Massive brain swelling after cranioplasty in the setting of severe sunken flap syndrome: illustrative case

BACKGROUND: Massive brain swelling after cranioplasty (MBSC) is a rare but frequently fatal complication characterized by cerebral edema following cranial vault reconstruction. Evidence suggests that MBSC occurs in patients with chronic intracranial hypotension, often in the setting of severe sinking skin flap (SSF) syndrome after decompressive craniectomy (DC). OBSERVATIONS: The authors report a case of MBSC in a 42-year-old man who underwent custom cranioplasty 5 months after DC. Preoperative imaging demonstrated SSF syndrome with a 15-mm midline shift. Within 1 hour after surgery, the patient developed a seizure followed by rapid neurological deterioration. CT of the head revealed diffuse supratentorial and cerebellar edema with a new intraparenchymal hemorrhage. Despite aggressive medical therapy and emergency removal of the implant with re-decompression, the patient sustained profound neurological injury. LESSONS: This case highlights the malignant presentation pattern of MBSC and underscores severe SSF syndrome as a critical preoperative risk state. Sudden restoration of the cranial vault may precipitate a pathological intracranial pressure surge in patients chronically adapted to intracranial hypotension. Cranioplasty timing and seizure activity may impact MBSC risk. Further study is needed to clarify optimal timing strategies and prophylactic antiseizure therapy. Heightened perioperative vigilance and explicit risk counseling may be necessary, as outcomes remain poor once the MBSC cascade is established. https://thejns.org/doi/10.3171/CASE2659.