HE CMs encompass a spectrum of congenital hind- brain herniation syndromes. First described by the Austrian pathologist Hans Chiari in 1891, 11 there are at least two accepted variations, frequently referred to simply as CMI and CMII. Although both types character- istically involve some degree of cerebellar herniation; 1,7,13, 26,30 the CMII is more severe, involving a caudal displace- ment of the pons, medulla, and fourth ventricle; a strong association with various forms of myelodysplasia, spinal dysraphism, and hydrocephalus; and, almost invariably, an evident and/or symptomatic presentation at or soon after birth, which frequently occurs as a result of associat- ed central nervous system abnormalities (such as myelo- meningocele) rather than as a direct result of the CM. The CMI is recognized by caudal displacement of the cer- ebellar tonsils through the foramen magnum and into the cervical canal. The extent of tonsillar ectopia varies, rang- ing from a few millimeters to several centimeters. Associ- ated findings include a small posterior fossa, mild caudal displacement of the medulla and/or the fourth ventricle, angulation of the cervicomedullary junction, hydrocepha- lus, syringomyelia, and various osseous anomalies. Pres- entation frequently does not occur until the third decade of life. Although well characterized, the CMI remains poorly understood. Chiari malformation Type I is a congenital, yet slowly dynamic condition, and attempts to understand this condition have had to rely on static examinations across time, usually obtained in patients already symp- tomatic of their condition. To this end, researchers have examined the extent of tonsillar herniation; 1,7,13,26,30 mor- phological aspects of the posterior fossa; 4,29,38,40,44 and pres- sure differentials across 46,47 and CSF motion about 35,42,43 the foramen magnum. Despite the contributions of these stud- ies, a basic understanding of the pathogenesis and pro- gression of this condition remains elusive. This report is meant to serve as a contribution to the understanding of the progression of the CMI. Symptomat- ic cases have been well characterized. 13,21,27,29,41 However, because the CMI typically presents only in middle age, a significant interval remains during which individuals with a CMI are without symptomatic burden. Twenty-one pa- tients are presented here who were incidentally found to have tonsillar herniation significant enough to meet the current criteria for a CMI, but who were without clinical symptoms. The appearance of this malformation on MR imaging is presented, and factors that lead to progression of this condition are discussed in the context of current pathogenetic theories. Finally, strategies for treating pa- tients with asymptomatic CMIs are discussed. Clinical Material and Methods We performed a retrospective analysis of MR images J Neurosurg 92:920–926, 2000 920 Asymptomatic Chiari Type I malformations identified on magnetic resonance imaging JEFFERY MEADOWS, B.S., MICHAEL KRAUT , M.D., PH.D., MICHAEL GUARNIERI, PH.D., RAYMOND I. HAROUN, M.D., AND BENJAMIN S. CARSON, M.D. Departments of Neurological Surgery and Radiology, Johns Hopkins Hospital, Baltimore, Maryland Object. Chiari Type I malformation (CMI) is a congenital disorder recognized by caudal displacement of the cere- bellar tonsils through the foramen magnum and into the cervical canal. Frequently, associated findings include abnor- malities of nearby bony and neural elements as well as syringomyelia. Cerebellar tonsillar ectopia is generally consid- ered pathological when greater than 5 mm below the foramen magnum. However, asymptomatic tonsillar ectopia is an increasingly recognized phenomenon, the significance of which is poorly understood. Methods. The authors retrospectively reviewed the records of all brain magnetic resonance (MR) images obtained at our hospital over a 43-month period in an attempt to ascertain the relative prevalence and MR imaging characteristics of asymptomatic CMIs. Of 22,591 patients who underwent MR imaging of the head and cervical spine, 175 were found to have CMIs with tonsillar herniation extending more than 5 mm below the foramen magnum. Of these, 25 (14%) were found to be clinically asymptomatic. The average extent of ectopia in this population was 11.4  4.86 mm, and was significantly associated with a smaller cisterna magna. Syringomyelia and osseous anomalies were found in only one asymptomatic patient. Conclusions. The authors suggest that the isolated finding of tonsillar herniation is of limited prognostic utility and must be considered in the context of all available clinical and radiographic data. Strategies for treating patients with asymptomatic CMIs are discussed. KEY WORDS • Chiari malformation • Arnold–Chiari malformation • syringomyelia • syrinx • posterior fossa T J. Neurosurg. / Volume 92 / June, 2000 Abbreviations used in this paper: CM = Chiari malformation; CMI = CM Type I; CMII = CM Type II; CSF = cerebrospinal fluid; MR = magnetic resonance.