Failed Foramen Magnum Decompression in Chiari I Malformation Is Associated With Failure to Restore Normal Intracranial Compliance: An Observational Cohort Study

BACKGROUND: The standard treatment for symptomatic Chiari malformation (CM) I is foramen magnum decompression (FMD) to facilitate cerebral spinal fluid flow through the foramen magnum, improve intracranial compliance, and alleviate symptoms (commonly headache). This procedure has a variable success rate, with a significant proportion of patients having persistent symptoms after surgery. OBJECTIVE: To investigate intracranial pressure (ICP) hydrodynamics in symptomatic surgery-naive patients with CM I and symptomatic patients who have had prior FMD. METHODS: We undertook a retrospective, observational cohort study, extracting data from our departmental ICP database. Patients with untreated ("Virgin") Chiari malformations (VCM), patients with previous "failed" FMD (ie, with persistent classical Chiari symptoms) (fFMD) and a normal control group, all with existing continuous ICP monitoring date were included. Median ICP (mICP) and median pulse amplitude (mPA) were compared between the groups. RESULTS: A total of 33 CM patients (22 VCM and 11 fFMD) and 42 normal controls were included for analysis. mICP did not differ significantly between the normal control, VCM, and fFMD groups. mPA in the VCM and fFMD groups was significantly higher than the control group (P < .01 and P < .05, respectively). CONCLUSION: In this cohort, patients with persistent symptoms after FMD have persistently impaired intracranial compliance, similar to patients who have not undergone surgery at all when compared with a control group. The reasons for this are not clear, and further research is required to establish the causation and optimum management for failed FMD.