Hypereosinophilia or hypereosinophilic syndrome is a disease characterised by a marked increase in the eosinophil count in the bloodstream.
The eosinophil count in human blood is normally 0.4 × 109/L (0.1 - 0.6) and results from a balance between production of eosinophils and emigration through post-capillary venules (Yamaguchi et al. 1991). Eosinophils are only a small minority of peripheral blood leucocytes and in normal subjects, most are found in the tissues of the lung and gastro-intestinal tract (Beeken et al. 1987). Blood eosinophil counts are arbitrarily classified as mild - between 0.6 to 1.5 × 109/L; moderate between 1.5 to 5 × 109/L and severe when greater than 5 × 109/L.
An elevated blood eosinophil count may be associated with a number of reactive conditions and with clonal disorders of the bone marrow. However, when the blood eosinophil count is persistently greater than 1.5 × 109 /L, for a period of more than six months, damage to end organs such as the heart, lungs, skin, joints and nervous system can be demonstrated, and in the absence of any clonal or reactive cause, the term idiopathic hypereosinophilic syndrome (HES) is used. The three defining criteria of HES are therefore:
- Eosinophil count persistently greater than 1.5 × 109/L
- Damage to end-organs
- No ascertainable cause for the eosinophilia and no evidence of clonality.
There are three categories of blood eosinophilia: