Is defined as [Na] < 135 mmol/L. Symptoms include nausea, vomiting, confusion, lethargy, and disorientation. When [Na] < 120 mmol/L, seizures, central herniation, coma, and even death may occur.
It is important to note that low sodium concentration is not indicative of low total-body sodium content. There are three types of hyponatremia: euvolemic, hypovolemic, and hypervolemic. Unofficially, there can also be pseudohyponatremia where an increase in the concentration of other solutes cause the sodium concentration to appear lower.
Hypovolemia from renal causes (1’ adrenal insufficiency, hypoaldosteronism) and non-renal ones (GI loss, dehydration) can cause hyponatremia. Such hypovolemia will cause activation of the RAA, sympathetic, and ADH systems. Increased ADH will lead to retention of water and hyponatremia. Treatment is crystalloid solution.
Hypervolemic hyponatremia is associated with fluid-overloaded disorders such as CHF, cirrhosis, and nephrosis. Treatment is correction of the underlying cause.
Euvolemic hyponatremia is from SIADH, hypothroidism, and 2’ adrenal insufficiency. SIADH (syndrome of inappropriate ADH secretion) is caused by pulmonary and CNS diseases, as well as with malignancies (small cell carcinoma) and drugs. Treatment is targeted at the underlying cause.
Acute symptomatic hyponatremia. A sudden drop in serum [Na] leading to cerebral edema, seizures, and death, as well as N/V and respiratory failure. Often caused by excess intake of fluids with increased ADH, thiazide diuretic, colonoscopy prep, glycine irrigants in the OR. Treatment is hypertonic solution to increase sodium by 1-2mM/hr.
In general, treatment of hyponatremia should not exceed > 10-12 mM within the first 24 hours or by 18mM in the 1st 2 days.