Arachnoid granulations of the temporal bone: A histologic study of dural and osseous penetration

Hypothesis: Arachnoid granulations (AG) are more prevalent along the middle fossa surface of the temporal bone, where they produce larger bony defects than those occurring on the posterior surface. Background: Dural and bony defects formed by AGs are proposed to lead to spontaneous meningoencephaloceles and cerebrospinal fluid otorrhea. They most commonly occur at the tegmen and in individuals older than 40 years. Methods: Vertically sectioned temporal bones were evaluated using light microscopy to determine AG histology, distribution, and morphometry and to determine the prevalence of AG penetration in the donor population. Results: AGs were observed to penetrate the dura mater and make direct contact with cortical surfaces in 12.7% of donors in the Johns Hopkins Temporal Bone Collection. AGs occurred at middle fossa sites 13% more frequently than at posterior fossa sites. At middle fossa sites AGs produced significantly larger bony openings and were more likely to be associated with herniating brain tissue. Donors with AGs were significantly older, and all were in the late 30s or older. Conclusion: Erosion of the temporal bone by AGs is not a rare occurrence in the population and becomes increasingly prevalent with age. It is estimated that 14 in 1,000 donors were at greatest risk of eventual cerebrospinal fluid leakage at the tegmen. The age and anatomic distribution described in this study strengthens the notion that AG penetration plays a role in the pathophysiology of spontaneous cerebrospinal fluid leaks and meningoencephaloceles of the temporal bone.