Secondary Chiari-like Formation Caused by Grafted Bone Overgrowth Following Occipitocervical Fixation: A Rare Cause of Pediatric Quadriplegia

Secondary Chiari-like Formation Caused by Grafted Bone Overgrowth Following Occipitocervical Fixation: A Rare Cause of Pediatric Quadriplegia | Research Square window.SnipcartSettings = { analytics: { enabled: false } }; (function() { var accessVector = localStorage.getItem('access_vector') || ''; window.dataLayer = window.dataLayer || []; if (accessVector) { window.dataLayer.push({ user: { profile: { profileInfo: { snid: accessVector } } } }); } })(); (function(w,d,s,l,i){w[l]=w[l]||[];w[l].push({'gtm.start':new Date().getTime(),event:'gtm.js'});var f=d.getElementsByTagName(s)[0],j=d.createElement(s),dl=l!='dataLayer'?'&l='+l:'';j.async=true;j.src='https://www.googletagmanager.com/gtm.js?id='+i+dl;f.parentNode.insertBefore(j,f);})(window,document,'script','dataLayer','GTM-K279D39R'); Browse Preprints In Review Journals COVID-19 Preprints AJE Video Bytes Research Tools Research Promotion AJE Professional Editing AJE Rubriq About Preprint Platform In Review Editorial Policies Our Team Advisory Board Help Center Sign In Submit a Preprint Cite Share Download PDF Case Report Secondary Chiari-like Formation Caused by Grafted Bone Overgrowth Following Occipitocervical Fixation: A Rare Cause of Pediatric Quadriplegia Junji Koyama, Nobuyuki Akutsu, Atsufumi Kawamura This is a preprint; it has not been peer reviewed by a journal. https://doi.org/ 10.21203/rs.3.rs-8685776/v1 This work is licensed under a CC BY 4.0 License Status: Published Journal Publication published 27 Apr, 2026 Read the published version in Child's Nervous System → Version 1 posted 7 You are reading this latest preprint version Abstract Unlike Chiari malformation type I, which results from a congenitally small posterior fossa, secondary Chiari-like formation (SCLF) is acquired and arises from a craniospinal pressure gradient or reduced posterior fossa volume. We report a rare case of SCLF caused by massive bone overgrowth after occipitocervical fixation (OCF) in a child. An 8-year-old boy with Klippel–Feil syndrome and basilar invagination developed quadriparesis after OCF. His symptoms worsened following repeat OCF performed due to misdiagnosis. Four months later, the quadriparesis progressed further. Serial imaging demonstrated foramen magnum crowding caused by a hypertrophic graft, along with cerebellar tonsillar herniation. Foramen magnum decompression, performed 10 months after the second surgery, led to rapid motor improvement, allowing independent ambulation within 3 weeks. This case highlights massive graft overgrowth as a unique mechanical etiology of SCLF. Vigilance and careful comparison of serial craniovertebral junction imaging are crucial in pediatric patients with neurological deterioration after OCF. secondary Chiari-like formation acquired Chiari type 1 malformation quadriplegia foramen magnum decompression occipitocervical fixation grafted bone overgrowth Figures Figure 1 Figure 2 Figure 3 Figure 4 Introduction Chiari malformation Type I (CMI) typically results from congenital hypoplasia of the occipital bone [ 1 , 2 ]. By contrast, secondary Chiari-like formation (SCLF) refers to an acquired condition that develops morphological and symptomatic features similar to congenital CMI. It was formerly referred to as “acquired Chiari malformation” [ 3 , 4 ]. These acquired conditions arise either from a craniospinal pressure gradient (e.g., spinal cerebrospinal fluid hypotension [ 5 – 8 ] or intracranial hypertension due to various pathologies [ 9 – 17 ]) or from secondary reduction in posterior fossa (PF) volume (e.g., craniosynostosis or hyperostosis [ 18 – 22 ]). We report a rare case of SCLF in a pediatric patient following OCF for basilar invagination. The patient developed subacutely progressive quadriplegia. The etiology was massive overgrowth of grafted bone resulting in PF volume reduction, representing a unique mechanical cause and a significant diagnostic challenge. Case Presentation An 8-year-old boy presented with progressive gait disturbance. Three years earlier, he had undergone anterior decompression and occipitocervical posterior fusion with parietal bone grafting for symptomatic basilar invagination associated with Klippel–Feil syndrome [Fig. 1 a–c]. Serial head and neck computed tomography demonstrated regrowth of the decompressed occipital bone and moderate union of the grafted bone [Fig. 1 d]. Suspected minor instability prompted an additional instrumented posterior fusion. Postoperatively, his symptoms initially improved, and he regained independent ambulation. Four months later, he developed subacute quadriplegia, worsening gait disturbance, and sleep apnea. Although magnetic resonance imaging (MRI) at 8 years of age had shown no tonsillar herniation [Fig. 2 a], retrospective review of MRI obtained 2 months later revealed subtle tonsillar descent [Fig. 2 b]. Notably, the dorsal grafted bone had become markedly hypertrophic [Fig. 2 c], resulting in significant overcrowding at the foramen magnum [Fig. 2 d]. We hypothesized that this excessive bone overgrowth reduced PF volume, leading to acquired tonsillar herniation and spinal cord compression. Ten months after the previous fusion, foramen magnum decompression was performed. Intraoperatively, the excessive ossified graft was drilled out, and the dura was opened, revealing tonsils that occluded the subarachnoid space. Subpial resection of the tonsils and expansile duraplasty using autologous pericranium were performed [Fig. 3 a–d]. The patient’s recovery was excellent. Motor weakness improved within 1 week, and he was able to walk independently by 3 weeks. Postoperative computed tomography confirmed adequate decompression [Fig. 4 a, b]. At the 8-year follow-up, the patient was neurologically intact and leading an active life as a university student. Follow-up MRI confirmed maintenance of decompression [Fig. 4 c]. Discussion CMI is traditionally defined by descent of the cerebellar tonsils more than 5 mm below the foramen magnum [ 2 ]. While often congenital [ 1 ], secondary forms of hindbrain herniation (i.e., SCLF) have been increasingly recognized. The etiology is broadly classified into two mechanisms: an increased craniospinal pressure gradient or overcrowding due to secondary PF disproportion. Conditions producing a pressure gradient include spinal hypotension [ 5 – 8 ] or intracranial hypertension [ 10 – 17 ]. By contrast, PF disproportion is typically associated with secondary skull thickening [ 18 , 19 , 21 , 22 ] or craniosynostosis [ 20 ]. In the present case, secondary PF disproportion is the most plausible etiology. The patient had pre-existing basilar invagination associated with Klippel–Feil syndrome, which inherently narrowed the PF [ 23 ], creating a predisposition to tonsillar descent. Kimura et al. [ 24 ] reported a case of SCLF following atlantoaxial vertical subluxation in a patient with rheumatoid arthritis. In that report, dynamic vertical instability at the craniocervical junction led to upward migration of the dens and consequent PF volume reduction. Although the background circumstances were similar, our patient differed in that tonsillar herniation developed after internal fixation had already been achieved. We therefore hypothesize that subsequent marked overgrowth of grafted bone at the craniocervical junction further reduced the already compromised PF volume. This “second hit” likely precipitated tonsillar herniation and severe spinal cord compression. The mechanism underlying postoperative ectopic bone overgrowth warrants consideration. Although spinal instability is often associated with bone regrowth [ 25 ], mechanical instability is unlikely in this case given the solid fusion. Instead, considering the patient’s age, we speculate that the high osteogenic potential of the pediatric population contributed to the excessive bone formation. Diagnostic difficulty arose because the tonsillar descent was subtle and difficult to detect radiographically, leading to initial misdiagnosis and treatment delay. Although SCLF typically progresses gradually, it can result in acute, life-threatening deterioration [ 8 ]. This case highlights that exuberant grafted bone overgrowth can induce tonsillar herniation even after successful instrumented fusion. Clinicians must therefore maintain a high index of suspicion and carefully compare serial imaging when pediatric patients present with unexplained neurological decline following craniocervical surgery. Conclusion We report a rare case of SCLF caused by massive graft overgrowth following OCF. This case underscores that pediatric patients are at risk of PF volume reduction because of their high osteogenic potential. Clinicians should maintain a high index of suspicion and rigorously compare serial imaging when pediatric patients exhibit unexplained neurological deterioration after craniocervical surgery. Declarations Ethics declarations Funding No funding was secured for this study. Conflict of interest The authors have no conflicts of interest to declare. Ethics approval All procedures performed in this case report were in accordance with ethical standards of the institution and the 1964 Helsinki Declaration. Consent to participate Informed consent for participation was obtained from the patient and the patient’s parents. Consent for publication Informed consent for publication was obtained from the patient and the patient’s parents. Availability of data and material N/A Code availability N/A Author contributions Dr. Koyama composed and revised the manuscript and participated in the diagnosis and treatment of the patient. Drs. Akutsu and Kawamura participated in the diagnosis and treatment of the patient. Dr. Kawamura contributed to manuscript revision. All authors read and approved the final manuscript. References Shoja MM, Tubbs RS, Oakes WJ (2013) Embryology and pathophysiology of the Chiari I and II malformations. In: Tubbs RS, Oakes WJ (eds) The Chiari Malformations. Springer, New York, pp 13–54 Poretti A, Ashmawy R, Garzon-Muvdi T, Jallo GI, Huisman TA, Raybaud C (2016) Chiari Type 1 Deformity in Children: Pathogenetic, Clinical, Neuroimaging, and Management Aspects. Neuropediatrics 47:293-307. doi: 10.1055/s-0036-1584563 Payner TD, Prenger E, Berger TS, Crone KR (1994) Acquired Chiari malformations: incidence, diagnosis, and management. Neurosurgery 34:429–434 Sahuquillo J, Moncho D, Ferré A, López-Bermeo D, Sahuquillo-Muxi A, Poca MA (2023) A Critical Update of the Classification of Chiari and Chiari-like Malformations. J Clin Med 12:4626. doi: 10.3390/jcm12144626 Atkinson JL, Weinshenker BG, Miller GM, Piepgras DG, Mokri B (1998) Acquired Chiari I malformation secondary to spontaneous spinal cerebrospinal fluid leakage and chronic intracranial hypotension syndrome in seven cases. J Neurosurg 88:237–242 Peleggi AF, Lovely TJ (2012) Treatment of delayed Chiari malformation and syringomyelia after lumboperitoneal shunt placement: Case report and treatment recommendations. Surg Neurol Int 3:101. doi: 10.4103/2152-7806.100188 Sathi S, Stieg PE (1995) “Acquired” Chiari I malformation after multiple lumbar punctures: Case report. Neurosurgery 37:342-344 Sugrue PA, Hsieh PC, Getch CC, Batjer HH (2009) Acute symptomatic cerebellar tonsillar herniation following intraoperative lumbar drainage. J Neurosurg 110:800-803. doi: 10.3171/2008.5.17568 Thotakura AK, Marabathina NR (2017) Acquired Chiari I Malformation with Syringomyelia Secondary to Colloid Cyst with Hydrocephalus-Case Report and Review of Literature. World Neurosurg 108:995.e1-995.e4. doi: 10.1016/j.wneu.2017.09.012 Iampreechakul P, Wangtanaphat K, Hangsapruek S, Wattanasen Y, Lertbutsayanukul P, Siriwimonmas S (2022) Acquired Chiari malformation Type I and holocord syringomyelia associated with a high-flow supratentorial fistulous arteriovenous malformations: A case report and literature review. Surg Neurol Int 13:217. doi: 10.25259/SNI_11_2022 Chen KW, Kuo MF, Lee CW, Tu YK (2015) Acquired Chiari malformation type I associated with a supratentorial fistulous arteriovenous malformation: a case report. Childs Nerv Syst 31:499-501. doi: 10.1007/s00381-014-2508-2 Bahuleyan B, Rao A, Chacko AG, Daniel RT (2007) Supracerebellar arachnoid cyst - A rare cause of acquired Chiari I malformation. J Clin Neurosci 14:895-898. doi: 10.1016/j.jocn.2006.06.009 Galarza M, López-Guerrero AL, Martínez-Lage JF (2010) Posterior fossa arachnoid cysts and cerebellar tonsillar descent: short review. Neurosurg Rev 33:305-314. doi: 10.1007/s10143-010-0262-9 Lou C, Wang L, Pan X, Xu D, Chen Y (2025) Resolution of tonsillar herniation and syringomyelia after resection of supratentorial large meningioma. Brain Spine 5:104312. doi: 10.1016/j.bas.2025.104312 Weinberg JS, Rhines LD, Cohen ZR, Langford L, Levin VA (2003) Posterior fossa decompression for life-threatening tonsillar herniation in patients with gliomatosis cerebri: report of three cases. Neurosurgery 52:216-223 Muzumdar D, Ventureyra EC (2006) Tonsillar herniation and cervical syringomyelia in association with posterior fossa tumors in children: a case-based update. Childs Nerv Syst 22:454-459. doi: 10.1007/s00381-005-0027-x Raja AI, Adada B (2007) Immediate resolution of tonsillar herniation and severe cervicothoracic syringomyelia after third ventriculostomy for hydrocephalus caused by a brainstem tumor. Case report. J Neurosurg 106:44-47. doi: 10.3171/ped.2007.106.1.44 Albert L Jr, Hirschfeld A (2009) Acquired Chiari malformation secondary to hyperostosis of the skull: a case report and literature review. Surg Neurol 72:157-161. doi: 10.1016/j.surneu.2008.02.030 Rahme R, Koussa S, Samaha E (2009) C1 arch regeneration, tight cisterna magna, and cervical syringomyelia following foramen magnum surgery. Surg Neurol 72:83-85. doi: 10.1016/j.surneu.2008.01.041 Leikola J, Koljonen V, Valanne L, Hukki J (2010) The incidence of Chiari malformation in nonsyndromic, single suture craniosynostosis. Childs Nerv Syst 26:771-774. doi: 10.1007/s00381-009-1044-y Han Y, Chen M, Xu J, Wang Y, Wang H (2020) Acquired Chiari type I malformation managed by expanding posterior fossa volume and literature review. Childs Nerv Syst 36:235-240. doi: 10.1007/s00381-019-04437-0 D’Amico A, Giammalva GR, Furlanis GM, Emanuelli E, Maugeri R, Baro V, Denaro L (2023) Acquired Chiari type I malformation: a late and misunderstood supratentorial over-drainage complication. Childs Nerv Syst 39:343-351. doi: 10.1007/s00381-022-05775-2 Nishikawa M, Sakamoto H, Hakuba A, Nakanishi N, Inoue Y (1997) Pathogenesis of Chiari malformation: a morphometric study of the posterior cranial fossa. J Neurosurg 86:40-47. doi: 10.3171/jns.1997.86.1.0040 Kimura Y, Seichi A, Gomi A, Kojima M, Inoue H, Kimura A (2012) Acquired Chiari malformation secondary to atlantoaxial vertical subluxation in a patient with rheumatoid arthritis combined with atlanto-occipital assimilation. Neurol Med Chir (Tokyo) 52:683-686. doi: 10.2176/nmc.52.683 Chen Q, Baba H, Kamitani K, Furusawa N, Imura S (1994) Postoperative bone re-growth in lumbar spinal stenosis. A multivariate analysis of 48 patients. Spine (Phila Pa 1976) 19:2144-2149 Additional Declarations No competing interests reported. Cite Share Download PDF Status: Published Journal Publication published 27 Apr, 2026 Read the published version in Child's Nervous System → Version 1 posted Editorial decision: Revision requested 12 Mar, 2026 Reviews received at journal 12 Mar, 2026 Reviewers agreed at journal 03 Mar, 2026 Reviewers invited by journal 19 Feb, 2026 Editor assigned by journal 26 Jan, 2026 Submission checks completed at journal 26 Jan, 2026 First submitted to journal 24 Jan, 2026 You are reading this latest preprint version Research Square lets you share your work early, gain feedback from the community, and start making changes to your manuscript prior to peer review in a journal. As a division of Research Square Company, we’re committed to making research communication faster, fairer, and more useful. We do this by developing innovative software and high quality services for the global research community. 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Also discoverable on Platform About Our Team In Review Editorial Policies Advisory Board Help Center Resources Author Services Accessibility API Access RSS feed Manage Cookie Preferences © Research Square 2026 | ISSN 2693-5015 (online) Privacy Policy Terms of Service Do Not Sell My Personal Information {"props":{"pageProps":{"initialData":{"identity":"rs-8685776","acceptedTermsAndConditions":true,"allowDirectSubmit":false,"archivedVersions":[],"articleType":"Case Report","associatedPublications":[],"authors":[{"id":594154450,"identity":"aa4f6e7f-a05e-46a1-98f2-2d28af037bce","order_by":0,"name":"Junji Koyama","email":"data:image/png;base64,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","orcid":"","institution":"Hyogo Prefectural Kobe Children’s Hospital","correspondingAuthor":true,"prefix":"","firstName":"Junji","middleName":"","lastName":"Koyama","suffix":""},{"id":594154451,"identity":"e1ef2d41-c99f-4761-b65e-23c51de55ade","order_by":1,"name":"Nobuyuki Akutsu","email":"","orcid":"","institution":"Hyogo Prefectural Kobe Children’s Hospital","correspondingAuthor":false,"prefix":"","firstName":"Nobuyuki","middleName":"","lastName":"Akutsu","suffix":""},{"id":594154452,"identity":"55e5b8fe-b5d5-4d52-9fe1-7768297ab5ec","order_by":2,"name":"Atsufumi Kawamura","email":"","orcid":"","institution":"Hyogo Prefectural Kobe Children’s Hospital","correspondingAuthor":false,"prefix":"","firstName":"Atsufumi","middleName":"","lastName":"Kawamura","suffix":""}],"badges":[],"createdAt":"2026-01-24 10:11:58","currentVersionCode":1,"declarations":"","doi":"10.21203/rs.3.rs-8685776/v1","doiUrl":"https://doi.org/10.21203/rs.3.rs-8685776/v1","draftVersion":[],"editorialEvents":[{"content":"https://doi.org/10.1007/s00381-026-07272-2","type":"published","date":"2026-04-27T15:58:21+00:00"}],"editorialNote":"","failedWorkflow":false,"files":[{"id":103399977,"identity":"43bc781a-65bd-4742-b7b3-d9a1c0d2ea65","added_by":"auto","created_at":"2026-02-25 09:13:23","extension":"jpeg","order_by":1,"title":"Figure 1","display":"","copyAsset":false,"role":"figure","size":179724,"visible":true,"origin":"","legend":"\u003cp\u003ePreoperative imaging and initial surgical management of basilar invagination\u003c/p\u003e\n\u003cp\u003ea) Initial sagittal computed tomography (CT) image shows flattening of the clivus and superior migration of the odontoid process, resulting in severe foramen magnum stenosis.\u003c/p\u003e\n\u003cp\u003eb) Initial sagittal T2-weighted magnetic resonance imaging demonstrates posterior fossa crowding and ventral compression of the brainstem by the odontoid process.\u003c/p\u003e\n\u003cp\u003ec) Immediately postoperative sagittal CT image of the head and neck demonstrates decompression at the foramen magnum and placement of graft bone between the occipital bone and the axis.\u003c/p\u003e\n\u003cp\u003ed) Sagittal CT image obtained 3 months after fixation surgery reveals early graft bone union, along with regrowth of the decompressed occipital bone.\u003c/p\u003e","description":"","filename":"floatimage1.jpeg","url":"https://assets-eu.researchsquare.com/files/rs-8685776/v1/c60363624e13279927550b1a.jpeg"},{"id":103399975,"identity":"01a32860-36da-46ec-9410-d4f23e0d58c2","added_by":"auto","created_at":"2026-02-25 09:13:23","extension":"jpeg","order_by":2,"title":"Figure 2","display":"","copyAsset":false,"role":"figure","size":172558,"visible":true,"origin":"","legend":"\u003cp\u003eSerial magnetic resonance imaging (MRI) and computed tomography (CT) demonstrating progressive acquired tonsillar herniation\u003c/p\u003e\n\u003cp\u003ea) Sagittal T1-weighted MRI obtained 7 months prior to the final fusion surgery shows no evidence of cerebellar tonsillar herniation.\u003c/p\u003e\n\u003cp\u003eb) Sagittal T1-weighted MRI obtained 5 months prior to the final fusion surgery reveals mild caudal displacement of the cerebellar tonsils.\u003c/p\u003e\n\u003cp\u003ec) Sagittal CT image obtained 4 months prior to the final fusion surgery demonstrates excessively thickened graft bone at the dorsal craniocervical junction at the craniovertebral junction.\u003c/p\u003e\n\u003cp\u003ed) Sagittal T1-weighted MRI obtained 4 months after the final fusion surgery shows more pronounced cerebellar tonsillar herniation.\u003c/p\u003e","description":"","filename":"floatimage2.jpeg","url":"https://assets-eu.researchsquare.com/files/rs-8685776/v1/21bef84b32f8f0e22e55e239.jpeg"},{"id":103399969,"identity":"961ac805-f7d9-4b6e-b9b8-bf28a02f8514","added_by":"auto","created_at":"2026-02-25 09:13:21","extension":"jpeg","order_by":3,"title":"Figure 3","display":"","copyAsset":false,"role":"figure","size":181467,"visible":true,"origin":"","legend":"\u003cp\u003eIntraoperative photographs illustrating posterior fossa decompression\u003c/p\u003e\n\u003cp\u003ea) Midline exposure of the occipital bone and upper cervical spine with partial removal of previous instrumentation, including the occipital plate.\u003c/p\u003e\n\u003cp\u003eb) Following an osteoplastic midline suboccipital craniotomy, the thickened bone graft at the craniovertebral junction was carefully drilled out.\u003c/p\u003e\n\u003cp\u003ec) Bilateral cerebellar tonsils (asterisk) protruding below the foramen magnum and occupying the spinal canal are visible.\u003c/p\u003e\n\u003cp\u003ed) After bilateral cerebellar tonsillectomy for foramen magnum decompression, the lower pole of the fourth ventricle and the obex (arrowhead) are clearly visualized.\u003c/p\u003e","description":"","filename":"floatimage3.jpeg","url":"https://assets-eu.researchsquare.com/files/rs-8685776/v1/db92a5eda91421ff5058ae62.jpeg"},{"id":103399974,"identity":"bd73b93f-0fb2-4af0-89b9-c16d9771be02","added_by":"auto","created_at":"2026-02-25 09:13:23","extension":"jpeg","order_by":4,"title":"Figure 4","display":"","copyAsset":false,"role":"figure","size":70596,"visible":true,"origin":"","legend":"\u003cp\u003eImaging findings following posterior fossa decompression (PFD)\u003c/p\u003e\n\u003cp\u003ea) Sagittal and b) axial computed tomography scans obtained shortly after PFD demonstrate sufficient bony resection at the craniovertebral junction.\u003c/p\u003e\n\u003cp\u003ec) Sagittal T1-weighted magnetic resonance imaging obtained 8 years after PFD shows maintained decompression of the craniovertebral junction, with no evidence of brainstem or spinal cord compression.\u003c/p\u003e","description":"","filename":"floatimage4.jpeg","url":"https://assets-eu.researchsquare.com/files/rs-8685776/v1/79e782805ff09413fbfe151c.jpeg"},{"id":108437677,"identity":"21ce12b4-26ab-4809-a21a-a461c6a555f7","added_by":"auto","created_at":"2026-05-04 16:02:00","extension":"pdf","order_by":0,"title":"","display":"","copyAsset":false,"role":"manuscript-pdf","size":750421,"visible":true,"origin":"","legend":"","description":"","filename":"manuscript.pdf","url":"https://assets-eu.researchsquare.com/files/rs-8685776/v1/323cd86a-8f02-4375-81e2-56ec4f2ef726.pdf"}],"financialInterests":"No competing interests reported.","formattedTitle":"Secondary Chiari-like Formation Caused by Grafted Bone Overgrowth Following Occipitocervical Fixation: A Rare Cause of Pediatric Quadriplegia","fulltext":[{"header":"Introduction","content":"\u003cp\u003eChiari malformation Type I (CMI) typically results from congenital hypoplasia of the occipital bone [\u003cspan class=\"CitationRef\"\u003e1\u003c/span\u003e, \u003cspan class=\"CitationRef\"\u003e2\u003c/span\u003e]. By contrast, secondary Chiari-like formation (SCLF) refers to an acquired condition that develops morphological and symptomatic features similar to congenital CMI. It was formerly referred to as “acquired Chiari malformation” [\u003cspan class=\"CitationRef\"\u003e3\u003c/span\u003e, \u003cspan class=\"CitationRef\"\u003e4\u003c/span\u003e]. These acquired conditions arise either from a craniospinal pressure gradient (e.g., spinal cerebrospinal fluid hypotension [\u003cspan class=\"CitationRef\"\u003e5\u003c/span\u003e–\u003cspan class=\"CitationRef\"\u003e8\u003c/span\u003e] or intracranial hypertension due to various pathologies [\u003cspan class=\"CitationRef\"\u003e9\u003c/span\u003e–\u003cspan class=\"CitationRef\"\u003e17\u003c/span\u003e]) or from secondary reduction in posterior fossa (PF) volume (e.g., craniosynostosis or hyperostosis [\u003cspan class=\"CitationRef\"\u003e18\u003c/span\u003e–\u003cspan class=\"CitationRef\"\u003e22\u003c/span\u003e]). We report a rare case of SCLF in a pediatric patient following OCF for basilar invagination. The patient developed subacutely progressive quadriplegia. The etiology was massive overgrowth of grafted bone resulting in PF volume reduction, representing a unique mechanical cause and a significant diagnostic challenge.\u003c/p\u003e "},{"header":"Case Presentation","content":"\u003cp\u003eAn 8-year-old boy presented with progressive gait disturbance. Three years earlier, he had undergone anterior decompression and occipitocervical posterior fusion with parietal bone grafting for symptomatic basilar invagination associated with Klippel–Feil syndrome [Fig.\u0026nbsp;\u003cspan class=\"InternalRef\"\u003e1\u003c/span\u003ea–c]. Serial head and neck computed tomography demonstrated regrowth of the decompressed occipital bone and moderate union of the grafted bone [Fig.\u0026nbsp;\u003cspan class=\"InternalRef\"\u003e1\u003c/span\u003ed]. Suspected minor instability prompted an additional instrumented posterior fusion. Postoperatively, his symptoms initially improved, and he regained independent ambulation.\u003c/p\u003e\u003cp\u003eFour months later, he developed subacute quadriplegia, worsening gait disturbance, and sleep apnea. Although magnetic resonance imaging (MRI) at 8 years of age had shown no tonsillar herniation [Fig.\u0026nbsp;\u003cspan class=\"InternalRef\"\u003e2\u003c/span\u003ea], retrospective review of MRI obtained 2 months later revealed subtle tonsillar descent [Fig.\u0026nbsp;\u003cspan class=\"InternalRef\"\u003e2\u003c/span\u003eb]. Notably, the dorsal grafted bone had become markedly hypertrophic [Fig.\u0026nbsp;\u003cspan class=\"InternalRef\"\u003e2\u003c/span\u003ec], resulting in significant overcrowding at the foramen magnum [Fig.\u0026nbsp;\u003cspan class=\"InternalRef\"\u003e2\u003c/span\u003ed]. We hypothesized that this excessive bone overgrowth reduced PF volume, leading to acquired tonsillar herniation and spinal cord compression.\u003c/p\u003e\u003cp\u003eTen months after the previous fusion, foramen magnum decompression was performed. Intraoperatively, the excessive ossified graft was drilled out, and the dura was opened, revealing tonsils that occluded the subarachnoid space. Subpial resection of the tonsils and expansile duraplasty using autologous pericranium were performed [Fig.\u0026nbsp;\u003cspan class=\"InternalRef\"\u003e3\u003c/span\u003ea–d].\u003c/p\u003e\u003cp\u003eThe patient’s recovery was excellent. Motor weakness improved within 1 week, and he was able to walk independently by 3 weeks. Postoperative computed tomography confirmed adequate decompression [Fig.\u0026nbsp;\u003cspan class=\"InternalRef\"\u003e4\u003c/span\u003ea, b]. At the 8-year follow-up, the patient was neurologically intact and leading an active life as a university student. Follow-up MRI confirmed maintenance of decompression [Fig.\u0026nbsp;\u003cspan class=\"InternalRef\"\u003e4\u003c/span\u003ec].\u003c/p\u003e"},{"header":"Discussion","content":"\u003cp\u003eCMI is traditionally defined by descent of the cerebellar tonsils more than 5 mm below the foramen magnum [\u003cspan citationid=\"CR2\" class=\"CitationRef\"\u003e2\u003c/span\u003e]. While often congenital [\u003cspan citationid=\"CR1\" class=\"CitationRef\"\u003e1\u003c/span\u003e], secondary forms of hindbrain herniation (i.e., SCLF) have been increasingly recognized. The etiology is broadly classified into two mechanisms: an increased craniospinal pressure gradient or overcrowding due to secondary PF disproportion. Conditions producing a pressure gradient include spinal hypotension [\u003cspan additionalcitationids=\"CR6 CR7\" citationid=\"CR5\" class=\"CitationRef\"\u003e5\u003c/span\u003e\u0026ndash;\u003cspan citationid=\"CR8\" class=\"CitationRef\"\u003e8\u003c/span\u003e] or intracranial hypertension [\u003cspan additionalcitationids=\"CR11 CR12 CR13 CR14 CR15 CR16\" citationid=\"CR10\" class=\"CitationRef\"\u003e10\u003c/span\u003e\u0026ndash;\u003cspan citationid=\"CR17\" class=\"CitationRef\"\u003e17\u003c/span\u003e]. By contrast, PF disproportion is typically associated with secondary skull thickening [\u003cspan citationid=\"CR18\" class=\"CitationRef\"\u003e18\u003c/span\u003e, \u003cspan citationid=\"CR19\" class=\"CitationRef\"\u003e19\u003c/span\u003e, \u003cspan citationid=\"CR21\" class=\"CitationRef\"\u003e21\u003c/span\u003e, \u003cspan citationid=\"CR22\" class=\"CitationRef\"\u003e22\u003c/span\u003e] or craniosynostosis [\u003cspan citationid=\"CR20\" class=\"CitationRef\"\u003e20\u003c/span\u003e].\u003c/p\u003e \u003cp\u003eIn the present case, secondary PF disproportion is the most plausible etiology. The patient had pre-existing basilar invagination associated with Klippel\u0026ndash;Feil syndrome, which inherently narrowed the PF [\u003cspan citationid=\"CR23\" class=\"CitationRef\"\u003e23\u003c/span\u003e], creating a predisposition to tonsillar descent. Kimura et al. [\u003cspan citationid=\"CR24\" class=\"CitationRef\"\u003e24\u003c/span\u003e] reported a case of SCLF following atlantoaxial vertical subluxation in a patient with rheumatoid arthritis. In that report, dynamic vertical instability at the craniocervical junction led to upward migration of the dens and consequent PF volume reduction. Although the background circumstances were similar, our patient differed in that tonsillar herniation developed after internal fixation had already been achieved. We therefore hypothesize that subsequent marked overgrowth of grafted bone at the craniocervical junction further reduced the already compromised PF volume. This \u0026ldquo;second hit\u0026rdquo; likely precipitated tonsillar herniation and severe spinal cord compression.\u003c/p\u003e \u003cp\u003eThe mechanism underlying postoperative ectopic bone overgrowth warrants consideration. Although spinal instability is often associated with bone regrowth [\u003cspan citationid=\"CR25\" class=\"CitationRef\"\u003e25\u003c/span\u003e], mechanical instability is unlikely in this case given the solid fusion. Instead, considering the patient\u0026rsquo;s age, we speculate that the high osteogenic potential of the pediatric population contributed to the excessive bone formation.\u003c/p\u003e \u003cp\u003eDiagnostic difficulty arose because the tonsillar descent was subtle and difficult to detect radiographically, leading to initial misdiagnosis and treatment delay. Although SCLF typically progresses gradually, it can result in acute, life-threatening deterioration [\u003cspan citationid=\"CR8\" class=\"CitationRef\"\u003e8\u003c/span\u003e]. This case highlights that exuberant grafted bone overgrowth can induce tonsillar herniation even after successful instrumented fusion. Clinicians must therefore maintain a high index of suspicion and carefully compare serial imaging when pediatric patients present with unexplained neurological decline following craniocervical surgery.\u003c/p\u003e"},{"header":"Conclusion","content":"\u003cp\u003eWe report a rare case of SCLF caused by massive graft overgrowth following OCF. This case underscores that pediatric patients are at risk of PF volume reduction because of their high osteogenic potential. Clinicians should maintain a high index of suspicion and rigorously compare serial imaging when pediatric patients exhibit unexplained neurological deterioration after craniocervical surgery.\u003c/p\u003e"},{"header":"Declarations","content":"\u003cp\u003e\u003cstrong\u003eEthics declarations\u003c/strong\u003e\u003c/p\u003e\n\u003cp\u003eFunding No funding was secured for this study.\u003c/p\u003e\n\u003cp\u003e\u003cstrong\u003eConflict of interest\u0026nbsp;\u003c/strong\u003e\u003c/p\u003e\n\u003cp\u003eThe authors have no conflicts of interest to declare.\u003c/p\u003e\n\u003cp\u003e\u003cstrong\u003eEthics approval\u0026nbsp;\u003c/strong\u003e\u003c/p\u003e\n\u003cp\u003eAll procedures performed in this case report were in accordance with ethical standards of the institution and the 1964 Helsinki Declaration.\u003c/p\u003e\n\u003cp\u003e\u003cstrong\u003eConsent to participate\u0026nbsp;\u003c/strong\u003e\u003c/p\u003e\n\u003cp\u003eInformed consent for participation was obtained from the patient and the patient\u0026rsquo;s parents.\u003c/p\u003e\n\u003cp\u003eConsent for publication Informed consent for publication was obtained from the patient and the patient\u0026rsquo;s parents.\u003c/p\u003e\n\u003cp\u003e\u003cstrong\u003eAvailability of data and material\u003c/strong\u003e\u003c/p\u003e\n\u003cp\u003eN/A\u003c/p\u003e\n\u003cp\u003e\u003cstrong\u003eCode availability\u003c/strong\u003e\u003c/p\u003e\n\u003cp\u003eN/A\u003c/p\u003e\n\u003cp\u003e\u003cstrong\u003eAuthor contributions\u003c/strong\u003e\u003c/p\u003e\n\u003cp\u003eDr. Koyama composed and revised the manuscript and participated in the diagnosis and treatment of the patient. Drs. Akutsu and Kawamura participated in the diagnosis and treatment of the patient. Dr. Kawamura contributed to manuscript revision. All authors read and approved the final manuscript.\u003c/p\u003e"},{"header":"References","content":"\u003col\u003e\n\u003cli\u003eShoja MM, Tubbs RS, Oakes WJ (2013) Embryology and pathophysiology of the Chiari I and II malformations. In: Tubbs RS, Oakes WJ (eds) The Chiari Malformations. Springer, New York, pp 13\u0026ndash;54\u003c/li\u003e\n\u003cli\u003ePoretti A, Ashmawy R, Garzon-Muvdi T, Jallo GI, Huisman TA, Raybaud C (2016) Chiari Type 1 Deformity in Children: Pathogenetic, Clinical, Neuroimaging, and Management Aspects. Neuropediatrics 47:293-307. doi: 10.1055/s-0036-1584563\u003c/li\u003e\n\u003cli\u003ePayner TD, Prenger E, Berger TS, Crone KR (1994) Acquired Chiari malformations: incidence, diagnosis, and management. Neurosurgery 34:429\u0026ndash;434\u003c/li\u003e\n\u003cli\u003eSahuquillo J, Moncho D, Ferr\u0026eacute; A, L\u0026oacute;pez-Bermeo D, Sahuquillo-Muxi A, Poca MA (2023) A Critical Update of the Classification of Chiari and Chiari-like Malformations. J Clin Med 12:4626. doi: 10.3390/jcm12144626\u003c/li\u003e\n\u003cli\u003eAtkinson JL, Weinshenker BG, Miller GM, Piepgras DG, Mokri B (1998) Acquired Chiari I malformation secondary to spontaneous spinal cerebrospinal fluid leakage and chronic intracranial hypotension syndrome in seven cases. J Neurosurg 88:237\u0026ndash;242\u003c/li\u003e\n\u003cli\u003ePeleggi AF, Lovely TJ (2012) Treatment of delayed Chiari malformation and syringomyelia after lumboperitoneal shunt placement: Case report and treatment recommendations. Surg Neurol Int 3:101. doi: 10.4103/2152-7806.100188\u003c/li\u003e\n\u003cli\u003eSathi S, Stieg PE (1995) \u0026ldquo;Acquired\u0026rdquo; Chiari I malformation after multiple lumbar punctures: Case report. 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Brain Spine 5:104312. doi: 10.1016/j.bas.2025.104312\u003c/li\u003e\n\u003cli\u003eWeinberg JS, Rhines LD, Cohen ZR, Langford L, Levin VA (2003) Posterior fossa decompression for life-threatening tonsillar herniation in patients with gliomatosis cerebri: report of three cases. Neurosurgery 52:216-223\u003c/li\u003e\n\u003cli\u003eMuzumdar D, Ventureyra EC (2006) Tonsillar herniation and cervical syringomyelia in association with posterior fossa tumors in children: a case-based update. Childs Nerv Syst 22:454-459. doi: 10.1007/s00381-005-0027-x\u003c/li\u003e\n\u003cli\u003eRaja AI, Adada B (2007) Immediate resolution of tonsillar herniation and severe cervicothoracic syringomyelia after third ventriculostomy for hydrocephalus caused by a brainstem tumor. Case report. 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Childs Nerv Syst 36:235-240. doi: 10.1007/s00381-019-04437-0\u003c/li\u003e\n\u003cli\u003eD\u0026rsquo;Amico A, Giammalva GR, Furlanis GM, Emanuelli E, Maugeri R, Baro V, Denaro L (2023) Acquired Chiari type I malformation: a late and misunderstood supratentorial over-drainage complication. Childs Nerv Syst 39:343-351. doi: 10.1007/s00381-022-05775-2\u003c/li\u003e\n\u003cli\u003eNishikawa M, Sakamoto H, Hakuba A, Nakanishi N, Inoue Y (1997) Pathogenesis of Chiari malformation: a morphometric study of the posterior cranial fossa. J Neurosurg 86:40-47. doi: 10.3171/jns.1997.86.1.0040\u003c/li\u003e\n\u003cli\u003eKimura Y, Seichi A, Gomi A, Kojima M, Inoue H, Kimura A (2012) Acquired Chiari malformation secondary to atlantoaxial vertical subluxation in a patient with rheumatoid arthritis combined with atlanto-occipital assimilation. Neurol Med Chir (Tokyo) 52:683-686. doi: 10.2176/nmc.52.683\u003c/li\u003e\n\u003cli\u003eChen Q, Baba H, Kamitani K, Furusawa N, Imura S (1994) Postoperative bone re-growth in lumbar spinal stenosis. A multivariate analysis of 48 patients. Spine (Phila Pa 1976) 19:2144-2149\u003c/li\u003e\n\u003c/ol\u003e"}],"fulltextSource":"","fullText":"","funders":[],"hasAdminPriorityOnWorkflow":false,"hasManuscriptDocX":true,"hasOptedInToPreprint":true,"hasPassedJournalQc":"","hasAnyPriority":false,"hideJournal":false,"highlight":"","institution":"","isAcceptedByJournal":true,"isAuthorSuppliedPdf":false,"isDeskRejected":"","isHiddenFromSearch":false,"isInQc":false,"isInWorkflow":false,"isPdf":false,"isPdfUpToDate":true,"isWithdrawnOrRetracted":false,"journal":{"display":true,"email":"[email protected]","identity":"childs-nervous-system","isNatureJournal":false,"hasQc":true,"allowDirectSubmit":false,"externalIdentity":"cnsy","sideBox":"Learn more about [Child's Nervous System](http://link.springer.com/journal/381)","snPcode":"381","submissionUrl":"https://submission.nature.com/new-submission/381/3","title":"Child's Nervous System","twitterHandle":"","acdcEnabled":true,"dfaEnabled":true,"editorialSystem":"em","reportingPortfolio":"Springer Hybrid","inReviewEnabled":true,"inReviewRevisionsEnabled":false},"keywords":"secondary Chiari-like formation, acquired Chiari type 1 malformation, quadriplegia, foramen magnum decompression, occipitocervical fixation, grafted bone overgrowth","lastPublishedDoi":"10.21203/rs.3.rs-8685776/v1","lastPublishedDoiUrl":"https://doi.org/10.21203/rs.3.rs-8685776/v1","license":{"name":"CC BY 4.0","url":"https://creativecommons.org/licenses/by/4.0/"},"manuscriptAbstract":"\u003cp\u003eUnlike Chiari malformation type I, which results from a congenitally small posterior fossa, secondary Chiari-like formation (SCLF) is acquired and arises from a craniospinal pressure gradient or reduced posterior fossa volume. We report a rare case of SCLF caused by massive bone overgrowth after occipitocervical fixation (OCF) in a child. An 8-year-old boy with Klippel\u0026ndash;Feil syndrome and basilar invagination developed quadriparesis after OCF. His symptoms worsened following repeat OCF performed due to misdiagnosis. Four months later, the quadriparesis progressed further. Serial imaging demonstrated foramen magnum crowding caused by a hypertrophic graft, along with cerebellar tonsillar herniation. Foramen magnum decompression, performed 10 months after the second surgery, led to rapid motor improvement, allowing independent ambulation within 3 weeks. This case highlights massive graft overgrowth as a unique mechanical etiology of SCLF. Vigilance and careful comparison of serial craniovertebral junction imaging are crucial in pediatric patients with neurological deterioration after OCF.\u003c/p\u003e","manuscriptTitle":"Secondary Chiari-like Formation Caused by Grafted Bone Overgrowth Following Occipitocervical Fixation: A Rare Cause of Pediatric Quadriplegia","msid":"","msnumber":"","nonDraftVersions":[{"code":1,"date":"2026-02-25 09:11:55","doi":"10.21203/rs.3.rs-8685776/v1","editorialEvents":[{"type":"communityComments","content":0},{"type":"decision","content":"Revision requested","date":"2026-03-13T00:55:23+00:00","index":"","fulltext":""},{"type":"editorInvitedReview","content":"","date":"2026-03-12T14:42:16+00:00","index":"hide","fulltext":""},{"type":"reviewerAgreed","content":"275283105228913272096104789008739831964","date":"2026-03-03T12:50:39+00:00","index":"hide","fulltext":""},{"type":"reviewersInvited","content":"","date":"2026-02-19T23:52:32+00:00","index":"","fulltext":""},{"type":"editorAssigned","content":"","date":"2026-01-27T04:51:53+00:00","index":"","fulltext":""},{"type":"checksComplete","content":"","date":"2026-01-27T04:49:04+00:00","index":"","fulltext":""},{"type":"submitted","content":"Child's Nervous System","date":"2026-01-24T09:57:22+00:00","index":"","fulltext":""}],"status":"published","journal":{"display":true,"email":"[email protected]","identity":"childs-nervous-system","isNatureJournal":false,"hasQc":true,"allowDirectSubmit":false,"externalIdentity":"cnsy","sideBox":"Learn more about [Child's Nervous System](http://link.springer.com/journal/381)","snPcode":"381","submissionUrl":"https://submission.nature.com/new-submission/381/3","title":"Child's Nervous System","twitterHandle":"","acdcEnabled":true,"dfaEnabled":true,"editorialSystem":"em","reportingPortfolio":"Springer Hybrid","inReviewEnabled":true,"inReviewRevisionsEnabled":false}}],"origin":"","ownerIdentity":"8e718563-ed86-4243-b2ec-bda8a61dd1b8","owner":[],"postedDate":"February 25th, 2026","published":true,"recentEditorialEvents":[],"rejectedJournal":[],"revision":"","amendment":"","status":"published-in-journal","subjectAreas":[],"tags":[],"updatedAt":"2026-05-04T16:00:56+00:00","versionOfRecord":{"articleIdentity":"rs-8685776","link":"https://doi.org/10.1007/s00381-026-07272-2","journal":{"identity":"childs-nervous-system","isVorOnly":false,"title":"Child's Nervous System"},"publishedOn":"2026-04-27 15:58:21","publishedOnDateReadable":"April 27th, 2026"},"versionCreatedAt":"2026-02-25 09:11:55","video":"","vorDoi":"10.1007/s00381-026-07272-2","vorDoiUrl":"https://doi.org/10.1007/s00381-026-07272-2","workflowStages":[]},"version":"v1","identity":"rs-8685776","journalConfig":"researchsquare"},"__N_SSP":true},"page":"/article/[identity]/[[...version]]","query":{"redirect":"/article/rs-8685776","identity":"rs-8685776","version":["v1"]},"buildId":"XKTyCvWXoU3ODBz1xrDgd","isFallback":false,"isExperimentalCompile":false,"dynamicIds":[84888],"gssp":true,"scriptLoader":[]}