Retroflexed dens in paediatric Chiari 1 patients and implications: Single centre retrospective study

Introduction Chiari 1 malformation and hind brain hernia can be associated with skull base and craniocervical anomalies. One of the more recently associated anomalies is a retroverted or retroflexed odontoid process or dens. We conducted a retrospective study of our operated symptomatic and conservatively managed asymptomatic Chiari cohort to assess the impact of dens retroflexion on rate of revision or cerebrospinal fluid diversion following primary foramen magnum decompression (FMD). Methods We undertook a retrospective study of all foramen magnum decompression (FMD) cases for Chiari type 1 malformation performed over a 15-year period in a single tertiary paediatric neurosurgical unit. For comparison, non-operated asymptomatic Chiari cases were considered as reference cohort. Information gathered included: demographics, age, sex, length of cerebellar tonsils below McRae’s line, pB-C2 distance (a line drawn perpendicular to one drawn between the basion and the posterior aspect of the C2 body), angle of retroflexion (angle formed between a line drawn through the odontoid synchondrosis and its intersection with a line drawn from the tip of the odontoid process) and angle of retroversion (angle formed between the line drawn from the base of C2 and its intersection with a line drawn from the tip of the odontoid process). Grade of retroflexion was measured using pre-operative mid-sagittal MR images and classified as grade 0 (> 90°), grade 1 (85°–89°); grade 2 (80°–84°) and grade 3 (