Chiari I malformation redefined: Clinical and radiographic findings for 364 symptomatic patients

OBJECTIVE: Chiari malformations are regarded as a pathological continuum of hindbrain maldevelopments characterized by downward herniation of the cere bellar tonsils. The Chiari I malformation (CMI) is defined as tonsillar her niation 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 so me were evaluated using CINE-magnetic resonance imaging and other neurodiag nostic tests. For 50 patients and 50 age- and gender-matched control subjec ts, the volume of the posterior cranial fossa was calculated by the Cavalie ri 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 2 4.9 +/- 15.8 years (mean +/- standard deviation), and 89 patients (24%) cit ed trauma as the precipitating event. Common associated problems included s yringomyelia (65%), scoliosis (42%), and basilar invagination (12%). Forty- three patients (12%) reported positive family histories of CMI or syringomy elia. Pedigrees for 21 families showed patterns consistent with autosomal d ominant 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) spin al cord disturbances in the absence of syringomyelia. The most consistent m agnetic resonance imaging findings were obliteration of the retrocerebellar cerebrospinal fluid spaces (364 patients), tonsillar herniation of at leas t 5 mm (332 patients), and varying degrees of cranial base dysplasia. Volum etric calculations for the posterior cranial fossa revealed a significant r eduction of total volume (mean, 13.4 mi) and a 40% reduction of cerebrospin al fluid volume (mean, 10.8 mi), 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 th e posterior cranial fossa and overcrowding of the normally developed hindbr ain. Tonsillar herniation of less than 5 mm does not exclude the diagnosis. Clinical manifestations of CMI seem to be related to cerebrospinal fluid d isturbances (which are responsible for headaches, pseudotumor-like episodes , endolymphatic hydrops, syringomyelia, and hydrocephalus) and direct compr ession of nervous tissue. The demonstration of familial aggregation suggest s a genetic component of transmission.