Objectives/HypothesisThe objective of this study was to utilize techniques of otopathology to gain insight into the pathogenesis, sites of origin, and associated findings in idiopathic lateral skull base defects.Study DesignHistopathologic analysis of temporal bones from an otopathology repository.MethodsSpecimens from a human temporal bone repository were investigated for clinical or otopathologic evidence of occult bony dehiscence indicating communication between the subarachnoid space and air cells of the temporal bone. Specimens were examined by light microscopy, organized by fistula site, and histopathologically described. Premortem patient demographics and clinical history was reviewed.ResultsSpecimens from 52 individuals met inclusion criteria. Three distinct fistula pathways were determined: transdural, labyrinthine, and perilabyrinthine. Transdural fistulae occur most commonly as the result of arachnoid granulations along the middle or posterior fossa dura (n = 30) and are frequently incidental findings in specimens of older individuals (median age at death: 81 years). Labyrinthine fistulae (n = 10) were noted with cochlear malformations when modiolar atresia permits cerebrospinal fluid (CSF) flow into a common intracochlear scala and oval window perilymphatic fistula results. Perilabyrinthine fistulae (n = 7) were observed through three congenitally unfused tracts: the tympanomeningeal fissure, the petromastoid canal, or an extension of the subarachnoid space into the fallopian canal.ConclusionsIdiopathic lateral skull base defects occur in three distinct anatomic locations with consistent histopathologic findings. In the absence of clear radiographic localization, patient age and associated cochlear defects may assist in the determination of the site of CSF leak. These data have implications for surgical approaches of CSF fistula repair.Level of EvidenceNA Laryngoscope, 125:1798–1806, 2015