Extensive peritruncal subarachnoid hemorrhage: a risk factor for the development of spinal arachnoid cysts. Illustrative cases.

backgroundSpinal arachnoid cysts (SACs) are a rare, delayed complication after aneurysmal subarachnoid hemorrhage (aSAH), potentially leading to spinal cord injury. Pathogenesis and preventive strategies remain poorly understood, and management is often frustrating. The authors reviewed their institutional 19-year aSAH patient registry to explore SAC prevalence and potential risk factors. OBSERVATIONS: Among 1470 aSAH admissions (2005-2023), 1098 of whom were 6-month survivors, 5 patients (0.46%) developed symptomatic, imaging-confirmed SAC. All featured ruptured posterior circulation aneurysms with extensive peritruncal blood distribution, yielding an 11.4% SAC rate in this subgroup. In the authors' clinic, active blood clearance for aSAH therapy was introduced in October 2015. Notably, all SAC cases occurred in patients who did not receive active blood clearance (3 before October 2015 and 2 who were not eligible), resulting in a prevalence of 15.2%. The median time to myelopathy onset was 28 months (range 5-76 months). Four patients underwent surgery, but neurological deficits persisted; 1 patient still walked independently, 3 needed assistance, and 1 was nonambulatory. LESSONS: SACs may represent an underrecognized complication after aSAH, observed exclusively in patients with extensive peritruncal hemorrhage from posterior circulation aneurysms. The fact that all cases occurred in the absence of active blood clearance suggests a potential preventive role of this intervention. https://thejns.org/doi/10.3171/CASE25470.