MRI morphometric analysis of patients with Chiari malformation type 1 without small posterior cranial fossa

Chiari malformation type 1 (СМ1) without a small posterior cranial fossa is a poorly studied variant of the «spectrum of classical primary СМ1», requiring careful differentiation with secondary forms of cerebellar ectopia. An MRI morphometric study was carried out in 59 patients with clinical symptoms of СМ1 (29 with syringomyelia) with ectopia of the cerebellar tonsils of 3 mm or more, a tight cisterna magna, the length of Clivus and Supraocciput of 40 mm or more. The comparison group consisted of 60 patients with classical СМ1 (30 with syringomyelia), and the control group consisted of 30 healthy subjects. As a result of the study, the features of bone phenotype of the pathology in question were revealed, and their pathogenetic role was discussed. The main features of the bone phenotype of clinically symptomatic CM1 without short bones and without signs of secondary cerebellar ectopia, along with the large size of the clivus, supraocciput and the volume of the posterior cranial fossa, are a decrease in the extra-bone part of the posterior cranial fossa, a sharpening of the angle of the bottom, or «funnel» of the posterior cranial fossa, narrowing of the anterior cerebrospinal spaces of the foramen magnum and a decrease in the anteroposterior diameter of the spinal canal at C2 and C6 levels.