Idiopathic Hypereosinophilic Syndrome

Definition

Persistent eosinophilia with tissue damage but without proven clonality or increase in blasts

All of the following must be demonstrated
- Persistent peripheral blood eosinophilia ≥1.5 x 10³/μL
  - Duration ≥6 months
  - Eosinophils may have hyper or hyposegmented nuclei
    - May have nuclear enlargement
- Reactive (allergy etc.) and secondary (T cell lymphoma etc.) eosinophilia should be excluded (see Differential Diagnosis)
- Evidence of organ involvement with tissue damage must be present
  - Endomyocardial fibrosis is the most significant complication
  - May also involve lungs, nervous system, skin and gastrointestinal tract
  - If no tissue damage, consider Idiopathic Hypereosinophilia
- No evidence of clonality on cytogenetic or molecular studies
  - Exclude BCR-ABL1, PDGFRA, PDGFRB FGFR1 and KIT abnormalities and inv(16) as indicated
    - If positive, consider CML or Hematolymphoid neoplasms with PDGFR/FGFR Abnormalities, systemic mastocytosis or AML with inv(16)
- No increase in myeloblasts or dysplastic features
  - Peripheral blood ≤2%
  - Bone marrow ≤5%
  - If increased, see Chronic Eosinophilic Leukemia
As defined, Hypereosinophilic Syndrome is probably a mixed group
- Some may be unrecognized reactive or secondary and some may be unrecognized clonal neoplasms
- Patients should be followed for subsequent development of evidence of clonality, an increase in myeloblasts or overt T cell lymphoma

Dita Gratzinger MD PhD
Tracy I George MD
Department of Pathology
Stanford University School of Medicine
Stanford CA 94305-5342

Original posting: 10/23/11