Hypereosinophilic syndrome (HES) is a rare blood disorder in which a type of white blood cell called eosinophils builds up to abnormally high levels and begins damaging organs throughout the body. To qualify as HES, the eosinophil count must reach at least 1,500 cells per microliter of blood on two separate tests at least a month apart, and there must be evidence that the excess eosinophils are causing organ damage. It’s not a single disease but an umbrella term covering several related conditions that share this pattern of eosinophil-driven harm.
How HES Differs From Regular Eosinophilia
Everyone has eosinophils. They’re part of the immune system, normally involved in fighting parasites and modulating allergic responses. Mild elevations are common and often harmless, triggered by allergies, asthma, or a temporary infection. Doctors classify eosinophilia by severity: mild (up to 1,500 cells per microliter), moderate (1,500 to 5,000), and severe (above 5,000).
The jump from “elevated eosinophils” to HES requires two things beyond a high count. First, the elevation has to persist, not just show up on a single blood draw. Second, and most importantly, there must be clear evidence that eosinophils are infiltrating tissues and causing damage. Someone with a count of 2,000 who feels fine and has no organ involvement has hypereosinophilia, not hypereosinophilic syndrome. The distinction matters because HES requires active treatment.
What Causes It
In many cases, the underlying cause remains unknown even after extensive testing. These cases are called idiopathic HES, and they’re diagnosed only after other explanations have been ruled out.
When a cause can be identified, it generally falls into one of two categories. In the myeloid (or neoplastic) form, a genetic mutation causes the bone marrow itself to overproduce eosinophils as part of a clonal, cancer-like process. The most well-known mutation is a fusion gene called FIP1L1-PDGFRA, found in roughly 14% to 60% of HES patients depending on the study population. This form is significant because it responds well to a targeted medication originally developed for certain leukemias. When this mutation is present, the condition is technically reclassified as chronic eosinophilic leukemia rather than idiopathic HES.
In the lymphocytic form, abnormal immune cells (specifically T cells) produce chemical signals that tell the bone marrow to churn out eosinophils. The eosinophils themselves are normal; they’re just being summoned in excess by a malfunctioning part of the immune system. This is considered a reactive process, similar in mechanism to how parasitic infections or severe allergies cause eosinophil spikes, though in this case the trigger is an abnormal T-cell population rather than an external threat.
Which Organs Are Affected
Eosinophils release toxic proteins meant to kill parasites. When these cells accumulate in tissues where they don’t belong, those same proteins damage healthy tissue. Almost any organ can be affected, but some are more vulnerable than others.
The heart is the most dangerous target. Eosinophils can infiltrate the inner lining of the heart, causing inflammation and scarring that restricts the heart’s ability to fill and pump blood. This can mimic the symptoms of a heart attack, with chest pain, shortness of breath, and reduced blood flow. In a 19-year review at Mayo Clinic involving 247 HES patients, cardiac dysfunction was the leading identified cause of death, accounting for a third of cases where cause of death could be determined.
The skin is frequently involved, with rashes, itching, hives, and swelling that can be among the earliest noticeable symptoms. The lungs may develop inflammation leading to persistent cough and difficulty breathing. The gastrointestinal tract can become inflamed from the esophagus through the colon, causing difficulty swallowing, nausea, abdominal pain, and weight loss. The nervous system is also at risk: eosinophil-driven inflammation of blood vessels can lead to strokes or clots in multiple areas of the brain. One case report documented a patient who developed cerebral infarctions across different vascular territories alongside severe inflammation of the esophagus, stomach, and colon, all driven by eosinophil infiltration.
Recognizing the Symptoms
HES symptoms are notoriously nonspecific, which is part of what makes the condition difficult to diagnose. What you experience depends entirely on which organs are under attack. Common presentations include:
- Skin: persistent rashes, hives, itching, or unexplained swelling
- Heart: chest pain or discomfort, shortness of breath, fatigue
- Lungs: chronic cough, wheezing, difficulty breathing
- Digestive system: abdominal pain, nausea, vomiting, diarrhea, difficulty swallowing, unexplained weight loss
- Nervous system: numbness, tingling, confusion, behavioral changes, vision problems
Many people experience symptoms in multiple organ systems simultaneously. Because the symptoms overlap with so many other conditions, from allergies to heart disease to cancer, HES is often only considered after initial diagnoses don’t fully explain the clinical picture.
How It’s Treated
Treatment depends on how severe the symptoms are, how quickly they’re progressing, and whether a specific underlying cause has been identified.
For most patients without a known genetic driver, corticosteroids are the first-line treatment. In moderate to severe cases, treatment typically starts at a dose equivalent to about 1 mg per kilogram of body weight daily; milder cases may start at half that. The goal is to suppress the immune system’s production of eosinophils and reduce inflammation quickly. If symptoms don’t improve within two to three days on high-dose corticosteroids, a second medication is usually added.
For patients who carry the FIP1L1-PDGFRA fusion gene, a targeted therapy that blocks the abnormal protein produced by this mutation has become the first-line treatment. These patients’ eosinophils show remarkable sensitivity to this approach, often with rapid and dramatic responses.
For patients with idiopathic HES lasting six months or more who don’t have an identifiable cause, an injectable biologic medication called mepolizumab is FDA-approved for patients 12 and older. It works by blocking a key chemical signal that drives eosinophil production. The treatment is given as an injection every four weeks. Because many HES patients need long-term corticosteroids, which carry significant side effects over time (bone thinning, weight gain, blood sugar problems, immune suppression), having an alternative that directly targets eosinophils is a meaningful option for reducing steroid dependence.
Long-Term Outlook
The prognosis for HES has improved substantially over the past few decades, largely because of better diagnostic tools and more targeted treatments. In the Mayo Clinic review spanning 19 years, 23 out of 247 patients died during the study period. Among those deaths where the cause was identified, cardiac dysfunction led (33%), followed by infection and unrelated cancers (each 20%), and blood clots (13%).
The pattern of mortality highlights why monitoring the heart is so critical in HES. Blood clot risk is also elevated, as eosinophil activity can damage blood vessel walls and promote clotting. Early detection and consistent treatment to keep eosinophil levels controlled significantly reduce the risk of irreversible organ damage. Patients with the FIP1L1-PDGFRA mutation tend to respond especially well to targeted therapy, while those with idiopathic HES may require ongoing management with combinations of corticosteroids, biologics, or other immune-modulating treatments to maintain eosinophil counts in a safe range.