Syndrome of inappropriate antidiuretic hormone (SIADH) is an oversecretion of the antidiuretic (water-retaining) hormone (ADH) called vasopressin. This occurs because some cancer cells have the ability to manufacture, store and secrete ADH. The excess ADH results in more water in the cells and extreme dilution of the sodium concentration, causing hyponatremia (low blood sodium). SIADH causes changes to cell function. The central nervous system is the most sensitive to the changes. The presence of SIADH is not related to the stage or prognosis of the cancer.
Hyponatremia may be called water intoxication. SIADH that develops suddenly can be life-threatening.
Causes
SIADH is caused by medical conditions, drugs and some cancers. SIADH is most often associated with lung cancer, especially small-cell lung cancer. Other cancers associated with SIADH include:
- brain
- pancreas
- prostate
- ovary
- lymphoma
- leukemia
- thymoma
Those at highest risk of developing SIADH have squamous cell or neuroendocrine tumours.
SIADH can also be triggered by drugs that are used to treat people with cancer, including:
- chemotherapy drugs
- vincristine
- vinblastine
- vinorelbine
- cyclophosphamide
- ifosfamide
- cisplatin
- docetaxel
- narcotics, including morphine
Signs and symptoms
Symptoms of SIADH are related to hyponatremia. The severity of the symptoms depends on the blood level of sodium and the rate of onset of hyponatremia.
- anorexia
- nausea
- vomiting
- fatigue
- headache
- muscle cramps
- weakness
- weight gain without edema
- change in mental status
- irritability
- disorientation
- confusion
- lethargy
- combativeness
- psychotic behaviour
- problems walking (gait disturbances)
- seizures
- coma
Diagnosis
Tests that help diagnose SIADH include:
- neurological examination
- blood tests
- Blood chemistry tests will show low levels of sodium, blood urea nitrogen (BUN), creatinine, albumin and uric acid.
- Blood osmolality (osmolality is the measure of the concentration of particles in a solution) will be decreased.
- urine tests
- The sodium level will be increased.
- Osmolality of the urine will be greater than blood osmolality.
Treatment
The underlying cause of SIADH needs to be identified and treated. If SIADH is caused by cancer or cancer treatment, the hyponatremia is treated. The only potential cure for SIADH is successful treatment of the cancer.
Treatment to stabilize the hyponatremia depends on its severity:
- moderate SIADH
- fluid restriction (drinking very little) – may be the only treatment that is needed
- severe SIADH
- intravenous infusion of saline solution to increase sodium level
- medication (diuretic) to increase urine output