Anatomy and significance of the subarachnoid space in the fallopian canal.

OBJECTIVE: To describe the anatomic variations of the subarachnoid space (SAS) in the facial nerve canal from human temporal bone specimens. BACKGROUND: The SAS in the facial canal usually is assumed to terminate in the petrosal segment of the canal. This temporal bone study indicates that in 12% of temporal bones the SAS extends laterally into or beyond the tympanic segment of the facial canal. METHODS: Histologic sections through the petrosal and tympanic segments of the facial canal were examined by light microscopy in 1 fetal (30 weeks) and 163 adult human temporal bones. The arachnoidal membrane of the SAS was judged to fall into three types according to its lateral extension in the fallopian canal. RESULTS: One hundred forty-four (88%) of the adult temporal bones contained an SAS limited to the petrosal fallopian canal (type I). Thirteen temporal bones (8%) demonstrated an SAS that extended into the tympanic or geniculate portions of the facial nerve (type II). There were six temporal bones (4%) in which the SAS extended lateral to the tympanic facial nerve or into a separate bony compartment (type III). CONCLUSIONS: Most (88%) temporal bones contain an SAS that is limited to the petrosal fallopian canal. In 12% of temporal bones, however, the SAS may extend laterally in the fallopian canal and present clinically as an asymptomatic enlargement of the canal by computed tomography or as cerebrospinal fluid otorrhea.