Real-World Use of Hybrid Closed-Loop System in Very Young Children with Type 1 Diabetes: Daily Glycemic Patterns Support Glycemic Improvements and Highlight Emerging Challenges for Hypoglycemia

<b><i>Background and Aims:</i></b> The management of very young children with type 1 diabetes (T1D) presents a considerable challenge for health care teams and the families from glycemic and psychosocial perspectives. The aim of this study was to evaluate the impact of hybrid closed-loop (HCL) system on glycemic control, with a particular focus on the patterns of glycemic parameters throughout the day. <b><i>Methods:</i></b> Seventy-two children under the age of six diagnosed with T1D (diabetes duration ≥6 months) who transitioned to CamAPS-FX-HCL were included. Data were collected prior to (pre-HCL) and at 1, 3, 9, and 12 months following HCL initiation. Pre-HCL and post-HCL periods were compared using linear mixed models. <b><i>Results:</i></b> Time in range (TIR_70-180) increased by 11% ± 8% from the pre-HCL to 1 month post-HCL (<i>P</i> < 0.001), with a concomitant decrease in target above range (TAR₁₈₀) of 13% ± 9% (<i>P</i> < 0.001). The improvement in glycemic control was sustained through 12 months, with the greatest differences observed overnight (<i>P</i> < 0.001). Hypoglycemic metrics (TBR₇₀/TBR₅₄) remained similar before and after HCL (all <i>P</i> > 0.05). However, hourly glycemic patterns indicated that the late-morning period after HCL initiation was a high-risk period for hypoglycemia with TBR₇₀ peaking at 11% ± 9% between 11 AM and 1 PM. This elevated risk of hypoglycemia persisted after 1 year. <b><i>Conclusions:</i></b> In real-life setting, HCL was associated with increased TIR and reduced TAR in very young children with T1D from the first month of use. The present study suggests that late morning is a high-risk window for hypoglycemia in this age group. This finding highlights the importance of optimizing HCL therapy settings and educating parents about specific times of the day that carry a higher risk of glycemic instability.