The actual cause of Churg-Strauss-syndrome (CSS) remains unknown, although researchers have determined that it is not inherited. There is some speculation that the syndrome may be caused by an overactive immune system that was set in motion by an acquired infection. The presence of asthma, eosinophilia and increased immunoglobulin E (IgE) levels suggest it is an allergic process. Some authors have suggested that drug sensitivities to penicillin, penicillamine, iodides, and leukotriene modifiers may lead to CSS. More research is necessary to confirm the cause of the disease.

Churg-Strauss syndrome (CSS) is difficult to diagnose because symptoms vary widely among patients. Some patients experience mild symptoms, while others suffer from a wide variety of medical problems, including sinus problems, rashes, lung involvement, peripheral neuropathy, gastrointestinal problems, and heart involvement.

In 1990, the American College of Rheumatology (ACR) developed criteria that distinguished CSS from other vasculitic diseases. These criteria included: (1) asthma, (2) eosinophilia greater than 10% on a differential white blood cell (WBC) count, (3) mononeuropathy (nerve disorder affecting one nerve trunk) or polyneuropathy (disorder that affects multiple nerves), (4) non-fixed pulmonary infiltrates (fluid in the lungs), (5) paranasal sinus abnormalities, and (6) biopsy containing a blood vessel with extravascular granulomas (clumps of cells that forms lumps on the outside of blood vessels). The presence of four or more of the six criteria yielded a sensitivity of 85% and a specificity of 99.7%. This means that the presence of four or more of the six criteria will correctly identify the disease 85 out of 100 times. When CSS is diagnosed using these standards, 99.7% of those individuals will actually have the disease. Many doctors diagnose CSS when only two or three of these criteria are present. However, for purposes of a clinical trial or study, four out of six must be met.

General: Churg-Strauss syndrome (CSS) is a progressive disease consisting of three phases: the allergic stage, the eosinophilic stage, and the systemic vasculitic stage. Each stage is characterized by the presentation of different symptoms. On average, progression from the first phase of CSS to the systemic vasculitis phase takes about three years. However, these stages may or may not occur sequentially, and not everyone experiences all three phases.

During any phase of CSS, patients may also experience the general effects of chronic illness, including depression, fatigue, and malaise (general feeling of discomfort).

Allergic phase: The allergic phase is characterized by allergic inflammation of the nose, sinuses, lungs, and the skin. Patients are often diagnosed with late-onset asthma during this phase. Others may have childhood-onset asthma and allergies that suddenly become worse. Sometimes asthma symptoms improve as the disease intensifies in other areas of the body. The allergic phase typically lasts from four to 27 months, but some individuals may remain in this phase for several years.

Asthma symptoms, including bronchospasms (abnormal contraction of the bronchi, resulting in airway obstruction), coughing (constantly or intermittently), wheezing or whistling sounds when exhaling, shortness of breath or rapid breathing, chest tightness or chest pain, and fatigue, almost always occur during this phase. Rhinitis (allergic inflammation of the nose), sinus pain, nasal polyps, headache, itchy or runny nose, recurrent pneumonia, and bronchitis (inflammation of the bronchial tubes) may also occur during the allergic phase.

Eosinophilic phase: During the eosinophilic phase, there is an overabundance of eosinophils in the blood or tissues. Symptoms during this stage may be recurrent. This stage can last anywhere from months to years.

Common symptoms may include weight loss, fever, sweating, abdominal pain, diarrhea, hemoptysis (blood in the sputum), rales (crackling sound that occurs when air moves through fluid-filled airways), gastrointestinal obstruction, eosinophilic gastroenteritis (infection and swelling in the gastrointestinal tract), ascites (fluid in the abdomen), gastrointestinal bleeding, and cough.

Systemic vasculitic phase: During the systemic vasculitic phase, the blood vessels may become swollen reducing blood flow to organs and possibly leading to permanent organ damage. Since CSS can affect many different organs during this stage, symptoms vary depending on the organ affected. This phase can last from months to years.

In general, symptoms often include fever, weight loss, and adenopathy (enlarged lymph nodes). Patients may be anemic or have low platelet counts. The skin, heart, lungs, central nervous system, peripheral nervous system and gastrointestinal tract are often affected. Occasionally, the kidneys, eyes, and musculoskeletal system are affected.

CSS involving the skin often cause rashes, purpura, and nodules. In addition, livedo reticularis (constriction of blood vessels) and hives may occur.

If CSS involves the lungs, common symptoms include, cough, shortness of breath, hemoptysis (blood in sputum), rales (crackling sound when air moves through fluid-filled airways), and a feeling of pressure in the chest.

Heart failure is leading cause of death in CSS. If the disease progresses to the heart, symptoms may include fatigue, dyspnea (shortness of breath), chest pain, irregular heartbeat, hypertension (high blood pressure), difficulty breathing, swollen legs, loss of appetite, fainting, and heart attack.

If CSS involves the central nervous system or the brain, symptoms may include cognitive impairment, motor problems, seizures, difficulty speaking, and headache. CSS patients who have hypertension are at risk of developing a cerebral hemorrhage (bleeding in the brain), which may be fatal.

CSS may cause peripheral neuropathy. Other symptoms that affect the peripheral nervous system include numbness, hyperesthesia (abnormal sensitivity to touch), abnormal sensations, and difficulty moving. Some patients experience foot drop and they are unable to lift the ankle, straighten or extend the toes, or turn the foot outward.

Gastrointestinal problems occur in about 40-60% of CSS patients in the vasculitic phase. Symptoms generally include severe abdominal pain, bloody diarrhea, vomiting, and nausea. The symptoms may be the same or similar to those of eosinophilic gastroenteritis. Some patients may also develop obstructions or perforated intestines.

Musculoskeletal symptoms may include swollen joints, muscle pain, and arthritis.

In rare cases, CSS may affect the kidneys and cause symptoms such as fever (secondary to infection), urinary tract infection, nausea, edema (swelling), and difficulty or pain during urination.

In rare instances, the optic nerve may be affected by vasculitis, resulting in eye pain and vision problems.