Case Report of Overlap of Diabetic Ketoacidosis and Hyperosmolar Hyperglycemic State in a 5-Year-Old with New-Onset Type 1 Diabetes Mellitus: Diagnostic and Management Considerations

<b>Background and Clinical Significance:</b> Overlap of diabetic ketoacidosis (DKA) and hyperosmolar hyperglycemic state (HHS) in children is a rare but life-threatening metabolic emergency. The coexistence of hyperosmolality and ketoacidosis increases neurologic vulnerability and complicates fluid and insulin management. Early identification and osmolality-guided therapy are essential to prevent cerebral edema and other complications. This case describes a 5-year-old boy with new-onset type 1 diabetes mellitus (T1D) presenting with DKA/HHS overlap two weeks after influenza vaccination-an unusual temporal association without proven causality. <b>Case Presentation:</b> A previously healthy 5-year-old presented with progressive polyuria, polydipsia, nocturnal enuresis, fatigue, and drowsiness. Two weeks earlier, he had received the influenza vaccine. Examination revealed moderate dehydration without Kussmaul respiration or altered consciousness. Laboratory evaluation showed glucose 45.9 mmol/L (826 mg/dL; reference 3.9-7.8 mmol/L), venous pH 7.29 (reference 7.35-7.45), bicarbonate 12 mmol/L (reference 22-26 mmol/L), moderate ketonuria, and measured serum osmolality 344 mOsm/kg (reference 275-295 mOsm/kg), fulfilling diagnostic criteria for DKA/HHS overlap. After an initial 20 mL/kg 0.9% NaCl bolus, fluids were adjusted to maintenance plus approximately 10% deficit using 0.45-0.75% NaCl according to sodium/osmolality trajectory. Intravenous insulin (approximately 0.03-0.05 IU/kg/h) was initiated once blood glucose no longer decreased adequately with fluids alone and had stabilized near 22.4 mmol/L (≈400 mg/dL). Dextrose was added when glucose reached 13.9 mmol/L (250 mg/dL) to avoid rapid osmolar shifts. Hourly neurological and biochemical monitoring ensured a glucose decline of 2.8-4.2 mmol/L/h (50-75 mg/dL/h) and osmolality decrease ≤3 mOsm/kg/h. The patient recovered fully without cerebral edema or neurologic sequelae. IA-2 antibody positivity with low C-peptide and markedly elevated HbA1c confirmed new-onset T1D. <b>Conclusions:</b> This case highlights the diagnostic and therapeutic challenges of pediatric DKA/HHS overlap. Osmolality-based management, conservative insulin initiation, and vigilant monitoring are crucial for preventing complications. The temporal proximity to influenza vaccination remains incidental.