Analysis of persistent sunken brain after cranioplasty as a cause of extracerebral fluid collection

Extracerebral fluid (ECF) collection is a complication of cranioplasty following decompressive craniectomy. The perioperative course of brain sinking as a cause of ECF was analyzed in patients with stroke and trauma. Patients who underwent cranioplasty with an autologous preserved cranial flap between 2015 and 2025 at the Yamaguchi University Hospital were included. Brain sinking distances were measured before surgery and on postoperative days (POD) 1, 7, and 90. Sunken brain incidence, risk factors, natural course, and requirements for additional treatment were evaluated. The total number of patients was 112, aged 65.6 ± 16.7 years, 43 of them had acute subdural hematomas, 30 cerebral infarctions, and 28 subarachnoid hemorrhages. Preoperative brain sinking distance of 4.0 ± 8.0 mm significantly increased to 7.5 ± 7.7 mm at POD 1 (p = 0.00004). Sunken brain incidence was 10.7% preoperatively, increased to 22.3% on POD 1, did not decrease by POD 7 (20.5%), and reached 1.8% on POD 90. Re-craniectomy or shunt interruption was required in four (3.6%) patients, with the sunken brain developing into a shifted brain. A risk factor for postoperative sunken brain was preoperative sunken brain (p = 0.005), and individual pre- and postoperative sunken brains were mildly correlated (r = 0.42). Brain volume was not restored immediately after cranioplasty. ECF filled the gap between the cranium and the sunken brain, persisted in volume beyond POD 7, and possibly increased when the sinking deepened due to accelerated venous return and cerebrospinal fluid absorption during postoperative care.