OBJECTIVE: Chiari malformations are regarded as a pathological continuum of hindbrain maldevelopments characterized by downward herniation of the cerebellar tonsils. The Chiari I malformation (CMI) is defined as tonsillar herniation of at least 3 to 5 mm below the foramen magnum. Increased detection of CMI has emphasized the need for more information regarding the clinical features of the disorder.

METHODS: We examined a prospective cohort of 364 symptomatic patients. All patients underwent magnetic resonance imaging of the head and spine, and some were evaluated using CINE-magnetic resonance imaging and other neurodiagnostic tests. For 50 patients and 50 age- and gender- matched control subjects, the volume of the posterior cranial fossa was calculated by the Cavalieri method. The families of 21 patients participated in a study of familial aggregation.

RESULTS: There were 275 female and 89 male patients. The age of onset was 24.9+/-15.8 years (mean +/- standard deviation), and 89 patients (24%) cited trauma as the precipitating event. Common associated problems included syringomyelia (65%), scoliosis (42%), and basilar invagination (12%). Forty-three patients (12%) reported positive family histories of CMI or syringomyelia. Pedigrees for 21 families showed patterns consistent with autosomal dominant or recessive inheritance. The clinical syndrome of CMI was found to consist of the following: 1) headaches, 2) pseudotumor-like episodes, 3) a Meniere's disease-like syndrome, 4) lower cranial nerve signs, and 5) spinal cord disturbances in the absence of syringomyelia. The most consistent magnetic resonance imaging findings were obliteration of the retrocerebellar cerebrospinal fluid spaces (364 patients), tonsillar herniation of at least 5 mm (332 patients), and varying degrees of cranial base dysplasia. Volumetric calculations for the posterior cranial fossa revealed a significant reduction of total volume (mean, 13.4 ml) and a 40% reduction of cerebrospinal fluid volume (mean, 10.8 ml), with normal brain volume.

CONCLUSION: These data support accumulating evidence that CMI is a disorder of the para-axial mesoderm that is characterized by underdevelopment of the posterior cranial fossa and overcrowding of the normally developed hindbrain. Tonsillar herniation of less than 5 mm does not exclude the diagnosis. Clinical manifestations of CMI seem to be related to cerebrospinal fluid disturbances (which are responsible for headaches, pseudotumor-like episodes, endolymphatic hydrops, syringomyelia, and hydrocephalus) and direct compression of nervous tissue. The demonstration of familial aggregation suggests a genetic component of transmission.