Chiari type I malformation with high foramen magnum anomaly.

A 14-year-old male with a neck pain and hypoesthesia in the upper extremities was diagnosed with Chiari type I malformation (CMI) and syringomyelia. The posterior part of the occipital bone was removed via cranio-cervical decompression. The accuracy of measuring the posterior cranial fossa (PCF) and foramen magnum (FM) dimensions were evaluated and compared with the literature. The linear PCF and FM dimensions as well as volumes were measured using computed tomography (CT) images with different techniques. The volume data were compared with similar data from literature. Use of the posterior fossa approach remains controversial when treating patients with minor little brain stem dislocation, small PCF, and or incomplete C1, but the approach can easily be applied if FM and PCF sizes are known. Linear measurements that were assessed for concordance with CT measurements had the best agreement. Quantification of PCF volume and high FM should be taken into consideration for differential diagnosis of tonsillar herniation and prediction of surgical outcome in CMI.