Incidence and predictors of contralateral surgery after initial unilateral evacuation of bilateral chronic subdural hematomas: A population-based cohort study.

Introduction: The optimal surgical management of bilateral chronic subdural hematoma (bCSDH) remains debated, particularly whether and when to evacuate unilaterally or bilaterally at index surgery. Research question: This study aimed to determine the incidence and predictors of later contralateral surgery after initial unilateral evacuation of bCSDH. Material and methods: We conducted a population-based retrospective cohort study of adults (≥18 years) treated for bCSDH at Karolinska University Hospital (2006 - 2023). Standard surgical approach was single burr-hole craniostomy followed by 24-h subgaleal drainage. The primary outcome was evacuation of the initially non-operated hematoma within 6 months. Candidate predictors included preoperative clinical and radiological variables, as well as intraoperative treatment-related variables. Univariable analyses and backward stepwise multivariable logistic regression identified independent predictors. Results: Of 861 bCSDH cases, 401 underwent initial unilateral evacuation. Of these, later contralateral surgery was required in 46/401 (11%) at a median of 21 days (IQR 12 - 31) after the index procedure. Non-operated hematoma volume at index surgery was the only predictor of later contralateral evacuation (adjusted OR 1.01 per mL, p = 0.022), with a Nagelkerke pseudo-R Discussion and conclusion: In this population-based study, 11% of patients with bCSDH required contralateral surgery following unilateral evacuation, with a median time to surgery of 21 days. Non-operated hematoma volume at index surgery was statistically associated with later contralateral surgery, although its predictive performance was limited. These findings may help inform individualized postoperative surveillance strategies, particularly in patients with larger non-operated hematoma volumes.