The syndrome of inappropriate antidiuretic hormone production (SIADH) is a condition associated with an increased water retention in the body derived from excessive production of antidiuretic hormone (ADH).
SIADH syndrome is characterized by hyponatremia (low concentration of sodium in the blood) caused by the excess of renal water reabsorption that results in a reduced urine output (antidiuresis).
The body maintains a balance between the total amount of water and the concentration of sodium. Many organs participate in this regulation such as the kidneys, the thyroid, the pituitary gland or the adrenal glands. Specifically, the pituitary gland release a hormone called antidiuretic hormone (ADH) or arginine vasopressin when the body has too little water or there is a high concentration of sodium. ADH causes the kidneys to increase the water retention and decreases the urine output.
In SIADH, there is an excessive and inappropriate production of antidiuretic hormone (ADH). The kidneys overreact to the ADH, so the body retains excess water and the sodium concentration in the blood becomes diluted and falls to abnormal levels.
SIADH (syndrome of inappropriate antidiuretic hormone production) is considered a paraneoplastic syndrome. A paraneoplastic syndrome causes symptoms in patients with a cancerous tumor that are not directly related to the tumor invasion. The main types of cancer known to cause SIADH are:
However, some conditions not related to cancer may cause the disorder. The most common ones are:
A variety of drugs such as antiepileptic, antipsychotic or antineoplastic drugs may also cause SIADH:
Patients with SIADH may not experience any symptom in the early stages. When the symptoms appear, it is usually due to hyponatremia (low concentration of sodium in the blood).
If there is a mild or moderate hyponatremia (125-135 mEq/l), symptoms are vague and nonspecific:
If there is a marked hyponatremia (< 125 mEq/l) the patient may experience:
In case of severe hyponatremia (< 115 mEq/l):
SIADH may be considered in patients with hypoosmolar hyponatremia (sodium in the blood below 135 mEq/l with a low plasmatic osmolality).
Other findings that are useful for the diagnosis are:
The SIADH diagnosis is based on the exclusion of other possible causes of hyponatremia.
A water loading ADH suppression test may be helpful to diagnose SIADH.
The treatment depends on the underlying cause. It is necessary to know the cause for a proper treatment. Sometimes it is due to a tumor that can be treated with radiotherapy or chemotherapy.
In addition, it is necessary a fluid restriction. All patients require a sharp restriction of daily water intake following the medical prescription.
Severe cases may require medication. The most used are:
It is also available the administration intravenously of a hypertonic saline solution in case of suffering from brain swelling, seizures or coma.