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Year : 2018  |  Volume : 66  |  Issue : 4  |  Page : 940--942

Basilar invagination, syringomyelia and Chiari formation and their relationship with atlantoaxial instability

Atul Goel 
 Department of Neurosurgery, K.E.M. Hospital and Seth G.S. Medical College, Parel, Mumbai, Maharashtra, India

Correspondence Address:
Dr. Atul Goel
Department of Neurosurgery, K.E.M. Hospital and Seth G.S. Medical College, Parel, Mumbai - 400 012, Maharashtra

How to cite this article:
Goel A. Basilar invagination, syringomyelia and Chiari formation and their relationship with atlantoaxial instability.Neurol India 2018;66:940-942

How to cite this URL:
Goel A. Basilar invagination, syringomyelia and Chiari formation and their relationship with atlantoaxial instability. Neurol India [serial online] 2018 [cited 2022 Oct 7 ];66:940-942
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The clinical entity of Chiari formation has been under discussion for over a century. Despite several publications on the subject, its exact pathogenesis has not been identified and no treatment protocol has been universally accepted. The term 'tonsillar herniation' and its correlation with 'uncal herniation' has resulted in the erroneous belief that it is a pathological and dangerous phenomenon. In the year 2014, we proposed that Chiari formation is not a primary event but is secondary to potential or manifest longstanding atlantoaxial instability.[1] It was speculated that Chiari formation is a natural protective maneuver wherein the tonsils act as a cushion and prevent pinching of critical neural structures between the abnormally moving odontoid process and the posterior arch of atlas. We put forward the proposal that the presence of Chiari formation was suggestive of atlantoaxial instability and indicated the need for atlantoaxial stabilization. Our proposition has been discordant with the popular concept that Chari formation is the outcome of a large cerebellar mass in the relatively small posterior cranial fossa, which results in tonsillar herniation and consequently, cervico-medullary cord compression. Foramen magnum decompression, aimed at widening the space for free traverse of the spinal cord has been widely regarded as standard treatment for this phenomenon. As our conclusions had repercussions regarding the treatment of Chiari formation, our article invited extensive academic discussion. Our continuing clinical experience and gratifying surgical outcomes reinforce our views about the pathogenesis and function of Chiari formation. Accordingly, we have proposed to change the existing nomenclature of Chiari 'malformation' to Chiari 'formation'.[2]

For several decades, basilar invagination was associated with the presence of 'fixed' or 'irreducible' atlantoaxial dislocation. As the anomaly was considered 'fixed', decompression of the bones and making space for the traverse of neural structures was the standard treatment. In the year 1998, we divided basilar invagination into two groups based on the absence (Group 1) or presence (Group 2) of Chiari formation.[3] We discussed transoral decompression for Group 1, and foramen magnum decompression for Group 2 basilar invagination. Our article was the first in the literature wherein basilar invagination and Chiari formation were associated with reduction in the posterior cranial fossa bone volume. It was proposed that dural decompression was not necessary.

In the year 2004, we proposed that in a group of cases with basilar invagination (Group A basilar invagination), direct manual distraction of the facets of atlas and axis resulted in craniovertebral junction realignment.[4] Essentially, it was identified that in Group A basilar invagination, the atlantoaxial joint was unstable and not fixed or irreducible. Our surgical procedure discussed the use of intra-articular spacers, in addition to bone graft, to distract and stabilize the facets and to achieve craniovertebral realignment by bringing the odontoid process inferiorly into its normal position. We now propose that even more important than craniovertebral realignment is the firm stabilization and ultimate bone fusion of the atlantoaxial joint. Group A basilar invagination patients were earlier recommended transoral decompression or resection of the odontoid process. The treatment thus changed from decompression to stabilization for cases with Group A basilar invagination. This concept has relegated the commonly performed transoral surgery into the realm of history. Cases with Group B basilar invagination were still considered to have 'fixed' atlantoaxial instability and foramen magnum bone decompression was recommended as treatment.

As we matured further in our understanding of the craniovertebral junction, we identified that atlantoaxial instability was the basis of pathogenesis of Group A as well as Group B basilar invagination.[5] The atlantoaxial joints are not fixed or irreducible, but are pathologically mobile and unstable. Accordingly, atlantoaxial fixation was identified to be the mode of treatment of basilar invagination. Our remarkable clinical results following atlantoaxial fixation are a testimony to this hypothesis. Our treatment aims, thus, changed from 'decompression' to 'atlantoaxial stabilization'. We identified that foramen magnum decompression was not necessary in any clinical condition related to atlantoaxial instability and is a potentially harmful surgical procedure in the long run.[6]

Atlantoaxial dislocation has been classified into the mobile (reducible) and the partially or completely irreducible types. Our technique of direct facetal manipulation and reduction of the dislocation relegated the entity of 'irreducible' atlantoaxial dislocation irrelevant.[7] The technique of facetal manipulation introduced the possibility of reduction and realignment of rotatory atlantoaxial dislocation. Prior to our publication on the subject, no definitive treatment for rotatory atlantoaxial dislocation was available.[8] For the first time in literature, we introduced the entities of 'vertical' dislocation,[9] 'lateral'[10] and 'translatory' atlantoaxial dislocation.[11] Conventionally, the only parameter to diagnose atlantoaxial instability was the abnormality of atlantodental interval and compression of the dural tube or neural structures by the odontoid process. We introduced a new classification of atlantoaxial instability on the basis of facetal mal-alignment on lateral radiograph with neutral head position and on the evaluation of instability on manual manipulation of the facets; Goel Type 1 atlantoaxial facetal instability, when the facet of atlas is located anterior to the facet of axis, and Goel Type 2 atlantoaxial facetal instability, when the facet of atlas is located posterior to the facet of axis.[12] In Goel Type 3 atlantoaxial facetal instability, the facets of atlas and axis are in alignment and the instability is diagnosed based on clinical parameters and on direct manual manipulation of the bones during surgery. Atlantoaxial instability in Type 2 and Type 3 cases was classified as central or axial as there may or may not be any dural or neural compression related to the odontoid process.

In general, clinical symptoms are relatively acute and neural deficits dominate in Type 1 atlantoaxial instability, wherein the odontoid process compression of the dural/neural tube is significant. It was observed that symptoms in cases with axial or central (Types 2 and 3) atlantoaxial instability are chronic or longstanding, probably because the odontoid process related dural/neural compression is not the primary feature. We identified that axial or central atlantoaxial dislocation is commonly associated with chronic or longstanding dislocations accompanying basilar invagination, Chiari formation and syringomyelia. Central or axial atlantoaxial dislocation can also be a feature in cases with multi-segmental cervical spondylosis,[13] ossified posterior longitudinal ligament [14] and Hirayama disease.[15]

Basilar invagination is a complex of alterations that is more often associated with central or axial atlantoaxial instability. Musculoskeletal alterations in basilar invagination are in the form of short neck, short head, short spine, torticollis, platybasia, bone fusions in the form of assimilation of atlas, C2-3 fusion, Klippel-Feil abnormalities, bifid posterior arch of atlas, os-odontoideum and a host of other features. Neural manifestations are in the form of Chiari formation and external syringomyelia, syringomyelia, external syringobulbia and syringobulbia.[5] We described Chiari formation as nature's 'air-bag' – a protective cushion to prevent compression of critical neural structures between bones in the event of potential or manifest atlantoaxial instability. Excessive presence of water inside and outside the neural structures has the function of making the neural structures float within a pool of cerebrospinal fluid such that trauma to the neural structures at the tip of the odontoid process is minimised. All these maneuvers are protective responses to stall or delay the compressive effects related to the odontoid process in the presence of atlantoaxial instability.

Surgical treatment that is focussed on atlantoaxial fixation results in immediate postoperative recovery from clinical symptoms. More importantly, musculoskeletal and neural alterations like Chiari formation and syringomyelia are likely to reverse in the early postoperative period.

Essentially, atlantoaxial instability forms the basis of the pathological entities of basilar invagination, Chiari formation and syringomyelia. We propose that atlantoaxial stabilization is the treatment, even in cases with assimilation of atlas. Large facets of atlas and axis provide solid ground for screw insertion. Inclusion of the occipital bone and subaxial spinal segments are not necessary and are suboptimal forms of stabilization. Although our articles were amongst the first to describe occipital screw fixation for craniovertebral junction stabilization, we have now completely abandoned the inclusion of occipital bone in the fixation construct.[16] The thin occipital bone provides a weak point of fixation that can easily give way on neck turning. Moreover, the occipitocervical fixation severely restricts neck movements. In cases with basilar invagination, atlantoaxial fixation can be a complex and difficult surgical operation. However, successful conduct of surgery can lead to a remarkable clinical recovery. Presence of large venous plexuses in the region and a complex course of vertebral artery can lead to difficulty in exposure of the atlantoaxial joint and in conducting the screw fixation. Avoiding the defined steps of atlantoaxial fixation and resorting to suboptimal fixation techniques for stabilizing the most mobile joint of the body can lead to failure of the entire surgical procedure.


1Goel A: Chiari malformation – Is atlantoaxial instability the cause? Outcome analysis of 65 patients with Chiari malformation treated by atlantoaxial fixation. J Neurosurg Spine2015;22:116-27.
2Goel A. Is Chiari a “formation” or a “malformation”. J Craniovertebr Junction Spine 2017;8:1-2.
3Goel A, Bhatjiwale M, Desai K. Basilar invagination: A study based on 190 surgically treated cases. JNeurosurg 1998; 88:962-8.
4Goel A: Treatment of basilar invagination by atlantoaxial joint distraction and direct lateral mass fixation. J Neurosurg Spine 2004; 1:281-6.
5Goel A, Sathe P, Shah A. Atlantoaxial fixation for basilar invagination without obvious atlantoaxial instability (Group B-basilar invagination): Outcome analysis of 63 surgically treated cases. World Neurosurg 2017;99:164-70.
6Goel A. Can foramen magnum decompression surgery become historical? J Craniovertebr Junction Spine. 2015;6:49-50.
7Goel A, Kulkarni AG, Sharma P. Reduction of fixed atlantoaxial dislocation in 24 cases: Technical note. J Neurosurg Spine. 2005;2:505-9.
8Goel A, Shah A. Atlantoaxial facet locking: Treatment by facet manipulation and fixation. Experience in 14 cases. J Neurosurg Spine 2011;14:3-9.
9Goel A, Shah A, Rajan S. Vertical mobile and reducible atlantoaxial dislocation. Clinical article. J Neurosurg Spine 2009;11:9-14.
10Goel A, Shah A. Lateral atlantoaxial facetal dislocation in craniovertebral region tuberculosis: Report of a case and analysis of an alternative treatment. Acta Neurochir (Wien) 2010;152:709-12.
11Goel A, Nadkarni T, Shah A, Ramdasi R, Patni N. Bifid anterior and posterior arches of atlas: Surgical implication and analysis of 70 cases. Neurosurgery 2015;77:296-306.
12Goel A: Goel's classification of atlantoaxial “facetal” dislocation. J Craniovertbr Junction Spine 2014;5(1):15-9.
13Goel A. Posterior atlantoaxial 'facetal' instability associated with cervical spondylotic disease. J Craniovertebr Junction Spine. 2015;6:51-5.
14Goel A. Is atlantoaxial instability the cause of “high” cervical ossified posterior longitudinal ligament? Analysis on the basis of surgical treatment of seven patients. J Craniovertebr Junction Spinev 2016;7:20-5.
15Goel A, Dhar A, Shah A. Multilevel spinal stabilization as a treatment for hirayama disease: Report of an experience with five cases. World Neurosurg 2017;99:186-91.
16Goel A, Laheri V. Plate and screw fixation for atlanto-axial subluxation. Acta Neurochir (Wien) 1994;129:47-53.