Thrombophlebitis in an Adolescent Patient With Primary Carnitine Deficiency: A Case Report

Primary carnitine deficiency (PCD) is a rare autosomal recessive disorder caused by biallelic pathogenic variants in SLC22A5, leading to defective carnitine transport and impaired mitochondrial long-chain fatty acid β-oxidation. Lifelong L-carnitine supplementation constitutes the mainstay of therapy and has been shown to effectively prevent metabolic decompensations. However, oral carnitine is partially metabolized by gut microbiota to trimethylamine, which is subsequently converted to trimethylamine N-oxide (TMAO), a metabolite that has been associated with increased thrombotic risk in both experimental and observational studies. We report the case of a 14-year-old boy with PCD on long-term, high-dose L-carnitine who developed acute superficial thrombophlebitis of the left great saphenous vein in the presence of previously undiagnosed congenital infrarenal inferior vena cava (IVC) agenesis with extensive collateral varices. No conventional prothrombotic risk factors were identified. The patient was successfully treated with anticoagulation while L-carnitine therapy was continued. This case illustrates that congenital IVC anomalies may predispose patients with PCD to venous thrombosis and raises the hypothesis that chronic L-carnitine supplementation may contribute as an additional prothrombotic factor through TMAO elevation.