Idiopathic hypereosinophilic syndrome (HES) is an uncommon condition characterized by an unexplained elevation of absolute eosinophil count (AEC) to ≥1.5x109/l for at least six months, and is frequently associated with eosinophil-mediated end organ damage. Idiopathic HES, as with secondary HES and primary hypereosinophilic clonal hematopoietic disorders, has a high incidence of myocardial, pulmonary, neurological and other organ injury. Myocardial fibroelastosis and valvular lesions are common, and successful treatment with valve replacement or resection of fibrotic myocardium has

been reported. The case is described of a patient with idiopathic HES and multi-organ complications including severe mitral valve disease, in whom a functionally obstructive thrombosis of a newly inserted prosthetic mitral valve occurred despite adequate anticoagulation, when the AEC was profoundly elevated. Recurrent thrombosis has not occurred over a substantial period following AEC reduction with corticosteroids, and subsequent maintenance at normal levels.

The Journal of Heart Valve Disease 2006;15:721-725