J Neurosurg 111:1046–1052, 2009 1046 J Neurosurg / Volume 111 / November 2009 C hiari malformation Type I is characterized by chronic herniation of the cerebellar tonsils into the upper cervical spinal canal. 5 According to a recent classifcation, 19 CTH can be encountered in vari- ous etiopathogenic conditions: global cranial constriction occurring with multiple craniosynostosis, acromegaly, or achondroplasia; downward traction of the spinal cord oc- curring with tethered cord syndrome; cranial settling and cerebellar ptosis occurring with hereditary disorders of the connective tissue; elevated intracranial pressure from hydrocephalus and space-occupying lesions; and intraspi- nal hypotension occurring with CSF leaks and lumbo- peritoneal shunting. In any other situation, CTH seems to result from isolated constriction of a PCF that is not capa- cious enough to house the developing hindbrain, which defnes the classic form of CMI. 19 Experimental mod- els 14,15,23 supported by morphometric studies 2,19,20,25,26,29,33 have shown that the CTH occurring in classic CMI mainly results from overcrowding of a normally devel- oping hindbrain within a congenitally small and shallow PCF due to occipital bone underdevelopment. Given the somitic origin of the occipital bone, a very early paraxial mesodermal defect of the parachordal plate was once considered the main cause of CMI. 15 However, extreme variations in the ages at presentation, with pediatric 27,28,35 and adult 17,19–21,25,29,33 types of presentation, and the pref- erential involvement of the basioccipital portion in previ- ously published morphometric studies 2,11,19–21,25,26,33 have suggested the occurrence of later growth impairment in the basichondrocranium. Incidence of basioccipital hypoplasia in Chiari malformation Type I: comparative morphometric study of the posterior cranial fossa Clinical article Rémy Noudel, m.d., 1 Nicolas JoveNiN, m.d., 2 cRistophe eap , m.d., 1 BeRNaRd scheRpeReel, m.d., 1 lauReNt pieRot , m.d., ph.d., 3 aNd p ascal Rousseaux, m.d. 1 Departments of 1 Neurosurgery, 2 Medical Informatics and Healthcare, and 3 Neuroradiology, Maison Blanche Hospital, University of Reims, France Object. The chronic tonsillar herniation defning Chiari malformation Type I (CMI) is thought to result from overcrowding of a normally developing hindbrain within a congenitally small posterior cranial fossa (PCF) due to occipital hypoplasia. The goals in the present study were to authenticate the cranioencephalic disproportion in a group of patients with CMI and to discuss new developmental aspects according to which part of the occipital bone was underdeveloped. Methods. The authors retrospectively examined a group of 17 patients with CMI. Measurements of osteotento- rial and neural structures of the PCF were made on MR images of the brain. The results were compared with fndings in 30 healthy controls by using the Mann-Whitney U-test. Results. Dimensions of the neural structures did not differ between the 2 groups of patients. The mean length of the basiocciput was signifcantly shorter in the CMI group (19.4 mm) compared with the control group (25.7 mm; p = 0.0003). The mean diameter of the foramen magnum was larger in the CMI group, but this difference was not statistically signifcant. The dimensions of the supraocciput and the mean angle of the cerebellar tentorium were identical in the 2 groups. Conclusions. Data in this study support the idea that occipital hypoplasia is the main cause of overcrowding within the PCF. Basioccipital shortness is a cardinal feature of the resultant shallow PCF and could proceed from a congenital disorder of the cephalic mesoderm of the parachordal plate or occur later in the infancy because of prema- ture stenosis of the sphenooccipital synchondrosis. (DOI: 10.3171/2009.2.JNS08284) Key WoRds • Chiari malformation Type I • posterior cranial fossa • morphometric study • basioccipital hypoplasia • hindbrain-related syringomyelia Abbreviations used in this paper: CM0 = Chiari malformation Type 0; CMI = CM Type I; CTH = chronic tonsillar herniation; PCF = posterior cranial fossa. See the corresponding editorial in this issue, pp 1043–1045.