Syndrome of inappropriate antidiuretic hormone secretion (SIADH) is a common complication of small cell lung cancer (SCLC). Between 10 percent and 45 percent of people with SCLC develop SIADH, which throws off the body’s balance of water and sodium. The result is a range of symptoms like muscle cramps, nausea, and mood swings.

If you or a loved one has small cell lung cancer, continue reading to learn about SIADH, what to watch for, and how it’s treated.

What Is SIADH?

If you have SIADH, it means your body is making too much of a hormone called antidiuretic hormone (ADH), also known as vasopressin. This hormone helps your kidneys manage the amount of water and salt in your body and controls your blood pressure and how well your kidneys work.

ADH is made in a part of your brain called the hypothalamus and is stored and released by the pituitary gland, which is just below the hypothalamus. Normally, your body releases ADH when you need to conserve water, like when you’re dehydrated, telling your kidneys to hold on to water. When you drink enough fluids, your body stops releasing ADH, which lets your kidneys get rid of the extra water.

However, with SIADH, your body keeps releasing ADH even if you don’t need it, which makes your kidneys hold on to more water than necessary. This extra water can lower the amount of sodium in your blood. This condition is called hyponatremia, which means your blood sodium levels are unusually low.

What Are the Symptoms of SIADH?

The symptoms of SIADH are different from the symptoms of SCLC. Talk to your cancer care team as soon as possible if you notice any new or worsening symptoms.

If you have mild hyponatremia, you may not experience any symptoms. However, moderate to severe hyponatremia can cause SIADH symptoms, such as:

• Muscle cramps
• Muscle weakness
• Lack of energy
• Nausea
• Vomiting
• Headache
• Balance problems
• Mood changes

A MyLungCancerTeam member explained that hyponatremia, in addition to chemotherapy, caused them to “lose weight and stamina and strength.”

Why Does SIADH Occur in People With SCLC?

People with SCLC can develop SIADH if their cancer cells make ADH. If cancer cells make ADH without regard to how much water and sodium your body needs, it can lead to SIADH.

SIADH is considered a paraneoplastic syndrome in people with lung cancer. A paraneoplastic syndrome is a group of signs and symptoms associated with certain cancers. SCLC is the most common type of cancer associated with paraneoplastic syndromes.

Other Causes of SIADH

Although SIADH is often associated with SCLC, it has also been reported in people with non-small cell lung cancer (NSCLC), according to the journal Respiratory Medicine Case Reports. Other types of cancer have also been known to cause SIADH, including extrapulmonary small cell carcinomas, head and neck cancers, and lymphoma.

People without cancer can also develop SIADH. Other possible causes of SIADH include:

• Head injury or trauma
• Stroke
• Infection
• Certain medications
• Surgery
• Pneumonia
• Hormone problems

Some symptoms of SIADH are serious and need emergency treatment. It’s important to go to your nearest emergency department or hospital if you experience any of the following symptoms of SIADH:

• Confusion
• Disorientation
• Seizures

How Is SIADH Diagnosed?

Your health care provider can diagnose SIADH by looking at your fluid status (the amount of water in your body), serum sodium level (the amount of sodium in your blood), and urine. Tests for SIADH may include:

• Comprehensive metabolic panel — This test measures the level of sodium and other substances in your blood. People with SIADH usually have low levels of sodium in their blood.
• Serum osmolality — This blood test checks the balance of water and other substances in your blood. People with SIADH usually have low serum osmolality (more diluted blood).
• Urine osmolality — This urine test checks the balance of water and other substances in your urine. People with SIADH usually have high urine osmolality (more concentrated urine).
• Urine sodium test — This test measures the amount of sodium in your urine. People with SIADH usually have increased sodium in their urine.

Your cancer care team may continue to monitor your fluid and electrolyte balance with blood and urine tests throughout your SCLC treatment.

How Is SIADH Treated?

Treatment for SIADH depends on the cause. If you have SIADH because your cancer cells are making too much ADH, your treatment will involve treating your cancer with surgery, radiation therapy, chemotherapy, immunotherapy, or a combination of treatments.

Other treatment options for the management of hyponatremia caused by SIADH include fluid restriction and medications.

Fluid Restriction

Fluid restriction involves limiting the amount of fluid you drink each day. Restricting how much fluid you have each day can help prevent extra fluid from building up in your body. Your health care provider will tell you exactly how much fluid you can have each day. In most cases of SIADH, your fluid intake will be limited to 1 liter or less per day.

Fluid restriction doesn’t just involve the water you drink — it includes all fluids you consume, including:

• Water
• Coffee
• Tea
• Ice
• Soup

In addition to limiting your fluid intake, your health care provider may make changes to the amount of protein and sodium you eat every day. These changes in your diet can help to make sure you have the right amount of fluid in your body.

Medications

Severe hyponatremia is usually treated in the hospital. To correct hyponatremia, your doctor may give you an intravenous (IV) solution with a high sodium concentration. You may also need medications that block the signal from excess ADH on your kidneys, such as tolvaptan (Samsca), conivaptan (Vaprisol), or demeclocycline (Declomycin).

Does SIADH Affect Outlook?

SIADH is considered a poor prognostic factor for people with SCLC. This means having SIADH can predict a worse outcome in people with SCLC. Studies have found that SIADH is associated with worse survival in people with SCLC. A 2016 study in China found that in people with SCLC, SIADH was associated with a shorter progression-free survival (how long you live with cancer but don’t get worse) and overall survival (how long you live after diagnosis).

If you develop hyponatremia from SIADH, it could mean you have to stay in the hospital for a longer time or delay your scheduled chemotherapy. One member of MyLungCancerTeam who cares for their father living with SCLC wrote, “Every time my dad goes in to get chemo, his sodium is so low that he is put in the hospital.” Another, whose wife has SCLC, said, “My wife had a rough week. She was hospitalized for three days for dangerously low sodium and came home with severe back pain.”

However, oncology researchers don’t know exactly how SIADH affects your prognosis. One possible reason SIADH is associated with a worse prognosis is that it could indicate you have kidney or hormone problems. Cancer cells that make ADH and cause SIADH may also be more aggressive compared to cancer cells that don’t release ADH.

Speak With Your Health Care Provider

Treating hyponatremia may help people with SIADH and cancer live longer. Talk to your oncologist about your risk of SIADH. Keeping sodium levels balanced is important for your health. Your doctor can explain different ways to keep an eye on and manage SIADH, making sure your treatment fits your health needs. Catching SIADH early and managing it actively are key steps to avoid further health issues and improve your life quality during treatment.

Talk With Others Who Understand

On MyLungCancerTeam, the social support network for people with lung cancer and their loved ones, more than 12,000 members come together to ask questions, give advice, and share their stories with others who understand.

Have you or a loved one developed SIADH with SCLC? Share your insights in the comments below, or start a conversation by posting on your Activities page.