Syndrome of Inappropriate Anti-Diuretic Hormone secretion, or SIADH, is a known complication of small-cell lung cancer (SCLC). SIADH makes your body produce too much of a hormone that causes your kidneys to hold onto water. The extra fluid leads to a low sodium (salt) concentration in the blood. A really low sodium concentration can lead to symptoms like nausea, tiredness, and confusion.
SIADH usually isn't life-threatening, but it can be severe enough to require a hospital stay if your sodium level gets too low. You may have to pause your cancer treatment until your doctor can get your sodium under control.
What Is SIADH?
SIADH, as its name suggests, is an inappropriate secretion of antidiuretic hormone (ADH). ADH is a hormone that helps control your sodium concentration and your blood pressure by adjusting the amount of water reabsorption by your kidneys.
ADH is normally released by the pituitary gland in your brain. The primary trigger for its release is a high sodium concentration. Your body maintains a very constant sodium concentration because so many chemical reactions rely on sodium. Another trigger for ADH release from the pituitary is when your blood pressure is low. This hormone reduces the amount of water your kidneys release into your urine. Retaining more fluid in your blood raises your blood pressure.
SIADH causes your body to secrete too much ADH. When your ADH levels are high, your kidneys hold onto extra water even though you don't need it. The higher the ADH the more concentrated your urine.
The added fluid dilutes the sodium in your blood, causing a drop in blood sodium level. You need sodium for your muscles and nerves to work properly, and to maintain a balance of fluids and minerals in your body. A drop in sodium is called hyponatremia. Your doctor may suspect SIADH from a low sodium level (hyponatremia) on a routine blood test.
Why Do People With Small-Cell Lung Cancer Get SIADH?
SIADH is part of a group of rare conditions called paraneoplastic syndromes that affect some people with lung cancer. "Neoplasm" is another name for a cancerous tumor.
Small-cell lung cancer tumors are made up of neuroendocrine cells – the same kind of cells that release hormones. In the case of SIADH, cancer cells release ADH, causing a spike in levels of this hormone. Unlike ADH release controlled by the pituitary, ADH released from cancer cells is not suppressed by low sodium levels or normal blood volume. That’s what makes cancer-related ADH release inappropriate.
Some chemotherapy drugs used to treat lung cancer, including platinum-based chemo and methotrexate, also increase the amount of ADH released.
Who Does SIADH Affect?
Small-cell lung cancer is the type of cancer most likely to cause SIADH. Between 10% to 15% of people with SCLC develop SIADH. Nearly 50% may have a milder form of the condition. You're more likely to have this complication if your cancer has spread outside of your lungs.
Less often, SIADH affects people with non-small-cell lung cancer or cancers like these:
What Symptoms Does SIADH Cause?
SIADH usually doesn't cause symptoms if the sodium level isn’t that low. Symptoms in SIADH go hand in hand with hyponatremia. The severity of symptoms corresponds to how quickly the sodium concentration changed and how low the sodium concentration dropped.
The most common symptoms are:
- Nausea or vomiting
- Muscle cramps
A quick drop in sodium levels can cause more serious symptoms like these:
- Slow heartbeat
- High blood pressure
How Is SIADH Diagnosed?
Your symptoms might point your doctor to SIADH, but often it is suspected on a routine blood test when the sodium concentration is low. Follow-up blood or urine tests can confirm the diagnosis. These tests measure sodium levels and osmolality – the concentration of particles – in your blood or urine.
The sodium level in your blood is measured in milliequivalents per liter (mEq/L).
- A normal blood sodium level is between 135 and 145 mEq/L
- A blood sodium level below 135 mEq/L is considered low sodium, or hyponatremia
- Blood sodium of less than 125 mEq/L is severe hyponatremia
How Do You Treat SIADH?
To diagnose and treat SIADH, it's helpful to see a doctor who specializes in hormone problems, called an endocrinologist. Your oncologist, pulmonologist (lung doctor), or primary care doctor might also be part of your treatment team.
Your doctor will tailor the treatment based on how low your sodium levels have dropped. Chemotherapy treatments might be enough to correct a mild case of low sodium from lung cancer. As the medicine kills lung cancer cells, your ADH production should drop and your blood sodium levels should go back to normal.
If SIADH is severe or it doesn't improve with chemotherapy, your doctor might look at other treatment options.
Bringing your sodium level back up to normal could reduce weakness and risk of falling.
Usually the low sodium levels improve by limiting water intake. This is the first step in mild SIADH. If you have SIADH and are dehydrated at the same time, your doctor may give you salt pills or a mixture of salt and water (saline) into a vein. Raising sodium levels too quickly can cause a serious complication called central pontine myelinolysis, which can lead to brain swelling, coma, and possibly death. Your medical team will raise your sodium level carefully and monitor you closely while doing it.