Syndrome of Inappropriate Ant... Health Article

Description

The syndrome of inappropriate antidiuretic hormone production (SIADH) is a condition in which the body develops an excess of water and a decrease in sodium (salt) concentration, as a result of improper chemical signals. Patients with SIADH may become severely ill, or may have no symptoms at all.

A syndrome is a collection of symptoms and physical signs that together follow a pattern. SIADH is one of the paraneoplastic syndromes, in which a cancer leads to widespread ill effects due to more than just the direct presence of tumor.

Normal physiology

The body normally maintains very tight control over its total amount of water and its concentration of sodium. Many organs including the kidneys, heart, and the adrenal, thyroid, and pituitary glands participate in this regulation. One important contribution is the release of a chemical substance, or hormone, by the pituitary gland into the bloodstream. This chemical substance, called antidiuretic hormone (ADH), is also known as arginine vasopressin, or AVP.

The pituitary releases ADH into the bloodstream when receptors in various organs detect that the body has too little water or too high a concentration of salt. ADH then affects the way the kidneys control water and salt balance. ADH causes the kidneys to decrease their output of urine. The body thus saves water by undergoing antidiuresis, that is, not excreting urine.

Simultaneously, the concentration of sodium in the body serum decreases. This decrease results from a second effect of ADH on the kidneys. When the kidneys retain extra water, the existing concentration of sodium in the body decreases slightly as a result of dilution. These functions are all part of the body's extremely precise control over water and salt balance in health.

Abnormal physiology in SIADH

Certain disease states can upset the delicate balance of water and salt in the body. If there is too much ADH in the body, or if the kidneys overreact to the ADH they receive, the body retains excess water and the serum sodium concentration becomes diluted and falls to abnormal levels. The patient with SIADH develops symptoms based on the degree of abnormality in the serum sodium concentration and the speed with which this concentration falls.

Normal serum sodium concentration is 135-145 mEq/L (milliEquivalents of sodium per liter of body fluid). When the sodium concentration is 125-135 mEq/L the patient may have mild nausea, loss of appetite, fatigue, headache, or still remain free of symptoms. As the sodium level drops below 120 mEq/L, the patient experiences greater weakness, confusion, sleepiness, vomiting, and weight gain. As the sodium concentration approaches 110 mEq/L, the patient may suffer seizures, coma, and death.