Idiopathic hypereosinophilic syndrome (HES) represents a group of leukoproliferative disorders characterized by overproduction of eosinophils due to an undetectable cause that results in organ damage. Chusid et al have defined 3 features required for diagnosis of HES.

	The eosinophil count exceeds 1500 cells/cumm and persists for at least 6 months.
	No etiology for eosinophilia is present.
	It leads to organ damage.

HES has been reported in women between 20 to 50 years and is extremely rare in children. Organ damage occurs due to increased survival of eosinophils in inflamed tissue for a prolonged time leading to release of toxic cationic proteins from its granules such as major basic protein, eosinophil peroxidase, eosinophil-derived neurotoxin and eosinophilic cationic protein. A study from NIH has found hematologic involvement in all, neurological in 64%, skin in 56%, splenomegaly in 45%, pulmonary in 40%, cardiovascular in 54%, hepatomegaly in 35% and ocular involvement in 18% of patients with HES. In our patient, bone marrow, liver and spleen were involved with no peripheral organ damage

Peripheral eosinophilia is associated with other disorders such as allergic diseases, parasitic infections, eosinophilia-myalgia syndrome, Churg-Strauss syndrome and malignancy which need to be excluded before a diagnosis of HES is made. Patients without organ involvement may have benign eosinophilia which may exist for years with no associated pathology. Eosinophilic leukemia needs to be excluded as both conditions tend to overlap. However, patients with eosinophilic leukemia have clonal abnormalities, >25% immature eosinophils in peripheral blood or more than 5% myeloblasts in bone marrow. Bone marrow cytogenetics in our patient was normal.

Clinical features may include cardiac symptoms in form of thrombosis, endomyocardial fibrosis; hematological manifestations in form of anemia, thrombosis, hypercoagulability; neurological manifestations in form of embolic episodes, encephalopathy, peripheral neuropathy; pulmonary manifestations in form of eosinophilic infiltrates in the lungs, dyspnea, pleural effusion, pulmonary fibrosis, rhinitis; dermatologic manifestations in form of pruritis and other features such as arthralgia, myalgia, diarrhea, fever, weight loss and night sweats.

Medical treatment is needed for symptomatic disease. Patients with no organ dysfunction despite eosinophilia need no treatment except close follow up at 3-6 intervals. Symptomatic patients should be treated with prednisolone therapy (1 mg/kg/d) till clinical improvement occurs followed by alternate day therapy and gradual tapering. Those with non-response to steroids can be treated with hydroxyurea, vincristine, 6-mercaptopurine, busulphan, and chlorambucil.