Relation Between Neutrophil Count and Left Ventricular Ejection Fraction Following Acute Myocarditis in Adolescents: A Preliminary Study

(1) Background: The clinical course of acute myocarditis in adolescents is heterogeneous, and reliable predictors of early functional changes remain limited, particularly in patients without severe systolic dysfunction. Routine hematologic parameters may reflect the early inflammatory response, but their prognostic relevance in pediatric non-fulminant myocarditis is poorly defined. This exploratory study aimed to assess whether admission inflammatory blood indices are associated with short-term changes in left ventricular systolic function in adolescents with acute myocarditis. (2) Methods: We retrospectively analyzed 44 adolescents (median age 16 years, 84% male) hospitalized with suspected acute non-fulminant myocarditis between 2020 and 2023. All patients had preserved or mildly reduced left ventricular ejection fraction (LVEF) at presentation. Clinical, laboratory, electrocardiographic, and echocardiographic data obtained at admission were analyzed. Changes in LVEF between the acute and post-acute phases during hospitalization were assessed using transthoracic echocardiography. Cardiac magnetic resonance imaging was performed in a subset of patients to support diagnosis but was not uniformly available for quantitative analysis. (3) Results: No in-hospital deaths occurred. A modest positive correlation was observed between neutrophil count at admission and improvement in LVEF during hospitalization (r = 0.348, <i>p</i> = 0.028). No significant associations were found between LVEF change and white blood cell count, lymphocyte count, monocyte count, neutrophil-to-lymphocyte ratio (NLR), troponin I, or NT-proBNP. (4) Conclusions: In adolescents with non-fulminant acute myocarditis and preserved or mildly reduced systolic function, admission neutrophil count was associated with short-term improvement in left ventricular ejection fraction. Given the retrospective design, limited sample size, and absence of mechanistic data, these findings should be interpreted as hypothesis-generating. Further prospective studies incorporating standardized cardiac magnetic resonance imaging and immunologic profiling are needed to clarify the clinical significance of this association.