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.
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.
SIADH has many known causes, some of which particularly relate to cancer or its treatment. These causes include specific types of cancer, drugs used to treat cancer itself, drugs used to treat the effects of cancer, and conditions that arise as a consequence of cancer or its treatment.
Specific types of cancer
SIADH results from numerous different types of cancer. The malignancies known to cause SIADH include:
- Lung cancer, small cell type
- Gastrointestinal cancers (pancreatic cancer, exocrine; duodenal or stomach cancer)
- Genitourinary cancer (bladder cancer, prostate cancer, ovarian cancer)
- Lymphoma, including Hodgkin's disease
- Head and neck cancers (oral cancers, laryngeal cancer, nasopharyngeal cancer)
- Brain and central nervous system tumors
- Breast cancer
Certain cancers produce and secrete ADH themselves. This production occurs without regard for the needs of the body. Thus, the kidneys receive repeated signals to save water, even when the body already has a marked excess of fluid. Of all the types of cancer that produce ADH themselves, small-cell lung cancer is by far the most common. Small-cell cancer of the lung is the cause in 75% of cases of SIADH caused directly by a tumor. In some cases, the appearance of SIADH may be the first indication that a cancer exists.
Also, primary or metastatic tumors in the brain may lead to SIADH. SIADH here results from an increase in intracranial pressure (pressure within the head), or from other effects of intracranial disease on the brain. Increased intracranial pressure commonly causes various parts of the brain to work improperly.
Drugs used to treat cancer itself
A variety of drugs used in cancer treatment may lead to SIADH. The mechanism of this effect may be that the drug causes the abnormal release of ADH, or that the drug makes existing ADH work in a stronger fashion than usual. Chemotherapy drugs that cause SIADH include:
- Vincristine, vinblastine, vinorelbine and other vinca alkaloids (Oncovin, Velban, Navelbine)
- Cyclophosphamide, ifosfamide, melphalan and other nitrogen mustards (Cytoxan, Ifex, Alkeran)
- Cisplatin (Platinol-AQ)
- Levamisole (Ergamisol)
Drugs used to treat the effects of cancer
SIADH may occur as a reaction to drugs used to treat effects of cancer such as pain, depression, or seizures. SIADH also may result from general anesthesia.
- Narcotic pain medications (morphine, Oramorph SR, fentanyl, Duragesic)
- Tricyclic antidepressants (amitriptyline, Elavil)
- Carbamazepine (Tegretol)
- General anesthetics
Conditions that arise as a consequence of cancer
SIADH may result from some of the debilitating consequences of cancer. For example, a person with cancer who is weak or unsteady will have a tendency to fall and hit the head. Skull fracture and other types of head injury may damage the brain or increase the intracranial pressure, and thus lead to SIADH.
Also, cancer patients who are weak, malnourished, receiving chemotherapy, or spending excessive time in bed have an increased risk of pneumonia and other infections. Infections including pneumonia, meningitis, and tuberculosis can cause SIADH.
The treatment of SIADH involves relief of the urgent symptoms and correction of the underlying problem. For immediate improvement, all patients with SIADH require sharp restriction of their daily water intake. As little as two cups of liquid, about 500 ml, may be the daily limit for some patients. In cases where the sodium concentration is already dangerously low, doctors may cautiously give an intravenous infusion of fluid with a high concentration of sodium (hypertonic saline solution). However, this treatment carries some risk of damaging the brain. Physicians may also use a medicine such as furosemide (Lasix) that promotes water excretion (diuresis). Another drug, demeclocycline, blocks the action of ADH in the kidney.
The most definitive way to relieve SIADH is to address the underlying problem itself. Thus, if a tumor produces abnormal ADH, then surgery, radiation therapy, or chemotherapy may help by reducing tumor size. If SIADH results from use of a drug, then the patient must discontinue the medicine. Finally, doctors try to identify and treat any other correctable cause, such as an infection.
The prognosis of SIADH depends largely on its cause. Until recently, many physicians believed that the appearance of SIADH indicated a poor prognosis for cancer. However, more recent reports contradict this idea. The patient's ability to observe severe restriction of fluid intake may determine the degree of ongoing symptoms. SIADH usually improves after stopping a drug or curing an infection when that is the cause. When cancer is the direct cause of SIADH, one hopes for similar improvement of SIADH from treatments that reduce the amount of cancer in the body.
- Antidiuretic hormone (ADH)
- A chemical hormonal signal sent by the pituitary gland to the kidneys through the bloodstream, telling the kidneys to conserve water in the body.
- The excretion of urine.
- A chemical signal released into the bloodstream that affects one or more other organs.
- Hypertonic saline solution
- Fluid that contains salt in a concentration higher than that of healthy blood.
- Within the head.
- Pituitary gland
- A small organ, located at the base of the brain, that regulates many body functions.
- Syndrome of inappropriate antidiuretic hormone production
- The clear yellowish liquid part of whole blood, after it is separated into solid and liquid components. It may be found within the vascular system or in body tissue itself.
- A collection of symptoms and physical signs that together follow a pattern.