Volumetric parameters of the posterior fossa in basilar impression.
Occipital bone, posterior fossa, basilar impression
The posterior cranial fossa is located in the posterior compartment of the skull and accommodates important structures of the nervous system such as the cerebellum, brainstem and some cranial nerves. Reduction of posterior fossa volume is reported in cases of basilar impression and Chiari malformation, accompanied by neurological symptoms. Historically, these abnormalities have been related to the brachycephalic phenotype. The objective of the study was to evaluate posterior fossa parameters in individuals with basilar impression. It was a case-control study, using exams of magnetic resonance imaging (MRI) from 126 adults of both sexes. The case (n=30) and control (n=96) groups were previously defined by two neuroradiologist doctors. Measurements were performed on MacBookpro computer and Osirix imaging software. The MRIs have 158 to 160 images in sagittal sections each. Posterior fossa volume, extra-posterior fossa volume and total volume were measured. Through the closed polygon tool it was possible to delimit the area, obeying the anatomical limits, later a 3D structure was generated corresponding to the posterior fossa volume and another to the extra-posterior fossa volume. In the statistical analysis the SPSS software (version 20) was used, all tests were calculated at the 95% confidence interval. It was verified that the posterior fossa volume is smaller in females. Considering the head phenotype, hyperbrachycephalic individuals with basilar impression suffered a greater volumetric loss of 1.22%, equivalent to 17.4 ml less. The posterior fossa volume was lower in individuals with basilar impression in comparison with the control group (P = 0,0001). There was no difference for extra-posterior fossa volume and total volume. It can be noticed that the denomination of shallow fossa may not be the most adequate, since the process of decrease in basilar elevation occurs in the anteroposterior diameter and not in depth.