Normal range – 135-145 mEq/L

Signs and Symptoms:
less than 125mmol/L – nausea, confusion, restlessness, wide QRS
less than 120mmol/L – somnolence, headache
less than 115mmol/L – seizures, Vtach/Vfib, respiratory arrest, cerebral edema/herniation

General Implications – Decreases MAC, can cause delayed emergence or emergence delirium, stimulates RAAS and ADH release.

Cancellation – There is an increase in mortality secondary to coronary events, wound infection, pneumonia, etc. Since correction is done over several days, ELECTIVE cases should be cancelled for a patient exhibiting signs or symptoms of hyponatremia until properly treated.

Correction – In general, serum sodium is corrected 4-6 mEq/L/day (MAX 8 mEq/L/day). Hypertonic saline is rarely used because of the concern for increasing the serum sodium level too quickly and precipitating osmotic demyelination. Hypertonic saline is typically reserved for those patients who have a serum sodium concentration less than 120 mEq/L and severe symptoms (such as seizures) attributable to abrupt hyponatremia. Correction starts with evaluating volume status. Hypervolumic hyponatremia, the most common case, is treated by restricting both diuretics and free-water (drinking and IV). In rarer cases, if the patient is hypovolemic (as well as hyponatremic), suspension of diuretics, careful volume resuscitation, and finding the root cause for hypovolemia (eg poor oral intake, diarrhea, etc.) should be the priority. In any case, correction of hypokalemia is important because this can result in improvement in serum sodium levels.

Osmotic Demyelination Syndrome (ODS) – The primary concern of sodium correction is that rapid increase can cause osmotic demyelination syndrome (ODS), leading to central pontine myelinolysis, which can cause seizures in the short term. ODS rates are highest in chronic and severely hyponatremic patients. Even after the hyponatremia is corrected, dysarthria, dysphagia, oculomotor dysfunction, and quadriparesis can develop. In fact, death or disability follows ~ 77% of those that develop ODS.

Sodium is the principal extracellular cation that maintains osmolarity, extracellular volume, and action potentials.

Hyponatremia Etiology

Hypovolemic:
Urine sodium less than 10mEq/L – dehydration (vomiting, diarrhea, sweating)
Urine sodium greater than 20mEq/L – diuretics, ketonuria, 3rd spacing (burns), cerebral salt wasting (e.g. subarachnoid hemorrhage), Addison’s disease

Isovolemic – renal failure, SIADH (reduced plasma osmolarity from dilution leads to increased urine osmo/sodium excretion), DDAVP treatment (e.g. Von Willebrand disease), hypothyroid myxedema, stress/pain, porphyria, adrenal insufficiency

Hypervolemic – congestive heart failure (CHF), liver failure, nephrotic syndrome, ESRD