Prevalence estimated at less than 1%: There are no population-based studies on the incidence or prevalence of Chiari malformations. From clinical series, prevalence has been estimated between 0.1 and 0.5% (43), but it is possible that higher rates could have resulted from more widespread recent use of MRI. Another study estimated a prevalence of 0.77% based on the total population of patients undergoing MRI of the head at a tertiary care center over a period of 3.5 years. Of these patients, 14% were clinically asymptomatic (44).
3.6% prevalence for radiographic Chiari I: In a pediatric study in which an 11-year brain MRI database of 14,116 children was studied, 509 patients (3.6%) met radiologic criteria for Chiari I malformation, with a mean tonsillar descent below the foramen magnum of 10.2 mm. Compared to previous work, this study comprised a much higher percentage of asymptomatic patients (45).
All ages with majority diagnosed in late childhood: Although most patients with Chiari I malformation are diagnosed in late childhood to early adulthood, it is not uncommon for young children and older adults to present with symptoms. Since Chiari II malformation is associated with myelomeningocele, patients are diagnosed at birth.
No difference: No population-based studies have been performed, but most clinical cohorts show equal incidence or a slight female predominance.
No distribution apparent: There is no particular ethnic or geographic distribution.
None known: There are no known risk factors, other than family history.
Relationships to Other Disease States and Syndromes
Chiari I: Several conditions are associated with Chiari I malformations (either incidental or pathophysiologically related). The most prominent ones are hydrocephalus, craniosynostosis, achondroplasia, acromegaly, and growth hormone deficiency. Several disorders that result in hyperostosis or bone mineral deficiency have been associated with Chiari I malformation, as well as spinal deformities such as atlantoaxial assimilation of the Klippel-Feil sequence. A more comprehensive list of associations can be found in the literature (46).
Chiari II: Chiari II malformations occur in association with myelomeningocele, with accompanying posterior fossa bony anomalies, hydrocephalus, and brainstem and cerebellar anomalies.