Eosinophilic Myocarditis Treated with IL-5 Blockade: An Integrated Case Report and Literature Review

Background/Objectives: Eosinophilic myocarditis (EM) is a rare, life-threatening inflammatory cardiomyopathy driven by eosinophil cytotoxicity and extracellular trap formation. Interleukin-5 (IL-5) inhibition may disrupt this pathogenic cascade. We reviewed contemporary evidence on IL-5 blockade in EM and contextualized it with an illustrative case. Methods: We searched PubMed through May 2025 for reports of EM treated with mepolizumab or benralizumab. Inclusion criteria were consistent with prior cohorts: acute cardiac symptoms with biomarker elevation plus abnormalities on transthoracic echocardiography and/or cardiac magnetic resonance imaging (CMR), along with documented IL-5-targeted therapy. We extracted clinical, imaging, biopsy, treatment-timing, and outcome data and included one institutional case. Results: Twenty-one episodes were analyzed (median age, 45 years; 10 men). Underlying conditions included eosinophilic granulomatosis with polyangiitis (10 cases; 48%), hypereosinophilic syndrome (5 cases; 24%), drug reaction with eosinophilia and systemic symptoms (DRESS, 3 cases; 14%), and eosinophilic asthma (3 cases; 14%). Treatments involved mepolizumab in 17 cases (81%) and benralizumab in 4 (19%); 4 patients received "early-start" therapy within 14 days of EM diagnosis. Among the 11 episodes with reported left ventricular ejection fraction (LVEF) at baseline and follow-up, the median baseline LVEF was 40% (range, 30-62), with 10 of 11 (91%) <50%. On follow-up, all 11 patients improved: 4 normalized (≥50%) and 7 improved to 40-49%. CMR (n = 18) demonstrated late gadolinium enhancement in 14 cases (78%), edema in 9 (50%), and intracardiac thrombus in 4 (22%). Endomyocardial biopsy confirmed eosinophilic infiltration in 13 of 15 cases (87%). Outcomes included one death (fulminant DRESS), one recovery following veno-arterial extracorporeal membrane oxygenation, and one successful heart transplantation. Illustrative case: A 24-year-old man on a steroid taper received mepolizumab 300 mg on Day 4. His LVEF improved from 47% to 59% by Day 15, accompanied by biomarker decline and successful steroid tapering. Conclusions: Across published cases and our institutional experience, IL-5-targeted therapy appears safe, steroid-sparing, and associated with rapid ventricular recovery, particularly when initiated early. Although limited, these findings support the need for prospective trials to define the optimal agent, dosing, timing, and integration with standard immunosuppression and anticoagulation.