SIADH (Syndrome of Inappropriate Antidiuretic Hormone Secretion)
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.
The body normally maintains very tight control over its total amount of water and its concentration of sodium. Many organs including the kidneys, heart, 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.
Syndrome of Inappropriate Antidiuretic Hormone Hypersecretion is a condition caused due to excessive secretion of the Antidiuretic Hormone (ADH) also known as vasopressin. This hormone is secreted by supraoptic and paraventricular nuclei in the hypothalamus and finally secreted via the posterior pituitary gland. The main function of the ADH is to regulate the water reabsorption and as a result influences urine concentration. As a consequence, hyponatremia is caused that in turn leads to fluctuation in the sodium concentration in the blood.
The main function of ADH is to control the amount of water reabsorbed by kidneys thus diluting the blood and decreasing the concentration of sodium in the blood. ADH prevents water loss in the kidneys. When water is consumed it is taken up in the blood which causes its dilution. Once this dilution which is also known as plasma osmolality is detected by osmoreceptors in the hypothalamus, ADH secretion is signaled to stop. The inhibition of ADH reduces reabsorption of water by kidneys thereby increasing the urine output. In SIADH when the blood water concentration increases ADH secretion is not stopped as a result the sodium in the blood dissolves and sodium concentration is considerably decreased.
Symptoms of SIADH Syndrome:
Loss of consciousness.
Causes and Risk Factors:
SIADH Syndrome Causes: There are a variety of causes for SIADH, including nervous system disorders, Rocky Mountain spotted fever, infection of the chest wall and lung disease. Certain cancers can also cause SIADH.
The four most common causes of SIADH are:
Cancer: SIADH is a common problem for people with lung cancer. Other types of cancer such as of the ovaries, pancreas, prostate, or stomach may also cause SIADH.
Lung problems: Lung infections such as pneumonia, tuberculosis, abscesses (pocket of infection), or aspergillosis (fungus) may cause this condition. Other lung problems such as asthma may also cause SIADH.
Medicine: Certain medicines used to treat cancer, diabetes, depression, pain, mental disorders, or seizures (convulsions) may cause SIADH. The recent use of a thiazide diuretic medicine may also cause the condition.
Brain and spinal cord problems: This includes infection, bleeding, or too much fluid and swelling in your brain. This may also include any direct injury or surgery to your brain or spine.
Other Causes Include:
Therapeutic agents such as nonsteroidal anti-inflammatory drugs (NSAIDs) inhibit prostaglandin E2, which impedes vasopressin's action. SIADH is also caused as a side effect of multiple psychotropic medications.
It can be caused due to meningitis or a head injury resulting in subarachnoid hemorrhage.
It may be caused due to post operative effects on patients hospitalized for injuries to the central nervous system.
It is also caused due to infections like brain and lung abscess or pneumonia.
Drugs that majorly cause this condition are: Clofibrate, Clorpropamide, Cyclophosphamide, Phenothiazine, Carbamazepine, Selective Serotonin Reuptake Inhibitors (SSRI's), and methylenedioxymethamphetamine (MDMA).
It may be caused due to lowered secretion of thyroid hormone, a condition called hypothyroidism.
The condition can be diagnosed with swelling, weight gain, lower levels of sodium along with lower plasma osmolality. The presence of higher concentration of sodium in urine is also a helpful guide and exclusion of other possibilities of similar presentation should also be done.
Blood tests: You may need blood taken for tests. The blood can be taken from a blood vessel in your hand, arm, or the bend in your elbow. It is tested to see how your body is doing. It can give your caregivers more information about your health condition. You may need to have blood drawn more than once.
Central venous pressure line: This is also called a CVP line or a central line. An intravenous (IV) catheter or tube is inserted into a large blood vessel near your collarbone, neck, or groin. The CVP line is hooked up to a monitor to take pressure readings so your caregiver can check the amount of fluid in your body.
Laboratory findings in diagnosis of SIADH include -
1. Hyponatremia <130 mEq/L, and POsm <270 mOsm/kg.
Other findings include -
1. Urine sodium concentration >20 mEq/L.
2. Suppression of renin-angiotensin system.
3. No equal concentration of atrial natriuretic peptide.
4. Low blood urea nitrogen (BUN).
5. Low creatinine.
6. Low uric acid.
7. Low albumin.
Osmolality: inappropriately elevated (>150 mosmol/kg).
Urine osmolality can occasionally be lower; free water clearance should be calculated in such cases.
Urine should be maximally diluted (< 100 mosmol/kg) in the presence of plasma hypo-osmolality.
Sodium level: Inappropriately elevated (>20 mEq/L).
Once diagnosed, the condition is usually treated with fluid restriction along with correcting any underlying disease states or conditions. Intravenous fluid infusions with normal saline or else with hypertonic saline would also be useful to get the sodium levels up but, it should be remembered that, rapid increase in the sodium levels can cause more damage than good and therefore needs to be controlled meticulously. Apart from these, certain medications would also be useful to tackle the ADH action and therefore bring about a normalcy in the plasma.
Other Treatment Options Include:
Demeclocycline: This is the most potential inhibitor of AVP action. It used in chronic situations when restricting fluid intake becomes difficult.
Conivaptan: It is a non-peptide inhibitor of vasopressin receptors which is an antidiuretic hormone. It also used in cases of heart failure other than being used in the treatment of SIADH.
Tolvaptan: It is used to treat low blood sodium levels associated with SIADH as well as congestive heart failure and cirrhosis. When administered into patients along with regular diuretics it increases the excretion of excess fluids and improve blood sodium levels in patients with heart failure without giving rise to side effects like hypotension.
DISCLAIMER: This information should not substitute for seeking responsible, professional medical care.
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