MEDICAL INFORMATION
HYPONATREMIA
Sodium (Na) is one of the major electrolytes in the body, along with others such as potassium, calcium, magnesium, chloride and other such chemicals. The normal level of serum (blood fraction) sodium is 135-145 mM. Sodium levels below 135 mM are designated hyponatremia, and values below 120 mM are often fatal. The kidney is responsible for most of the excretion of sodium from the body, although large amounts can be lost in sweat and feces. Vasopressin (AVP, ADH, or antidiuretic hormone} is a protein known as a hormone which regulates sodium by effects on the kidney: it increases permeability to water in the distal and collecting tubule, increases permeability to urea in the collecting tubule of the kidney, and increases sodium and chloride resorption of the ascending tubule. The effects of vasopressin occur through the action of vasopressin on receptors located in various cells in the body, and are designated V1, V2, V2a, V2b, etc. The result of vasopressin effects on V2 receptors in the kidney leads to a stimulation of the "countercurrent multiplier" results in a net water reabsorption known as a negative "Free Water" clearance. The balance of "Free Water" clearance is regulated by many physiological factors in which the effect of vasopressin is a central mechanism in the hour to hour regulation of blood levels of total water and sodium.
Vasopressin is manufactured by the brain (supraoptic and paraventricular nuclei) as a prohormone (glycopeptide, neurophysin, and vasopressin) which migrates inside the nerve fibers of the hypothalamic neurohypophyseal tract to the pituitary gland (neurohypophysis). Here it is stored as granules of neurophysin and vasopressin until it is released in the form of free vasopressin into the blood stream where it is transported to the kidney to affect "Free Water" excretion.
Hyponatremia may accompany many different medical conditions. Symptoms of hyponatremia may be mild weakness and fatigue with fluid retention in the legs and face, or symptoms may become very severe with confusion and disorientation, with the most severe situations resulting in death. Symptoms are usually mild or nonexistent with blood sodium levels of 128 to 134, and may be progressively more severe as levels range to 120 mM. One of the most common medical conditions is the excessive loss of sodium from diuretic medications. However, there are many conditions in which blood levels of vasopressin are elevated which may result in hyponatremia: syndrome of inappropriate ADH (SIADH), trauma to the brain, congestive heart failure, cirrhosis of liver disease, nephrotic syndrome of kidney disease and numerous drugs such as clofibrate, carbamazepine, cyclophosphamide, colchicine, morphine, nicotine, vincristine, and others. Various cancers, particularly lung cancer, may secrete large amounts of vasopressin and lead to hyponatremia. In contrast, some conditions decrease vasopressin and result in diuresis or excessive loss of water (diabetes insipides} and increased blood sodium levels (hypernatremia). Examples of possible hypernatremia include pituitary failure with decreased levels of vasopressin, and many drugs including morphine, ethanol, dilantin, steroids and other medications.
The primary treatment of hyponatremia is to correct the cause of the water retention, if possible. If the primary cause cannot be identified or treated, then the established therapy is fluid restriction. Some success in correction of hyponatremia is possible in certain medical conditions with the administration of lithium or demethylchlortetracycline or urea and furosemide, provided certain medical conditions do not prevent patients from receiving effective therapy with these drugs. Much research has been devoted to developing medications that could be taken orally and would have a specific effect to interfere with vasopressin on the V2 receptors of the kidney or the brain. Drugs that increase urine flow and excretion of sodium and other electrolytes are designated 'diuretics'. Drugs that increase urine flow without increasing excretion of electrolytes (i.e.: increase "Free Water"} are known as 'aquaretics'. Presently, there are no aquaretic medications available for treatment of hyponatremia which are effective orally and which have very few undesirable effects on patients.
The information provided on hyponatremia should not be taken as an authoritative source on the subject. Patients should talk to their physician for details of the signs and symptoms, and for the various treatments for hyponatremia.