Third ventricle width on routine MRI is associated with processing speed in multiple sclerosis: a single-centre retrospective observational study

Abstract Background: Cognitive impairment is common in multiple sclerosis (MS), and slowed information processing is among its most frequent manifestations. Although brain volumetry can quantify atrophy, automated volumetric tools are not universally available in routine practice. We investigated whether third ventricle width (3VW), a simple linear measure obtainable on conventional MRI, is associated with standardised Symbol Digit Modalities Test performance. Methods: This single-centre retrospective observational study included 100 adults with MS assessed in routine care between 2021 and 2023. Current MRI was acquired within one month of cognitive testing. A neuroradiologist blinded to cognitive results measured 3VW three times at the foramen of Monro, and the mean value was analysed. The primary analysis examined the association between current 3VW and age-standardised SDMT performance (SDMT-SD) using Spearman correlation with false-discovery-rate correction. Post hoc multivariable linear-regression sensitivity analyses adjusted for demographic and clinical covariates. MRI measurements approximately 3, 5 and 10 years earlier and analyses of SDMT impairment were exploratory. Results: Twenty-five participants (25%) met the prespecified definition of SDMT impairment (SDMT-SD ≤ -1). Current 3VW was inversely associated with SDMT-SD (rho = -0.556, 95% CI -0.678 to -0.404; q = 2.28 × 10⁻⁸). The association persisted in a fully adjusted linear-regression sensitivity model including age, sex, education and EDSS (β = -0.232 SDMT-SD units per 1-mm greater 3VW, 95% CI -0.391 to -0.074; p = 0.0045). Excluding 14 records previously labelled as CIS produced similar results. Analyses of prior MRI showed associations in selected subgroups but were limited by non-random availability of earlier scans. Conclusions: Greater third ventricle width on routine MRI is associated with worse processing-speed performance in MS. Because it is rapid to obtain and widely accessible, 3VW may serve as a simple imaging marker that prompts cognitive assessment when volumetric tools are unavailable. It should not replace cognitive testing or volumetric MRI, and predictive thresholds require prospective external validation.