J Neurosurg / Volume 111 / November 2009 J Neurosurg 111:1043–1045, 2009 1043 Basioccipital hypoplasia in Chiari malformation Type I Harold l. rekate, M.d. Barrow Neurological Institute, Phoenix, Arizona The understanding of chronic tonsillar herniation (CTH), which has been called Chiari malformation Type I (CMI), is hampered to some extent by the lack of an ac- cepted nomenclature and classifcation. Noudel and col- leagues 5 assessed the relationship between the descent of the cerebellar tonsils and dimensions of the basiocciput, specifcally the length of the clivus. They studied 17 pa- tients who had symptoms and signs related to the descent of the cerebellar tonsils more than 5 mm below the fora- men magnum. These patients were selected because they had no obvious causative factors that would otherwise explain their tonsillar herniation such as basilar invagi- nation or multisutural craniosynostosis. The authors re- ferred to these patients as having “classic” CMI. While training in neurosurgery at Case Western Re- serve University in Cleveland, Ohio, my thinking regard- ing the Chiari I or adult Chiari malformation was greatly infuenced by the work of our 2 neuropathologists, Drs. Uros Roessmann and Reinhart Friede. 3 They studied all cases of CTH that had been published to that point. They postulated that all of the cases of herniated cerebellar tonsils were related to distortion of the brainstem and cer- ebellum at the foramen magnum from multiple causes: brain distortion from above, as in the case of multiple su- tural closure in syndromic craniosynostosis; 2 suctioning of the brain from below, as in response to lumboperitone- al shunting; 1 or direct distortion by bony abnormalities of the craniovertebral junction, as in basilar invagination. 4 Based on their infuence, I have always thought that tonsillar herniation has a specifc cause and that with fur - ther study we would be able to defne the cause in any situation. To this point the cause of more than one-half of the cases of CMI remains unknown. The work presented by Noudel and colleagues is likely to lead to an under - standing of the cause of any tonsillar herniation. The pa- tients were all symptomatic, and all had a herniation of at least 5 mm. There is no description of the shape of the tonsils, which I have found to be related to symptoms. If substantiated by others, this fnding could improve our understanding of the mechanism underlying herniation in such patients and lead to an objective measurement to guide treatment and ensure that each investigator is actu- ally studying the same phenomena. Novegno and colleagues 6 have studied the natural history of hindbrain herniation and have proposed that the name be changed to “Chiari Type I anomaly.” In general, I favor this term for defned causes that are not clearly malformative such as lumboperitoneal shunting. I would like to see objective measurements of clival length in asymptomatic or minimally symptomatic patients to determine whether this parameter offers a prognostic beneft. References 1. Chumas PD, Armstrong DC, Drake JM, Kulkarni AV, Hoff- man HJ, Humphreys RP, et al: Tonsillar herniation: the rule rather than the exception after lumboperitoneal shunting in the pediatric population. J Neurosurg 78:568–573, 1993 2. Francis PM, Beals S, Rekate HL, Pittman HW, Manwaring K, Reiff J: Chronic tonsillar herniation and Crouzon's syndrome. Pediatr Neurosurg 18:202–206, 1992 3. Friede RL, Roessmann U: Chronic tonsillar herniation: an at- tempt at classifying chronic hernitations at the foramen mag- num. Acta Neuropathol 34:219–235, 1976 4. Kim LJ, Rekate HL, Klopfenstein JD, Sonntag VK: Treat- ment of basilar invagination associated with Chiari I malfor - mations in the pediatric population: cervical reduction and posterior occipitocervical fusion. J Neurosurg 101 (2 Sup- pl):189–195, 2004 5. Noudel R, Jovenin N, Eap C, Scherpereel B, Pierot L, Rous- seaux P: Incidence of basioccipital hypoplasia in Chiari mal- formation Type I: comparative morphometric study of poste- rior cranial fossa. Clinical article. J Neurosurg. Published online May 22, 2009; DOI 10.3171/2009.2.JNS08284. 6. Novegno F, Caldarelli M, Massa A, Chieffo D, Massimi L, Pettorini B, et al: The natural history of the Chiari Type I anomaly. J Neurosurg Pediatr 2:179–187, 2008 Response réMy Noudel, M.d. Departments of Neurosurgery, Medical Informatics and Health- care, and Neuroradiology, Maison Blanche Hospital, University of Reims, France We thank Dr. Rekate for the very thoughtful com- ments and perspective on the results of our study. Concerning the semantic discussion about CMI, we agree with Dr. Rekate about the need to defne more accu- rately and ubiquitously the clinical entity we are talking about. To this point, he refers to a discussion in a recently published editorial: should CTH be named an “anoma- ly” or a “malformation”? 7 The unaccepted terminology 1043 See the corresponding article in this issue, pp 1046–1052. Editorial