Fulminant Eosinophilic Myocarditis in Steroid-Refractory Hypereosinophilic Syndrome: Navigating the Thrombotic Stage With Targeted Biologic Therapy.

BACKGROUND: Hypereosinophilic syndrome is a rare hematologic disorder with multiorgan involvement. Its most lethal form, Loeffler endomyocarditis, often results in fatal cardiac or embolic events. CASE SUMMARY: A 57-year-old woman presented with chest pain, an absolute eosinophil count of 42 × 109/L, and a peak troponin of 47,000 ng/L. She developed acute encephalopathy from a cardioembolic storm and eosinophilic pleocytosis. Although echocardiography was inconclusive, cardiac magnetic resonance (CMR) identified a large laminar thrombus and subendocardial necrosis. Despite pulse-dose steroids and hydroxyurea, she suffered a hematologic relapse. Initiation of mepolizumab (anti-interleukin-5) led to rapid eosinophil normalization and clinical recovery. DISCUSSION: This case highlights the diagnostic superiority of CMR over echocardiography for detecting laminar thrombi. It illustrates the limitations of steroids in suppressing the myeloproliferative drive and demonstrates mepolizumab's efficacy as an acute rescue therapy. TAKE-HOME MESSAGES: CMR is mandatory in suspected eosinophilic myocarditis when echocardiography is negative. In steroid-refractory hypereosinophilic syndrome, early interleukin-5 inhibition is a life-saving intervention.