Epidemiology of chiari I malformation

Abstract

Almost 1% of normal adults studied with magnetic resonance imaging (MRI) of the brain will have a finding of Chiari I malformation based on 5 mm or more of cerebellar tonsillar ectopia, while as many as 3.6% of normal children will have a Chiari I malformation finding using the same criteria. The proportion of adults with symptoms and MRI evidence of Chiari I malformation is estimated to be in the range of 0.01–0.04%. Symptoms of Chiari I malformation occurred in 0.007% of children in one study. Symptoms occur most commonly when tonsillar ectopia exceeds 13 mm and when lesser degrees of ectopia are accompanied by narrowing of the cerebrospinal fluid (CSF) space posterior to the cerebellar tonsils, molding of the tonsils, and syringomyelia. Tonsillar ectopia and morphologic changes in the cerebellum and medulla found in Chiari I malformation are thought to arise from decreased development of the bones of the inferior part of the posterior fossa. Thus, factors that prevent normal development of these bones impact the development of Chiari I malformation. In monozygotic twins and triplets, the symptomatic index cases have considerably greater tonsillar ectopia than their generally asymptomatic monozygotic siblings, suggesting that a genetic predisposition to reduced development of the posterior fossa is not the only factor involved in creating tonsillar ectopia and in narrowing the CSF pathways. Factors reported to increase the likelihood of developing Chiari I malformation include birth injury, trauma, pseudotumor cerebri, hydrocephalus, ancestry from a Pacific island, female gender in adults in the United States, and male gender in the Tatarstan, Russia. Epigenetic changes due to environmental factors or normal aging and differences in the placenta and amniotic sac are also suspected of playing a role in the development of Chiari I malformation. Going forward, more rigorous measurements of the prevalence and incidence of Chiari I malformation would result from prospective epidemiologic studies of large populations designed for this purpose than from retrospective queries of the databases of private and governmental healthcare organizations.