The Idiopathic Hypereosinophilic Syndrome

THE IDIOPATHIC hypereosinophilic syndrome represents a leukoproliferative process likely caused by a number of disorders, all of which are marked by sustained overproduction of eosinophils.1In addition to the hallmark eosinophilia, this syndrome is distinctive because of its predilection for damaging specific organs, including the heart. The consequent endomyocardial fibrosis is not restricted to the idiopathic hypereosinophilic syndrome; identical cardiac damage may develop with eosinophilias secondary to other diseases with identifiable causes. Notably, not all patients with hypereosinophilia develop the organ damage characteristic of the hypereosinophilic syndrome. No specific tests are diagnostic of the hypereosinophilic syndrome; rather, the syndrome is defined by the combination of unexplained prolonged eosinophilia and evidence of organ involvement. DEFINITION OF THE SYNDROME From analyses of eosinophilic patients referred to the National Institutes of Health and other reported cases of hypereosinophilic syndrome, Chusid and colleagues2in 1975 identified the three defining features of the