Intracranial extradural arachnoid cyst in a child.

INTRODUCTION: Arachnoid cysts are benign developmental anomalies of arachnoid membrane origin that can occur anywhere along the neuro-axis. They are believed to develop from the splitting or duplication of the arachnoid membrane by CSF that is trapped by a ball-valve mechanism. Intracranial arachnoid cysts have only been described as intradural lesions while spinal arachnoid cysts can be both intradural or extradural. CASE REPORT: After an extensive literature review, we report the first case of an intracranial, extradural arachnoid cyst in a 5-yearold girl. The child presented with a 2-week history of suspected seizure-like activity and imaging revealed a large midline extradural CSF-containing arachnoid cyst causing severe compression of the superior sagittal sinus and underlying brain. Venous flow through the sagittal sinus was nearly obliterated. Osseous changes and bone growth adjacent to the cyst was also noted on imaging and intraoperatively. She underwent a bifrontal craniotomy and cyst excision with decompression of underlying brain and reestablishment of venous flow through the sagittal sinus.