Posterior Fossa CSF Leak and Encephalocele Secondary to Arachnoid Granulations-A Case Report and Review of The Literature

Spontaneous posterior fossa (PF) defects and encephaloceles are uncommonly encountered. Arachnoid granulations (AGs) have been increasingly recognized as a potential etiologic factor PF defects and cerebrospinal fluid (CSF) leaks. It has been proposed that aberrant AGs in contact with bone can cause gradual, progressive bony erosion over time. We present the case of a 66-year-old female with a prior hospitalization for meningitis and who at that time was noted on imaging to have otomastoiditis. Focal punctate bony erosion was identified anterior to the sigmoid sinus. Serial follow-up imaging demonstrated progressive bony erosion. The patient had no risk factors for a PF defect including no prior skull base surgery, signs of idiopathic intracranial hypertension or elevated BMI. The patient underwent a mastoidectomy to evaluate the PF plate. Intraoperatively, irregular bony dehiscence of the PF and sigmoid plate was observed, with herniation of arachnoid and glial tissue, and abnormal soft tissue overlying the expected location of the endolymphatic sac (ELS). Biopsy of this tissue revealed no definitive neoplasm. The ELS was exposed and appeared normal. The PF plate and dural defect were repaired using a multilayered reconstruction technique. PF CSF leaks arising from AGs are rare with only a handful reported in the literature. This case highlights the potential role of AGs in spontaneous PF defects with CSF leak and/or encephalocele and underscores the importance of careful imaging surveillance and intraoperative evaluation when these are suspected cases. Publication History Article published online: 27 February 2026 © 2026. Thieme. All rights reserved. Georg Thieme Verlag KG Oswald-Hesse-Straße 50, 70469 Stuttgart, Germany