Hypereosinophilic syndrome is a condition characterized by peripheral blood eosinophilia with manifestations of organ system involvement or dysfunction directly related to eosinophilia in the absence of parasitic, allergic, or other secondary causes of eosinophilia. Symptoms are myriad, depending on which organs are dysfunctional. Diagnosis involves excluding other causes of eosinophilia plus bone marrow and cytogenetic tests. Treatment usually begins with prednisone and, in one common subtype, includes imatinib.

Hypereosinophilic syndrome was previously considered to be idiopathic but is now known to result from various disorders, some of which have known causes. One limitation of the traditional definition is that it does not include those patients with some of the same abnormalities (eg, chromosomal defects) that are known causes of hypereosinophilic syndrome but who do not fulfill the traditional hypereosinophilic syndrome definition for degree or duration of eosinophilia. Another limitation is that some patients with eosinophilia and organ damage that characterize hypereosinophilic syndrome require treatment earlier than the 6 months necessary to confirm the traditional diagnostic criteria. Eosinophilia of any etiology can cause the same types of tissue damage.

The use of the term HES has evolved over the last 40 years since its first use by Hardy and Anderson to describe 3 patients with marked eosinophilia and eosinophilic cardiopulmonary involvement.  Not only have improved diagnostic techniques led to the identification of previously unrecognized causes of HES, but the availability of effective therapies has led to a marked decrease in morbidity and mortality in patients with HES who are treated early (before the development of irreversible complications). In an attempt to address these issues, updated definitions and classification systems for HES have been proposed by the World Health Organization (WHO),  consensus panels,  and other experts. Two major controversies remain: whether to include eosinophilic disorders of known etiology in the broad classification of HES and, if so, which disorders to include and how to define eosinophilic end organ damage.

Idiopathic hypereosinophilia (also termed hypereosinophilia of undetermined significance, i.e. HE_US) is a disorder characterized by an increase in eosinophil blood counts above 1,500/μL, as detected on at least 2 separate examinations. The disorder cannot be associated with eosinophil-based tissue damage or a primary or secondary cause of eosinophilia. That is, it is a diagnosis of exclusion and has no known cause. Over time, this disorder can resolve into a primary hypereosinphilia, typically clonal hyperesinophilia, chronic eosinphilic leukemia, or an eosinophilia associated with another hematological leukemia. The disorder may also become associated with tissue or organ damage and therefore be diagnosed as the hypereosinophilic syndrome. Idiopathic hyereosinophilia is treated by observation to detect development of the cited more serious disorders.

Regards
Rutherford
Managing Editor
Pharmacy Practice and Education.