Case Report: Neonatal-onset chylomicron retention disease presenting as isolated failure to thrive with compound heterozygous SAR1B variants: the value of early genetic testing and challenges of long-term management

Chylomicron retention disease (CMRD) is a rare autosomal recessive disorder caused by pathogenic variants of <i>SAR1B</i>, in which defective intestinal chylomicron secretion leads to fat malabsorption, hypocholesterolemia, and failure to thrive in infancy. Its diagnosis is typically challenging because of its rarity, nonspecific early symptoms, and overlap with other metabolic and malabsorptive disorders. The present case is notable for its neonatal onset with isolated failure to gain weight and the absence of persistent diarrhea or steatorrhea, complicating early clinical suspicion. Initial metabolic screening revealed overlapping abnormalities with urea cycle disorders and fatty acid oxidation defects, underscoring diagnostic complexity. A definitive diagnosis was achieved through the identification of compound heterozygous likely pathogenic <i>SAR1B</i> variants, c.258G > A (p.Trp86Ter) and c.442C > T (p.Arg148Ter), thereby expanding the known phenotypic and genotypic spectrum of CMRD. The patient exhibited marked clinical and biochemical improvements following timely intervention with a low-fat, medium-chain triglyceride(MCT)-enriched diet and fat-soluble vitamin supplementation. However, subsequent follow-up revealed suboptimal adherence to the dietary regimen, leading to the emergence of typical steatorrhea, persistent growth failure, and neurodevelopmental delay, highlighting the critical and sustained role of strict nutritional management. This case highlights the importance of heightened clinical vigilance, timely genetic testing, and the necessity of ensuring long-term treatment adherence in infants presenting with unexplained growth failure and subtle hepatic abnormalities even when classic symptoms are absent. Taken together, this study provides valuable insights into the diagnostic challenges, therapeutic pitfalls, and management strategies of CMRD, emphasizing the need for multidisciplinary collaboration, enhanced awareness among clinicians, and further research to elucidate genotype-phenotype correlations and optimize patient care.