Sucrase–Isomaltase Deficiency in Children with Functional Gastrointestinal Disorders

Background: Congenital sucrase–isomaltase deficiency (CSID) may mimic functional gastrointestinal disorders (FGIDs) and is likely underrecognized in pediatric practice. This study aimed to determine the frequency of sucrase–isomaltase (SI) gene variants among children with FGIDs and to evaluate genotype–phenotype associations and treatment-related quality-of-life outcomes. Methods: In this prospective cross-sectional study, children aged 0–18 years diagnosed with FGIDs according to Rome IV criteria were enrolled between May 2022 and January 2023. All patients underwent next-generation sequencing for SI gene variants. Clinical characteristics, FGID subtypes, and anthropometric data were recorded. Variant-positive patients received dietary sucrose restriction, and selected patients were treated with sacrosidase enzyme replacement. Symptom severity was assessed using the Numeric Rating Scale, and quality of life was evaluated with the Pediatric Quality of Life Inventory (PedsQL 4.0). Results: Among 290 children with FGIDs, SI gene variants were identified in 17 patients (5.9%). Variants were more frequently detected in children with irritable bowel syndrome–like symptoms. Clinical presentation was heterogeneous, and no consistent genotype–phenotype correlation was observed. Dietary intervention was associated with symptom improvement in compliant patients, while sacrosidase therapy led to significant improvements in both child- and parent-reported PedsQL scores. Conclusions: Sucrase–isomaltase deficiency is not uncommon among children with FGIDs and should be considered, particularly in those with IBS-like symptoms or diet-related complaints. Integrating genetic evaluation with targeted dietary and enzyme-based therapy may improve symptom control and quality of life in selected pediatric patients.