CLINICAL ARTICLE C HIARI malformations are rare hindbrain anomalies with reported prevalence rates between 0.1% and 1%. 1 To date, no consensus exists regarding the etiology or pathophysiological mechanism of Chiari mal- formation, 2 although several theories have been proposed for its pathogenesis, including the instability theory. 3–7 The most common subtype, Chiari malformation type I (CMI), is often identified during adulthood, and surgical treat - ment consists of foramen magnum decompression (FMD) with or without duraplasty to restore normal CSF flow. 8 ABBREVIATIONS ADI = atlantodental interval; BI = basilar invagination; CCOS = Chicago Chiari Outcome Scale; CMI = Chiari malformation type I; FMD = foramen mag- num decompression; NSS = Neurological Scoring System. SUBMITTED April 9, 2020. ACCEPTED June 12, 2020. INCLUDE WHEN CITING Published online October 16, 2020; DOI: 10.3171/2020.6.SPINE20544. * M.L.L. and T.V.B. contributed equally to this work. Prognostic significance of C1–C2 facet malalignment after surgical decompression in adult Chiari malformation type I: a pilot study based on the Chicago Chiari Outcome Scale *Michael Lumintang Loe, MD, 1 Tito Vivas-Buitrago, MD, 2 Ricardo A. Domingo, MD, 2 Johan Heemskerk, MD, 2 Shashwat Tripathi, BSA, 2 Bernard R. Bendok, MD, 3 Mohamad Bydon, MD, 4 Alfredo Quinones-Hinojosa, MD, 2 and Kingsley Abode-Iyamah, MD 2 1 Department of Neurosurgery, Faculty of Medicine, Universitas Sumatera Utara/H. Adam Malik General Hospital, Medan, Indonesia; 2 Department of Neurosurgery, Mayo Clinic, Jacksonville, Florida; 3 Department of Neurosurgery, Mayo Clinic, Phoenix, Arizona; and 4 Department of Neurosurgery, Mayo Clinic, Rochester, Minnesota OBJECTIVE The authors assessed the prognostic significance of various clinical and radiographic characteristics, in- cluding C1–C2 facet malalignment, in terms of surgical outcomes after foramen magnum decompression of adult Chiari malformation type I. METHODS The electronic medical records of 273 symptomatic patients with Chiari malformation type I who were treat- ed with foramen magnum decompression, C1 laminectomy, and duraplasty at Mayo Clinic were retrospectively reviewed. Preoperative and postoperative Neurological Scoring System scores were compared using the Friedman test. Bivariate analysis was conducted to identify the preoperative variables that correlated with the patient Chicago Chiari Outcome Scale (CCOS) scores. Multiple linear regression analysis was subsequently performed using the variables with p < 0.05 on the bivariate analysis to check for independent associations with the outcome measures. Statistical software SPSS version 25.0 was used for the data analysis. Significance was defined as p < 0.05 for all analyses. RESULTS Fifty-two adult patients with preoperative clinical and radiological data and a minimum follow-up of 12 months were included. Motor deficits, syrinx, and C1–C2 facet malalignment were found to have significant negative associa- tions with the CCOS score at the 1- to 3-month follow-up (p < 0.05), while at the 9- to 12-month follow-up only swallow- ing function and C1–C2 facet malalignment were significantly associated with the CCOS score (p < 0.05). Multivariate analysis showed that syrinx presence and C1–C2 facet malalignment were independently associated with the CCOS score at the 1- to 3-month follow-up. Swallowing function and C1–C2 facet malalignment were found to be independently associated with the CCOS score at the 9- to 12-month follow-up. CONCLUSIONS The observed results in this pilot study suggest a significant negative correlation between C1–C2 facet malalignment and clinical outcomes evaluated by the CCOS score at 1–3 months and 9–12 months postoperatively. Pro- spective studies are needed to further validate the prognostic value of C1–C2 facet malalignment and the potential role of atlantoaxial fixation as part of the treatment. https://thejns.org/doi/abs/10.3171/2020.6.SPINE20544 KEYWORDS facet malalignment; outcomes; Chiari malformation; foramen magnum decompression; syringomyelia; Chicago Chiari Outcome Scale; cervical J Neurosurg Spine October 16, 2020 1 ©AANS 2020, except where prohibited by US copyright law Brought to you by Mayo Clinic Library | Unauthenticated | Downloaded 10/19/20 01:40 PM UTC