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

## TL;DR

This study finds that Arnold–Chiari malformation is linked to a higher chance of needing medical procedures in patients with idiopathic intracranial hypertension.

## Contribution

The study identifies Arnold–Chiari malformation as a potential marker for predicting procedural need in idiopathic intracranial hypertension.

## Key findings

- Arnold–Chiari malformation was strongly associated with a 46% likelihood of requiring a procedure.
- Cerebellar tonsillar lowering was common in IIH patients and may act as a 'soft sign' of the disease.
- Observer agreement on tonsillar position measurements was high, indicating reliable assessment.

## Abstract

Approximately 18% of patients with idiopathic intracranial hypertension (IIH) prove medically refractory, and eventually require a procedure to manage their condition. An additional 2% have fulminant disease and require an immediate procedure to preserve vision. Identifying a neuroradiologic marker to stratify IIH patients more likely to require a procedure would assist clinical management and outcomes. Here, the authors explore whether cerebellar tonsillar position is such a marker.

This was a retrospective, single-center cohort study of 180 consecutive patients with IIH in Canberra, Australia. Patient outcomes were classified as 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 or above the line, or lowered if below. Subsets of lowering were defined as “descent” (<3mm below), “ectopia” (3–5 mm below), or “Arnold–Chiari malformation” (>5 mm below). Two observers independently assessed the patients’ initial neuroimaging, and a random sample of 20 was also assessed by a more senior radiologist. Measurement precision was assessed using intraclass correlation coefficients, and patient outcome was analyzed against tonsillar position using univariable penalized logistic regression modeling.

91% of patients were female. The tonsils were at or 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). Among those who underwent a procedure, the average tonsillar position was 1.94mm below the foramen magnum, whereas in those not requiring a procedure, it was 0.80mm. Across the whole cohort, the average tonsillar position was 1.0mm below the McRae line. The position of the cerebellar tonsils across the whole cohort was only mildly correlated with the likelihood of a procedural outcome (p = 0.04). However, true Arnold–Chiari malformation was strongly associated with procedural intervention at 46% (6/13), compared with 18% (30/167) in those without the malformation, with a relative risk of 2.57 and risk difference of 28% (odds ratio 5.15, 95% CI 1.45–18.52, p = 0.01). There was high concordance between the two observers’ measurements (0.89, 95% CI 0.81–0.93) and with the measurements obtained by the senior radiologist (0.97, 95% CI 0.93–0.99).

The presence of Arnold–Chiari malformation in patients with idiopathic intracranial hypertension is associated with an increased likelihood of requiring a procedure. Moreover, independent of procedural outcome, cerebellar tonsillar lowering occurs in IIH patients so frequently that it should be further investigated as a potential neuroradiologic “soft sign” of the disease.