[Clinical and radiological rationale for distinguishing subtypes of primary Chiari I malformation]

To clarify a role of distinct factors that form the morphological basis of the classical or primary Chiari type 1 malformation (CM1) in the development of its clinical manifestations and subtypes.

The main study group included 710 adult patients with cerebellar ectopia divided into subgroups according to the severity of cerebellar ectopia (less than 2 mm (CM0); 2-4 mm (CM0,5); 5 mm or more (CM1); 5 mm or more in combination with a pronounced prolapse of the brain stem below the foramen magnum (CM1,5)) as well as to the presence of «overcrowded» posterior cranial fossa (PF) and «small» PF. Clinical symptoms and bone phenotype of PF were analyzed.

With an increase of the degree of cerebellar tonsils ectopia, an increase in the proportion of patients with «overcrowded» PF, syringomyelia, otoneurological and lower cranial nerve, brain stem, cerebellar disturbances was revealed. The phenomenon of «small» PF was observed in 81% of the main group. «Small» PF was associated with a greater proportion of patients with syringomyelia, and did not correlate with the level of ectopia and «overcrowded» PF. «Overcrowded» PF was found in 51% of patients and was associated with a greater frequency of combined pseudotumor and suboccipital CM1-caused headaches, as well as a greater degree of cerebellar ectopia. An analysis of the ratio of morphometric components of the PF phenotype in the studied population revealed a combination of three factors in 37% of patients; the proportion of isolated cerebellar ectopia did not exceed 10%. Among patients with clinical symptoms of CM1, the classic CM1 was detected only in 37% of patients. When comparing CM0,5 and CM1 groups, similar clinical and radiological characteristics were revealed, which may indicate a pathogenetic relationship between CM1 and tonsillar ectopia 2-4 mm classified by the authors as CM0,5.

The study provided grounds for expanding the classification of CM1 in adults with the inclusion of CM0,5 form as well as for evaluation of the «small» and «overcrowded» PF phenomena in the diagnostic algorithm for patients with suspected Chiari malformation.