Chiari Formation, Basilar Invagination and Atlantoaxial Instability - Presentation as 6th Nerve Paresis

Sir, A number of forms of eye-movement disturbances have been identified in cases with Chiari formation. Incoordination, skew deviation or ataxia of eye movements is the more common form of disability.[1–3] Cranial nerve affection is relatively infrequent. Isolated sixth nerve paresis without any other major clinical symptom has been recorded rarely.[1,3] We report recovery from the sixth nerve weakness in the immediate postoperative period following atlantoaxial fixation. A 16-year-old female presented with diplopia on looking towards the left side for a period of about one month. She first noticed this when she was sitting in her classroom at school. The diplopia progressed and affected her routine activities to quite an extent and she preferred to keep her one eye closed and also tilted her neck when speaking. She had no other complaints. On neurological examination, she had left sixth nerve paresis. There was mild stiffness in all limbs with slightly exaggerated reflexes. Her motor power was normal. Investigations revealed Chiari formation, platybasia, basilar invagination and partial occipitalization of atlas. Dynamic CT scan of the craniovertebral junction did not reveal any alteration in the atlantodental or clivodental interval and the atlantoaxial facets were in alignment [Figure 1]. On the basis of our recent study on the subject, the instability in the case was classified as Type 2 wherein in neutral neck posture the facet of atlas was positioned posterior to the facet of axis [Figure 1b].[4] There was no alteration in the atlantodental interval and there was no direct dural or neural indentation by the odontoid process. The instability was classified as central or axial atlantoaxial facetal instability.[4,5] Additionally, the posterior arch of atlas was bifid [Figure 1c]. Atlantoaxial fixation was done by the techniques described by the senior author in the years 1994 and 2004.[6] The atlantoaxial joint was exposed and manual handling of bones identified it to be remarkably unstable. The articular cavity was widely opened, articular cartilage was denuded and bone graft was stuffed into the articular cavity. A C1 lateral mass and a C2 pedicle screw fixation was then performed. In the immediate postoperative phase, she identified improvement in her diplopia. The sixth nerve paresis rapidly recovered and at the end of about one week, the sixth nerve function was near normal [Figure 2]. Despite the fact that there was no complaint of limb weakness prior to surgery, she felt reduction in stiffness and improvement in her gait function after the surgical treatment. At a follow-up of 8 months, the patient was asymptomatic and there were no neurological deficits. The sixth nerve function was normal.Figure 1: Pre-operative Images. (a) T2 weighted sagittal MRI showing Group B basilar invagination and Chiari formation. (b) Sagittal MRI with cut passing through the facets showing Goel Type 2 atlantoaxial facetal instability. (c) Sagittal CT scan showing Group B basilar invagination. There is no abnormal alteration in the atlantodental interval. (d) 3D reconstructed image showing bifid arch of atlas. (e) Images of the eyes of the patient showing a left sixth nerve paresisFigure 2: Post-operative images. (a) Post-operative sagittal CT image showing the atlantoaxial fixation. (b) Sagittal CT image showing the implants. No foramen magnum decompression has been done. (c) Post-operative image of the patient showing resolution of the sixth nerve paresisAlthough infrequently, cranial nerve deficits have been identified in cases with Chiari formation.[1–3] Isolated sixth cranial nerve affection is rare and our literature search identified reports of 17 such cases.[3] Strabismus, oscillopsia, esotropia, divergence or convergence insufficiency and diplopia has been recorded and has been related to incoordination of eye movement and nystagmus, rather than named cranial nerve dysfunction.[1–3] As the number of reported cases having a combination of Chiari with sixth nerve paresis is distinctly infrequent exact cause-effect relationship is difficult to speculate. The suggested mechanisms have been compression of the abducens nuclei in the brainstem due to the Chiari formation. Diplopia due to divergence insufficiency is a rare presentation of Chiari formation. Divergence insufficiency is a clinical condition wherein the diplopia is more prominent when viewing distant objects. Ophthalmological conditions in children are more likely to cause diplopia when looking at near objects rather than distant objects.[3] This fact should be remembered in children presenting with acute onset of esotropia without any other abnormal ophthalmological findings. Many of these patients present after a preliminary surgery to correct the esotropia. Foramen magnum decompression has been the generally accepted surgical technique for management of the Chiari formation. Delayed correction of the esotropia has been reported in the majority of the cases after suboccipital decompression with or without duroplasty. None of the reported patients operated with posterior decompression reported an immediate improvement in the diplopia. Some patients additionally needed a corrective surgery for the esotropia.[3] In 2014, we identified atlantoaxial instability as the nodal point of pathogenesis of a cascade of secondary events that includes basilar invagination and Chiari formation.[7] Our earlier articles discuss that both Chiari formation and basilar invagination and a host of associated features like syringomyelia, Klippel-Feil abnormalities, assimilation of atlas and platybasia are secondary and natural protective phenomenon that ultimately aim to protect the neural structures from compression between bones in the event of potential or manifest atlantoaxial instability.[8,9] Considering this, we preferred the term Chiari ‘formation’ to Chiari ‘malformation’ and craniovertebral ‘alterations’ to craniovertebral ‘anomalies’. Atlantoaxial instability was diagnosed on the basis of observation of alignment of facets of atlas and axis on lateral profile imaging in neutral head position and on observations on manual manipulation of bones during surgery. As atlantodental interval may not be affected and there may not be any dural or neural compression, instability in such cases is labeled as central or axial atlantoaxial instability.[4,5] The more important observation was that following atlantoaxial fixation, all secondary events had the potential for reversal. Our positive clinical results in a large number of cases are a testimony of validity of the theory of atlantoaxial instability being the cause of Chiari formation. In our earlier article, we identified that presence of bifid posterior arch of atlas is by itself an indicator of atlantoaxial instability.[10] In the presented case, atlantoaxial fixation was done without any bone decompression. The exact cause of sixth nerve deficit and the mechanism of its recovery following fixation can only speculated. The presence of atlantoaxial instability, basilar invagination and its associated reduction in the posterior cranial fossa volume could ultimately result in the Chiari formation. The effect on the brainstem as a result of either compression or traction could eventually affect the sixth cranial nerve function. The validity of atlantoaxial instability as the nodal point of pathogenesis of Chiari formation is still under intense discussion. Even the entity of ‘central’ atlantoaxial instability is under evaluation. However, our increasing experience in the subject and uniformly gratifying clinical results has strengthened our enthusiasm to evaluate this issue further. Recovery from sixth nerve weakness in the immediate postoperative period is further evidence of validity of our concept. Isolated sixth nerve palsy can be a rare presentation of Chiari formation and basilar invagination. Surgical treatment of the basilar invagination and Chiari formation by atlantoaxial fixation can lead to immediate and sustained reversal of the sixth nerve weakness. Informed consent “Informed consent was obtained from the patient in the report.” Additional informed consent was obtained from the individual participant for whom identifying information is included in this article. Declaration of patient consent The authors certify that they have obtained all appropriate patient consent forms. In the form the patient(s) has/have given his/her/their consent for his/her/their images and other clinical information to be reported in the journal. The patients understand that their names and initials will not be published and due efforts will be made to conceal their identity, but anonymity cannot be guaranteed. Financial support and sponsorship Nil. Conflicts of interest There are no conflicts of interest.