Hypereosinophillic Syndrome

Educational clinical summary of Hypereosinophillic Syndrome for doctors and students: definition, workup, management, and key caveats. Not a substitute for guidelines.

Maintained by Dr. Vivek Karn Review process: editorial policy

Medical Disclaimer: This information is intended for educational purposes only and should not be used as a substitute for professional medical advice, diagnosis, or treatment. Clinical decisions must always be based on the individual circumstances of each patient, the treating physician's judgment, and current clinical guidelines. This information may not cover all possible aspects of the disease or its treatment. Always consult with a qualified healthcare professional for any health concerns or before making any decisions related to your health or treatment.

Overview

Definition: Hypereosinophilic Syndrome (HES) is a rare and heterogeneous group of disorders characterized by persistent marked peripheral blood eosinophilia (absolute eosinophil count [AEC] ≥ 1,500 cells/µL or 1.5 x 10^9/L) for at least six months (or for less than six months if signs/symptoms of organ damage are present), evidence of eosinophil-mediated organ damage, and exclusion of other known causes of eosinophilia. The clinical manifestations vary widely depending on the organs involved.
Epidemiology: HES is rare, with an estimated prevalence of 0.036 per 100,000 to 0.5 per 100,000 population. It can affect individuals of any age, but it is most commonly diagnosed in adults between 20 and 50 years old. There is a slight male predominance, particularly in the myeloproliferative variant (M-HES).
Classification: HES is classified based on its underlying cause or presumed mechanism. The 2022 World Health Organization (WHO) classification for myeloid neoplasms and acute leukemia, and the 2017 International Consensus Classification (ICC), provide frameworks for classifying eosinophilic disorders.
- Primary (Neoplastic) HES:
  - Myeloproliferative HES (M-HES): Associated with clonal abnormalities of myeloid cells, most notably the FIP1L1-PDGFRA fusion gene. Other mutations include PDGFRB, FGFR1, and PCM1-JAK2. Often characterized by elevated serum vitamin B12 and tryptase, splenomegaly, and myelofibrosis.
  - Chronic Eosinophilic Leukemia, NOS (CEL, NOS): Eosinophilia with evidence of clonality but without specific gene rearrangements (e.g., FIP1L1-PDGFRA) or other WHO-defined myeloid neoplasms. Blasts typically <20% in blood/bone marrow.
- Secondary (Reactive) HES: Eosinophilia caused by underlying conditions, such as parasitic infections, allergic reactions (e.g., drug hypersensitivity), certain solid tumors or lymphomas (e.g., Hodgkin lymphoma, T-cell lymphoma), and autoimmune diseases (e.g., Churg-Strauss syndrome/Eosinophilic Granulomatosis with Polyangiitis [EGPA]). Once the underlying cause is identified and treated, eosinophilia resolves.
- Idiopathic HES (I-HES): Diagnosis of exclusion where no underlying cause of eosinophilia or clonal abnormality can be identified, despite comprehensive investigation. This is the most common form of HES.
- Lymphocytic HES (L-HES): Associated with a clonal T-cell population (often CD3-CD4+), producing eosinophilopoietic cytokines (e.g., IL-5). Patients may have lymphadenopathy, skin lesions, and hypergammaglobulinemia.
- Familial HES: Rare, inherited forms of HES, often associated with specific genetic mutations (e.g., STAT5B gain-of-function mutation).
- Specific Organ HES: Eosinophilic infiltration and damage confined to a single organ (e.g., eosinophilic gastroenteritis, eosinophilic fasciitis). Not typically considered systemic HES unless the eosinophilia is sustained systemically.

Pathophysiology

Mechanism: The underlying mechanism involves the uncontrolled production, activation, and tissue infiltration of eosinophils, leading to inflammation and organ damage. Eosinophils release cytotoxic granule proteins (e.g., major basic protein, eosinophil cationic protein, eosinophil-derived neurotoxin, eosinophil peroxidase), cytokines, and chemokines, causing tissue damage, fibrosis, and thrombosis.
- M-HES: Driven by constitutive activation of tyrosine kinases (e.g., PDGFRA, PDGFRB, FGFR1) due to gene fusions or mutations, leading to uncontrolled myeloid proliferation, including eosinophils. FIP1L1-PDGFRA fusion gene, specifically, encodes an oncogenic tyrosine kinase that drives the proliferation and survival of eosinophil progenitors.
- L-HES: Caused by aberrant cytokine production, primarily IL-5, by an abnormal T-cell clone. IL-5 is a key cytokine for eosinophil proliferation, differentiation, and survival.
- I-HES: Pathogenesis is unclear but is presumed to involve dysregulation of eosinophil production or activation that cannot be linked to a defined clonal process or secondary cause.
Risk Factors:
- Genetic Predisposition: Specific gene mutations (FIP1L1-PDGFRA, STAT5B, others) are direct causes for primary HES subtypes.
- Male Gender: Slight male predominance, particularly in M-HES.
- Unknown: For I-HES, no clear environmental or lifestyle risk factors have been identified.
Protective Factors: None definitively identified for HES itself. Early diagnosis and appropriate treatment are crucial for preventing irreversible organ damage.

Clinical Presentation

Signs and Symptoms

Clinical presentation is highly variable and depends on the organs involved. Eosinophil-mediated damage can affect virtually any organ system. - Cardinal Features: * Non-specific symptoms: Fatigue, malaise, fever, weight loss, myalgia, arthralgia, cough. * Skin: Pruritic papules, nodules, plaques, erythema, angioedema, urticaria, non-specific rashes (most common organ involvement, 40-60%). * Cardiac: Dyspnea, chest pain, palpitations, syncope. Potentially life-threatening endomyocardial fibrosis leading to restrictive cardiomyopathy, valvular dysfunction, and heart failure. * Pulmonary: Cough, dyspnea, wheezing. Infiltrates, effusions. * Gastrointestinal: Abdominal pain, diarrhea, nausea, vomiting, dysphagia, GI bleeding, malabsorption. Eosinophilic gastroenteritis. * Neurological: Peripheral neuropathy (sensory or motor), encephalopathy (memory loss, confusion), ataxia, seizures, stroke-like episodes. * Hematologic: Anemia, thrombocytopenia, splenomegaly, hepatomegaly, elevated vitamin B12 and tryptase (in M-HES). - Early Signs: Often non-specific, such as fatigue, myalgia, fever, and skin rashes. Persistent unexplained eosinophilia on routine blood tests may be an early indicator. - Advanced Signs: Reflect significant organ damage, e.g., severe heart failure due to restrictive cardiomyopathy, profound neurological deficits, severe malabsorption. - Atypical Presentations: Eosinophilic involvement of unusual sites such as eyes, kidneys, bladder, or endocrine glands. Isolated organ involvement without marked systemic eosinophilia (e.g., eosinophilic fasciitis, eosinophilic cystitis) may be considered distinct entities but can rarely be part of the HES spectrum.

Physical Examination

Findings are diverse and depend on the affected organ systems. - Inspection: * Skin: Urticaria, angioedema, papules, nodules, hyperpigmentation, pruritic rashes, ulcers. * Eyes: Periorbital edema, conjunctivitis. * General: Cachexia, pallor (anemia). - Palpation: * Abdomen: Hepatosplenomegaly (especially in M-HES). * Lymph nodes: Lymphadenopathy (especially in L-HES). * Extremities: Edema. - Special Tests: * Cardiac auscultation: Murmurs (valvular involvement), gallop rhythm (heart failure). * Neurological exam: Sensory or motor deficits, ataxia, cranial nerve palsies, altered mental status. * Pulmonary auscultation: Rales, wheezes.

Diagnostic Approach

The diagnosis of HES requires a multi-step approach to confirm the presence of hypereosinophilia, demonstrate end-organ damage, and exclude other causes.

History Taking

Key Questions:
- Duration and maximum eosinophil count.
- Symptoms: Fatigue, fever, weight loss, rashes, cough, dyspnea, chest pain, abdominal pain, diarrhea, numbness, weakness.
- Travel history (parasitic infections).
- Medication history (drug reactions).
- Allergies, asthma, atopic history.
- Family history of eosinophilia or related disorders.
- Prior diagnoses of cancer, autoimmune diseases, or other conditions.
Red Flags:
- Rapidly worsening symptoms, especially cardiac (dyspnea, chest pain, syncope).
- New neurological deficits (weakness, numbness, vision changes, cognitive decline).
- Signs of active bleeding or thrombosis.

Laboratory Tests

First-Line:
- Complete Blood Count (CBC) with differential: Absolute eosinophil count (AEC) ≥ 1,500 cells/µL (1.5 x 10^9/L) on at least two occasions separated by at least 1 month, or evidence of organ damage attributable to eosinophilia.
- Erythrocyte Sedimentation Rate (ESR) and C-Reactive Protein (CRP): Often elevated, non-specific.
- Serum chemistry panel: Liver and renal function tests.
- Serum tryptase: Elevated in M-HES (FIP1L1-PDGFRA) due to mast cell proliferation.
- Serum Vitamin B12: Often elevated in M-HES.
- Troponin T and NT-proBNP: To assess for cardiac involvement/damage.
- Immunoglobulin E (IgE): Often elevated in allergic/atopic conditions.
- Parasite serology and stool ova and parasites: Essential to rule out parasitic infections (e.g., Strongyloides, Toxocara, filariasis).
- HIV testing: HIV can cause eosinophilia.
- Autoimmune serologies: ANA, RF, ANCA (to rule out EGPA/Churg-Strauss).
Confirmatory:
- Bone Marrow Biopsy with aspirate: Essential for assessing clonality and excluding other myeloid neoplasms. Look for increased eosinophils, myeloid hyperplasia, abnormal mast cells, and cytogenetics.
- Cytogenetics and FISH: Detection of FIP1L1-PDGFRA (by FISH or RT-PCR), PDGFRB, FGFR1, PCM1-JAK2 rearrangements. This is crucial for guiding treatment.
- Flow cytometry of peripheral blood and bone marrow: To detect aberrant T-cell populations (CD3-CD4+ in L-HES) or other myeloid aberrancies.
- T-cell Receptor (TCR) Gene Rearrangement studies: To confirm T-cell clonality in L-HES.
- Biopsies of affected organs (e.g., skin, GI tract, nerve, heart): To demonstrate eosinophilic infiltration and tissue damage.
Monitoring:
- Regular CBC with differential (AEC) to monitor response to therapy.
- Organ-specific tests: Troponin/NT-proBNP, liver/renal function, neurological assessments, pulmonary function tests, follow-up imaging as needed.
- Serum tryptase and Vitamin B12 (especially for M-HES).
- FIP1L1-PDGFRA quantification by PCR (for M-HES).

Imaging

Recommended:
- Echocardiogram: Essential to evaluate for cardiac involvement (e.g., endomyocardial fibrosis, restrictive cardiomyopathy, valvular abnormalities). Transesophageal echocardiogram may be more sensitive.
- Chest X-ray (CXR) / High-Resolution CT (HRCT) Chest: To assess for pulmonary infiltrates, effusions, interstitial lung disease.
- Abdominal Ultrasound/CT: To assess for hepatosplenomegaly, lymphadenopathy, or GI involvement.
- Brain MRI: If neurological symptoms are present, to evaluate for cerebrovascular events, white matter lesions, or demyelination.
Advanced:
- Cardiac MRI: Highly sensitive for detecting early endomyocardial inflammation and fibrosis. Gold standard for non-invasive cardiac assessment in HES.
- PET/CT Scan: Can be useful to identify areas of inflammation or occult malignancy, particularly in suspected L-HES or malignancy-associated eosinophilia.

Management

Management of HES is tailored to the specific HES subtype, the severity of organ involvement, and the presence of any life-threatening complications. The primary goals are to reduce eosinophil counts, prevent further organ damage, and reverse existing damage where possible.

Acute Management

Immediate:
- High-dose Corticosteroids: For life-threatening organ involvement (e.g., severe cardiac dysfunction, neurological deficits, rapidly progressive organ damage).
  - Methylprednisolone: 1 mg/kg/day to 2 mg/kg/day IV, or up to 500-1000 mg IV daily for 3-5 days in severe cases, followed by oral prednisone.
Stabilization:
- Oxygen therapy: For respiratory compromise.
- Diuretics: For fluid overload, especially with cardiac involvement.
- Anticoagulation: Consider for patients with high risk of thrombosis or documented thrombotic events, particularly in M-HES due to its myeloproliferative nature.
Emergency:
- Cardiac Catheterization/Surgery: In cases of severe restrictive cardiomyopathy or valvular dysfunction requiring intervention (e.g., valve replacement, endomyocardial stripping).
- Plasmapheresis: May be considered in very severe, steroid-refractory cases with high eosinophil counts and rapidly progressive organ damage, though evidence is limited.

Chronic Management

Pharmacological:
- Corticosteroids:
  - Prednisone: Initial oral dose of 0.5-1 mg/kg/day (typically 30-60 mg/day) tapered slowly over weeks to months to the lowest effective dose (e.g., 5-10 mg/day) or discontinued if possible. First-line for I-HES and L-HES.
- Tyrosine Kinase Inhibitors (TKIs):
  - Imatinib mesylate: First-line for FIP1L1-PDGFRA-positive M-HES. Highly effective and often curative.
    - Dosage: Initial dose 100 mg orally once daily. Response usually seen within weeks. Can be increased to 400 mg daily if needed, but often lower doses suffice. Life-long treatment typically required.
  - Other TKIs (e.g., nilotinib, dasatinib, ponatinib) for PDGFRB or FGFR1 rearrangements, or imatinib resistance.
- Immunosuppressants (for steroid-refractory or steroid-dependent HES, often I-HES or L-HES):
  - Hydroxyurea: 10-20 mg/kg/day, titrated to achieve AEC < 1,000 cells/µL. Myelosuppressive.
  - Interferon-alpha (IFN-α): 3-5 million units subcutaneously 3 times a week. Can be effective but has significant side effects (flu-like symptoms, depression).
  - Cyclosporine: 2-5 mg/kg/day. Nephrotoxic, requires therapeutic drug monitoring.
  - Azathioprine: 1-2 mg/kg/day. Myelosuppressive, hepatotoxic.
  - Methotrexate: 10-25 mg orally once weekly.
- Biologic Agents (Targeting IL-5):
  - Mepolizumab: Anti-IL-5 monoclonal antibody. Approved for HES.
    - Dosage: 300 mg subcutaneously once every 4 weeks. Can be highly effective in steroid-dependent or refractory HES, particularly in L-HES.
  - Reslizumab: Anti-IL-5 monoclonal antibody.
    - Dosage: 3 mg/kg IV once every 4 weeks.
  - Benralizumab: Anti-IL-5Rα monoclonal antibody, depletes eosinophils directly.
    - Dosage: 30 mg subcutaneously every 4 weeks for the first 3 doses, then every 8 weeks.
- Others:
  - Vincristine: Used rarely for rapidly progressive disease.
  - Chemotherapy (e.g., cladribine, cyclophosphamide): For aggressive cases, especially in CEL or advanced M-HES.
Non-pharmacological:
- Cardiac monitoring: Regular echocardiograms, ECGs, and clinical assessment for cardiac involvement.
- Dietary modifications: If GI involvement is prominent.
- Physical therapy/Occupational therapy: For neurological deficits.
- Psychological support: For chronic illness management.
Monitoring:
- Regular AEC, CBC, and differential.
- Monitoring for medication side effects (e.g., adrenal insufficiency with steroids, myelosuppression with hydroxyurea, kidney function with cyclosporine).
- Follow-up organ-specific tests (e.g., echo, PFTs, liver/renal panel) depending on affected systems.
- Molecular monitoring (e.g., FIP1L1-PDGFRA PCR) for M-HES.

Treatment Algorithms

First-Line:
1. Exclude secondary causes.
2. Determine HES subtype.
3. For FIP1L1-PDGFRA positive M-HES: Imatinib mesylate (100 mg/day initially).
4. For Idiopathic HES (I-HES) or Lymphocytic HES (L-HES): Oral corticosteroids (Prednisone 0.5-1 mg/kg/day).
Second-Line (for steroid-refractory/dependent I-HES/L-HES or if imatinib fails in M-HES):
1. For M-HES resistant to imatinib: Consider dose escalation, other TKIs (nilotinib, dasatinib), or allogeneic stem cell transplant.
2. For I-HES/L-HES:
  - Consider Mepolizumab: Especially if IL-5 pathway is suspected to be primary driver, or if steroid-sparing is desired.
  - Hydroxyurea: If biologics are not available or ineffective, or if quick cytoreduction is needed.
  - Interferon-alpha: Alternative immunosuppressant.
Refractory Cases:
- Consider alternative immunosuppressants (cyclosporine, azathioprine, methotrexate).
- Allogeneic hematopoietic stem cell transplantation (HSCT) for very aggressive, refractory forms, especially clonal myeloid HES, with high risk/benefit ratio.
- Clinical trials investigating novel agents.

Complications

Common:
- Skin lesions, pruritus.
- Gastrointestinal symptoms (abdominal pain, diarrhea).
- Fatigue, myalgia.
- Side effects of long-term corticosteroid use (osteoporosis, diabetes, hypertension, infections, cataracts).
Serious:
- Cardiac: Endomyocardial fibrosis leading to restrictive cardiomyopathy, heart failure, mural thrombi, valvular insufficiency. This is the leading cause of morbidity and mortality.
- Thromboembolism: Increased risk of both arterial and venous thrombosis (e.g., DVT, PE, stroke) due to hypercoagulability.
- Neurological: Peripheral neuropathy, encephalopathy, cognitive dysfunction, seizures, stroke.
- Pulmonary: Pulmonary fibrosis, acute respiratory distress syndrome (ARDS).
- Transformation: Rarely, M-HES can transform to acute myeloid leukemia (AML) or myelofibrosis.
Long-term:
- Chronic organ dysfunction (e.g., cardiac failure, permanent neurological deficits, pulmonary fibrosis).
- Immunosuppression-related infections.
- Long-term sequelae of chronic inflammation and fibrosis.

Prognosis

Factors: Prognosis varies significantly depending on the HES subtype, the degree of organ involvement at diagnosis, and response to therapy.
- Favorable: FIP1L1-PDGFRA-positive M-HES treated with imatinib generally has an excellent prognosis, with most patients achieving complete remission and normalization of life expectancy.
- Variable: I-HES and L-HES prognosis is more variable. Patients who respond well to steroids or biologics generally have a good prognosis.
- Poor: Poor prognostic factors include severe cardiac involvement (especially restrictive cardiomyopathy), advanced neurological damage at presentation, resistance to therapy, and transformation to AML.
Survival:
- Untreated HES, especially with significant cardiac involvement, had a 5-year survival rate of less than 10%.
- With modern treatment, overall 5-year survival rates range from 70% to 90%.
- For FIP1L1-PDGFRA HES treated with imatinib, survival is close to that of the general population.
Quality of Life: Can be significantly impacted by chronic symptoms, organ damage, and side effects of long-term therapy. Regular monitoring and supportive care are crucial for maintaining QoL.

Prevention

Primary: No known primary prevention strategies for HES, as its etiology is often genetic or idiopathic.
Secondary: Early diagnosis of HES is critical to prevent irreversible end-organ damage. Prompt investigation of unexplained persistent eosinophilia is important.
Tertiary:
- Aggressive management of eosinophilia: To prevent progression of organ damage.
- Regular monitoring: For organ dysfunction (especially cardiac, neurological).
- Prophylactic anticoagulation: May be considered in high-risk patients with M-HES or those with a history of thrombotic events.
- Close follow-up: To detect and manage complications of the disease and its treatment.

Special Populations

Pediatric: HES can occur in children, though it is less common. Diagnosis and management principles are similar, but careful consideration of growth, development, and long-term side effects of medications (e.g., corticosteroids) is necessary. Genetic causes may be more prevalent.
Geriatric: Older patients may have more comorbidities, affecting treatment choices and tolerance. Renal and cardiac function should be carefully assessed before initiating therapies, and drug dosages may need adjustment. Risk of medication side effects (e.g., osteoporosis with steroids) is higher.
Pregnancy: Management of HES in pregnancy is challenging. Corticosteroids can be used, but risks (e.g., gestational diabetes, pre-eclampsia, fetal growth restriction) must be balanced against disease activity. Imatinib is generally contraindicated, requiring careful risk-benefit assessment or alternative therapies. Mepolizumab's safety in pregnancy is not fully established but considered low risk. Close obstetric and hematology/immunology collaboration is essential.
Comorbidities:
- Cardiac disease: Requires careful management of HES to prevent exacerbation of heart failure or arrhythmias.
- Renal/hepatic impairment: May necessitate dose adjustments for medications.
- Immunocompromised states: Increased risk of opportunistic infections with immunosuppressive therapy.

Clinical Pearls

Diagnostic:
- Always thoroughly exclude secondary causes of eosinophilia, especially parasitic infections and drug reactions, before diagnosing HES.
- Don't wait for 6 months of eosinophilia if there's clear evidence of eosinophil-mediated organ damage.
- FIP1L1-PDGFRA testing is paramount in all patients suspected of HES, as it dictates highly effective targeted therapy (imatinib).
- Cardiac MRI is the most sensitive non-invasive test for early cardiac involvement.
Treatment:
- Imatinib is a game-changer for FIP1L1-PDGFRA positive HES; ensure patients receive it.
- Corticosteroids are the cornerstone for idiopathic and lymphocytic HES, but aim for the lowest effective dose to minimize side effects.
- Consider biologic agents (e.g., mepolizumab) early for steroid-dependent or refractory cases to reduce steroid burden.
- Prophylactic anticoagulation may be necessary, especially in M-HES due to increased thrombosis risk.
Pitfalls:
- Misdiagnosing HES as a simple allergic reaction or asthma, delaying crucial diagnosis and treatment.
- Failing to screen for the FIP1L1-PDGFRA fusion gene, leading to inappropriate and less effective treatment.
- Underestimating the severity of cardiac involvement; it's often asymptomatic until late stages but is a major cause of mortality.
- Abruptly stopping corticosteroids without a proper taper, leading to adrenal crisis.

Recent Updates

Guidelines: The 2022 WHO classification and 2017 International Consensus Classification (ICC) have refined the diagnostic criteria and classification of eosinophilic disorders, emphasizing molecular diagnostics.
Evidence: Increased understanding of the role of IL-5 in eosinophil biology has led to the development and approval of targeted biologic therapies (e.g., mepolizumab for HES), offering new steroid-sparing options.
Controversies:
- The optimal duration of imatinib therapy for FIP1L1-PDGFRA positive HES, particularly concerning the potential for discontinuation in patients with sustained molecular remission.
- The precise role and optimal sequencing of newer biologic agents (mepolizumab, reslizumab, benralizumab) in the treatment algorithm for I-HES and L-HES, especially in comparison to conventional immunosuppressants.
- The utility of routine prophylactic anticoagulation in all HES patients vs. risk-stratified approaches.

Key References

Guidelines:
- International Consensus Classification of Myeloid Neoplasms and Acute Leukemias. Blood. 2022.
- World Health Organization (WHO) Classification of Tumours of Haematopoietic and Lymphoid Tissues (5th Edition). 2022.
- American Academy of Allergy, Asthma & Immunology (AAAAI) and American College of Allergy, Asthma & Immunology (ACAAI) guidelines on eosinophilic disorders.
Studies:
- Landmark studies on imatinib for FIP1L1-PDGFRA positive HES (e.g., Cools J et al., N Engl J Med 2003; Pardanani A et al., Lancet 2006).
- Clinical trials for mepolizumab in HES (e.g., Chupp GL et al., N Engl J Med 2020).
Reviews:
- Contemporary reviews on the diagnosis and management of HES from major hematology and immunology journals (e.g., Blood, Leukemia, Journal of Allergy and Clinical Immunology).