Arnold-Chiari malformation is associated with increased likelihood of a procedure in idiopathic intracranial hypertension

Jiawen Ma^1*Philip Nguyen¹Jade Lee¹Chaturica Athukorala¹Kate Reid²

• ¹Department of Medical Imaging, The Canberra Hospital, Canberra, Australia
• ²ANU College of Health & Medicine, Australian National University, Canberra, Australia

Background: Some 20% of patients with idiopathic intracranial hypertension require a procedure. A neuro-radiologic marker identifying those more likely to require a procedure would assist clinical management and outcomes. Here the authors explore whether cerebellar tonsillar position is such a marker. Methods: This is a retrospective, single-center, cohort study of 180 consecutive patients with IIH in Canberra, Australia. Patient outcomes were a procedural intervention versus medical therapy alone. Cerebellar tonsillar position was measured relative to the foramen magnum to the nearest millimeter, as defined by the McRae line. The tonsillar position was classified as at/above the line, or lowered if below. The patient outcome was assessed against tonsillar position by logistic regression modelling. Results: The tonsils were at/above the McRae line in 36% (65/180) of patients, and lowered in 64% (115/180). In 7%, lowering amounted to Arnold Chiari malformation (13/180). The position of the cerebellar tonsils across the whole patient cohort was only mildly correlated with the likelihood of a procedural outcome (p = 0.04). However, true ACM was strongly associated with procedural intervention at 46% (6/13), compared with 18% (30/167) in those without ACM, with a relative risk of 2.57 and absolute risk difference of 28% (OR 5.15, 95% CI 1.45 – 18.52, p = 0.01). Additionally, lowering occurred at a prevalence far greater than seen in the general adult population. Conclusion: The finding of Arnold Chiari malformation in patients with IIH is associated with increased likelihood of requiring a procedure.