The natural history of the Chiari Type I anomaly

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Since the advent of MR imaging, an increasing number of asymptomatic or oligosymptomatic patients have been diagnosed with Chiari malformation Type I (CM-I). The decision of whether or not to operate is more difficult in these patients than in those with clear symptoms because of the lack of information about the natural course of this disease.


The authors report on their experience in a series of 22 patients with CM-I who were evaluated at the authors' institution, and for whom a conservative approach to treatment was adopted. The patients ranged in age from 1 to 16 years (mean 6.3 years) at diagnosis. Neuroradiological and complete clinical examinations were performed in all patients 6 months after the first observation and every year thereafter. The follow-up period ranged from 3 to 19 years (mean 5.9 years).


Chiari malformation Type I was incidentally detected on MR images in 11 of 22 patients. The remaining 11 patients had minimal clinical signs at presentation that were not regarded as necessitating immediate surgical treatment. Seventeen patients (77.3%) showed progressive improvement in their symptoms or remained asymptomatic at the last follow-up whereas 5 patients (22.7%) experienced worsening, which was mild in 2 cases and required surgical correction in the remaining 3 cases. On MR imaging a mild reduction in tonsillar herniation was appreciated in 4 patients (18.18%), with complete spontaneous resolution in 1 of these. In 16 patients, tonsillar herniation remained stable during follow-up.


The authors' data confirm the common impression that in both asymptomatic and slightly symptomatic patients with CM-I, a conservative approach to treatment should be adopted with periodic clinical and radiological examinations.

Abbreviations used in this paper: CM = Chiari malformation; CM-I = CM Type I.

Article Information

Address correspondence to: Federica Novegno, M.D., Department of Pediatric Neurosurgery, Catholic University Medical School, Largo A. Gemelli 1, 00168 Rome, Italy. email:

© AANS, except where prohibited by US copyright law.



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    Case 1. Representative T1-weighted MR images. Tonsillar herniation is demonstrated in the image in panel A obtained in 1997 and performed for gait instability. Additional images were obtained in 2000 (B), 2003 (C), 2005 (D), and 2007 (E). In 2007 the neuroradiological findings changed totally, showing complete resolution of the malformation (E).

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    Case 19. Representative MR images. Sagittal T1-weighted (A) and axial T2-weighted (B) images obtained at diagnosis, showing hindbrain herniation associated with mild ventriculomegaly. After 3 years, the patient presented with progression of hydrocephalus associated with an increase in tonsillar herniation shown on sagittal T1-weighted (C) and axial T2-weighted (D) images.



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