Click on Images for Enlarged View
Findings: On Sagittal T1 weighted MR images, the cerebellar tonsils are seen to extend approximately 1 cm below the foramen magnum.
Diagnosis: Chiari I malformation.
Discussion: A Chiari I malformation involves caudal displacement of the cerebellar tonsils below the foramen magnum (tonsillar ectopia). The cerebellar tonsils can protrude minimally below the foramen magnum in normal subjects. The amount of normal extension below the foramen magnum varies with age. The maximum normal level for the cerebellar tonsils to extend below the foramen magnum in the first decade of life is 6 mm, the second and third decade of life 5 mm, and the fourth through eighth decades of life 4 mm, and the ninth decade and beyond 3 mm. The fourth ventricle is usually normal in position but attenuated in shape. The cisterna magna is usually small or absent.
Chiari I malformations are associated with hydrocephalus in 35% of cases. Hydromeylia has been reported in 30 to 70% of patients. Chiari I malformations can also be associated with basilar impression, atlanto-occipital fusion, spina bifida of C1, as well as Klippel-Feil syndrome.
References:
Taveras JM, Brain Congenital Anomalies. Neuroradiology.
Taveras JM. Williams and Wilkins, Baltimore; 1996:166-175.
Mikulis DG, Diaz O, Egglin TK, Sanchez R, et al. Variance of the
Position of the Cerebellar Tonsils with Age: Preliminary Report.
Radiology 1992;183:725-728.
Woodruff WW. Pediatric Neuroimaging. Fundamentals of Neuroimaging.
Woodruff WW. W. B. Saunders Co., Philadelphia; 1993:513-517.
Submitted by: