Natural history of Chiari malformation Type I following decision for conservative treatment

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The natural history of the Chiari malformation Type I (CM-I) is incompletely understood. The authors report on the outcome of a large group of patients with CM-I that were initially selected for nonsurgical management.


The authors identified 147 patients in whom CM-I was diagnosed on MR imaging, who were not offered surgery at the time of diagnosis, and in whom at least 1 year of clinical and MR imaging follow-up was available after the initial CM-I diagnosis. These patients were included in an outcome analysis.


Patients were followed clinically and by MR imaging for a mean duration of 4.6 and 3.8 years, respectively. Of the 147 patients, 9 had new symptoms attributed to the CM-I during the follow-up interval. During this time, development of a spinal cord syrinx occurred in 8 patients; 5 of these patients had a prior diagnosis of a presyrinx state or a dilated central canal. Spontaneous resolution of a syrinx occurred in 3 patients. Multiple CSF flow studies were obtained in 74 patients. Of these patients, 23 had improvement in CSF flow, 39 had no change, and 12 showed worsening CSF flow at the foramen magnum. There was no significant change in the mean amount of cerebellar tonsillar herniation over the follow-up period. Fourteen patients underwent surgical treatment for CM-I. There were no differences in initial cerebellar tonsillar herniation or CSF flow at the foramen magnum in those who ultimately underwent surgery compared with those who did not.


In patients with CM-Is that are selected for nonsurgical management, the natural history is usually benign, although spontaneous improvement and worsening are occasionally seen.

Abbreviations used in this paper: CM-I = Chiari malformation Type I; EMERSE = Electronic Medical Record Search Engine.

Article Information

Address correspondence to: Cormac O. Maher, M.D., Department of Neurosurgery, University of Michigan, 1500 East Medical Center Drive, Room 3552 Taubman Center, Ann Arbor, Michigan 48109-5338. email:

© AANS, except where prohibited by US copyright law.



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    Bar graph illustrating change in tonsillar herniation in 147 patients with CM-I.

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    Scatterplot demonstrating change in tonsillar herniation by age at the time of CM-I diagnosis (regression slope estimate −0.15; p = 0.007).

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    A: Sagittal MR imaging study obtained in a 6-year-old boy with CM-I but no spinal syrinx. B: A new syrinx is seen in the same patient on a follow-up MR imaging study obtained 4 years later. C: Sagittal MR imaging study obtained in a 5-year-old girl showing CM-I but no syrinx within the spinal cord. D: In the same patient, a new syrinx is noted on a follow-up MR imaging study obtained 4 years later. E: Following CM-I decompression in this patient, the syrinx has resolved.

  • View in gallery

    Left: Bar graph depicting change in syrinx length by number of levels in patients with spinal cord syrinx at any time over duration of follow-up. Right: Bar graph depicting change in syrinx width (mm) in patients with syrinx at any time over duration of follow-up.

  • View in gallery

    Bar graph illustrating time to surgery after initial diagnosis of CM-I.



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