A Rapid Pathological Diagnosis Based on Unfixed Frozen Tissue Section Leads to the Misdiagnosis of Eosinophilic Myocarditis as Lymphocytic Myocarditis.

Acute myocarditis requires an urgent endomyocardial biopsy for an accurate diagnosis and appropriate therapeutic decision-making. We herein report the case of a 64-year-old man who presented with fever and progressive dyspnea and was initially diagnosed with acute myocarditis based on the clinical and imaging findings. A rapid pathological analysis of unfixed frozen tissue obtained from an endomyocardial biopsy suggested lymphocytic myocarditis; however, a subsequent examination of formalin-fixed paraffin-embedded tissue revealed extensive eosinophilic infiltration, thus confirming eosinophilic myocarditis. Clinical and functional improvements were observed after immunosuppressive therapy. This case highlights the diagnostic challenges associated with different tissue preservation techniques, emphasizing the limitations of a frozen section analysis.