Case Presentation: A 51 y/o male active smoker, presented with sudden onset of dizziness, confusion, right arm weakness, and numbness. He also reported left-sided chest pain and mild headache five days ago. The examination revealed decreased sensation and strength (3/5) in the right upper extremity. Labs notable for leukocytosis=22k/uL (60% eosinophils), high sensitivity troponin 440ng/L, and CRP 47mg/L. EKG and head CT were unremarkable. MRI brain showed multiple small acute cerebral infarcts in different vascular territories consistent with a cardio-embolic source. A peripheral blood smear revealed isolated eosinophilia. TEE revealed thickening of aortic valve leaflets. Subsequent cardiac MRI revealed significant myocardial edema/infiltrate and subendocardial enhancement of many segments with mild microvascular obstruction, suggestive of early endomyocardial fibrosis. He had normal B12, IgE and Tryptase levels and Hepatitis C, HIV, serum immunoglobulins, c-ANCA, p-ANCA, blood cultures, and stool tests were unremarkable ruling out possible etiologies of hyper-eosinophilia. Given the infiltrative pattern on cardiac MRI and concern for HES, Prednisone therapy was started pending results of bone marrow and right ventricular (RV) endomyocardial biopsies. RV biopsy showed focal endocardial mural thrombosis. Bone marrow (BM) aspirate had 22% eosinophils without atypia. BM biopsy reported as normocellular with trilineage hematopoiesis. Flow cytometry of BM was negative for B- or T-cell neoplasm, abnormal blasts, or aberrant CD56 expression on the myeloid cells. FISH was negative for PDGFR, FGFR or JAK2 mutations. With a presumed diagnosis of idiopathic HES, the patient was discharged on prednisone and anticoagulation

Discussion: HES is a heterogeneous disease with varied manifestations. Cardiovascular lesion pathologically described as eosinophilic myocarditis or Loeffler’s endocarditis is a major cause of morbidity and mortality in the majority of patients (1). Cardiac injuries related to eosinophilia occur in three chronological phases: eosinophilic infiltration, thrombosis, and fibrosis (2,3). Brain infarcts in HES occur due to thromboembolism from endomyocardial fibrosis or vascular endothelial toxicity of eosinophilic cells (4). Immunosuppressive therapy is the mainstay of treatment, and if administered early in the course of the disease, can improve prognosis.

Conclusions: Clinicians should be vigilant about thromboembolic complications of HES and initiate early corticosteroid and anticoagulant therapies, without waiting for tissue confirmation.