Churg-Strauss syndrome is a highly variable illness. Some people have only mild symptoms. Others experience severe or life-threatening complications.

Churg-Strauss syndrome has three stages, or phases, each with its own signs and symptoms. Not everyone develops all three phases. Nor do the phases always arise in the same order. This is especially true when the disease is caught and treated before the most serious damage occurs.

Allergic stage

This is usually the first stage of Churg-Strauss syndrome. It's marked by a number of allergic reactions, including:

• Asthma. The primary sign of Churg-Strauss syndrome, asthma develops on average three to nine years before other signs and symptoms appear. In people with Churg-Strauss syndrome who already have asthma, symptoms usually become worse and may require steroids for control.

Other people develop what is known as late-onset asthma. Developing asthma, even later in life, doesn't necessarily mean that you have Churg-Strauss syndrome.

• Hay fever (allergic rhinitis). This affects the mucous membranes of your nose, causing a runny nose, sneezing and itching.
• Sinus pain and inflammation (sinusitis). You may experience facial pain and develop nasal polyps, which are soft, noncancerous (benign) growths that develop as a result of chronic inflammation.

Eosinophilic stage

An eosinophil is a type of white blood cell that helps your immune system fight certain infections. Normally, eosinophils make up only a small percentage of white blood cells. In people with Churg-Strauss syndrome, abnormally high numbers of these cells (hypereosinophilia) are found in the blood or tissues, where they can cause serious damage.

Signs and symptoms of hypereosinophilia depend on which part of your body is affected. Your lungs and digestive tract — including your stomach and esophagus — are involved most often.

Signs and symptoms of the hypereosinophilia phase may include:

• Fever
• Loss of appetite and weight loss
• Night sweats
• Joint pain
• Asthma
• Fatigue
• Cough
• Abdominal pain and gastrointestinal bleeding

Vasculitic stage

The hallmark of this stage of Churg-Strauss syndrome is severe blood vessel inflammation (vasculitis). By narrowing blood vessels, inflammation reduces blood flow to vital organs and tissues throughout your body, including your skin, heart, peripheral nervous system, muscles, bones and digestive tract. Occasionally, your kidneys also may be affected.

Depending on which organs are affected, you may experience:

• Weakness, fatigue or a general feeling of being unwell
• Unintended weight loss
• Swollen lymph nodes
• Rash or skin sores
• Joint aches and swelling
• Severe pain, numbness and tingling in your hands and feet (peripheral neuropathy)
• Severe abdominal pain
• Diarrhea, nausea and vomiting
• Shortness of breath (dyspnea) from asthma or congestive heart failure
• Cough
• Chest pain
• Irregular heartbeat
• Blood in your urine (hematuria)

The exact cause of Churg-Strauss syndrome is unknown. It's likely that an overactive immune system response is triggered by a combination of genes and environmental factors, such as allergens or certain medications. Instead of simply protecting against invading organisms such as bacteria and viruses, the immune system overreacts and targets healthy tissue, causing widespread inflammation.

Some people have developed Churg-Strauss syndrome after using an asthma and allergy medication called montelukast or after switching from low-dose oral systemic steroids to inhaled steroid medications. But no clear connection between Churg-Strauss syndrome and any medication has been proved.

Possible risk factors for Churg-Strauss syndrome include:

• Age. On average, people with Churg-Strauss syndrome are diagnosed between 30 and 50 years of age.
• History of asthma or nasal problems. Most people diagnosed with Churg-Strauss syndrome have a history of nasal allergies, chronic sinusitis or asthma, which is often severe or hard to control.

Churg-Strauss syndrome can affect many organs, including your lungs, skin, gastrointestinal system, kidneys, muscles, joints and heart. Without treatment, the disease may be fatal. Complications depend on the organs involved and may include:

• Peripheral nerve damage. Peripheral nerves extend throughout your body, connecting your organs, glands, muscles, and skin with your spinal cord and brain. Churg-Strauss syndrome can damage the nerves in your hands and feet (peripheral neuropathy), leading to numbness, burning and loss of function.
• Skin scarring. The inflammation may cause sores to develop that can leave scars.
• Heart disease. Heart-related complications of Churg-Strauss syndrome include inflammation of the membrane surrounding your heart (pericarditis), inflammation of the muscular layer of your heart wall (myocarditis), heart attack and heart failure.
• Kidney (renal) damage. If Churg-Strauss syndrome affects your kidneys, you may develop glomerulonephritis. This is a kidney disease that hampers your kidneys' filtering ability, leading to a buildup of waste products in your bloodstream (uremia). Kidney failure is uncommon.