The unique features of middle cranial fossa and Sylvian fissure arachnoid cysts in children: MRI evaluation

Purpose The purpose of the study is to evaluate MRI findings of middle fossa arachnoid cysts in children according to Galassi classification and determine the differences between types and to assess the morphological changes in cysts during follow-up imaging. Methods MR images of 266 middle fossa arachnoid cysts of 255 pediatric patients were evaluated by two experienced radiologists retrospectively. MRI features including the sidedness of the cyst, Galassi type, parenchymal compression findings (cortical flattening and white matter compression), bone remodeling, and midline shift were evaluated on axial T2- and T1-weighted images. Follow-up MRI and available CSF flow MR imaging data were evaluated for change in cyst size and cisternal connections, respectively. Results The most common type was type 1 according to Galassi classification. The accompanying bone remodeling and white matter compression had a higher incidence in Galassi type 2 and 3 groups than Galassi type 1. Mean age of patients with bone remodeling and white matter compression was significantly higher in patients with Galassi type 1. All patients with cyst enlargement were younger than 2 years of age, and all of them were Galassi type 1. Cisternal connection was demonstrated in 7 patients. Conclusion While parenchymal compression and bone remodeling are expected findings in Galassi type 2 and 3 cysts, these features can also be seen in smaller Galassi type 1 cysts, regardless of size. Most of the middle fossa arachnoid cysts remain stable on follow-up imaging, and the increase in size is not an expected finding, especially in older children.