenal insufficiency, thyroid deficiency, medication and SIADH. Those with increased extracellular volume (edema on exam; water retention>sodium retention) occur secondary to congestive heart failure, cirrhosis, nephrotic syndrome and renal failure.
Hyponatremia primarily manifests with neurological symptoms. Severity is often related to whether it is an acute or chronic process as well as the degree of hyponatremia, with Na+>125 mEq/L often being asymptomatic. Treatment involves both restoration of plasma Na+ as well as treating the underlying cause.