Hypereosinophilic syndrome

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The hypereosinophilic syndrome is a disease process characterized by a persistently elevated eosinophil count (≥ 1500 eosinophils/mm³) in the blood for at least six months without any recognizable cause after a careful workup, with evidence of involvement of either the heart, nervous system, or bone marrow.

There are two forms of the hypereosinophilic syndrome: Endomyocardial fibrosis and Loeffler's endocarditis. Endomyocardial fibrosis (also known as Davies disease) is seen in Africa and South America, while Loeffler's endocarditis does not have any geographic predisposition.

In both forms of the hypereosinophilic syndrome, the eosinophilia causes infiltration of the myocardium of the heart, which leads to fibrotic thickening of portions of the heart. The portions of the heart most effected by this disease are the apex of the left and right ventricles, fibrotic infiltrations may involve the mitral or tricuspid valves. Because of the infiltrative nature of the disease process, the cavity of the ventricles of the heart diminish in size, causing an obliterative cardiomyopathy and restriction to the inflow of blood in to the chambers of the heart. Ventricular mural thromb may develop.

Chronic eosinophilic leukemia (CEL) is a myeloproliferative disease which shares many common characteristics with hypereosinophilic syndrome. Many cases of CEL have a characteristic gene rearrangement , FIP1L1/PDGFRA, caused by a sub-microscopic deletion of ~800 thousand base pairs of DNA on chromosome 4. The FIP1L1/PDGFRA fusion gene causes constitutive activation of the platelet derived growth factor receptor - alpha (PDGFRA). FIP1L1/PDGFRA-positive patients respond well to treatment with tyrosine kinase inhibitor drugs, such as imatinib mesylate (Gleevec® or Glivec®).