Bicaudate ratio as a magnetic resonance imaging marker of brain atrophy in multiple sclerosis

Context: Brain atrophy has emerged as a useful surrogate marker of disease involvement in multiple sclerosis (MS). The relationship between whole-brain or regional atrophy and cognitive dysfunction is poorly understood. Objectives: To determine whether the bicaudate ratio (BCR)-the minimum intercaudate distance divided by brain width along the same line-is increased in MS and to compare the ability of the BCR, whole-brain atrophy, and other magnetic resonance imaging markers to predict cognitive dysfunction. Design: Case-control study. Setting: University-affiliated clinic. Participants: Sixty patients with MS and 50 age- and sex-matched control subjects. Main Outcome Measures: Bicaudate ratio, whole-brain atrophy, T2 lesion load, T1 ("black hole") lesion load, and caudate volume were measured quantitatively using fluid-attenuated inversion recovery, T1-weighted, and gradient-echo magnetic resonance imaging scans. Symbol Digit Modalities Test was used to assess cognitive function. Results: The BCR (mean [SD]) was higher in patients with MS (0.11 [0.03]) than in controls (0.09 [0.02]) (P<.001), suggesting subcortical atrophy in MS. The BCR was related to total T2 (r=0.56, P<001) and T1 (r=0.40, P<002) lesion volumes, but not caudate volume in patients with MS. Regression modeling selected BCR (P<.05), but not whole-brain atrophy, T1 or T2 lesion volume, or caudate volume as predictive of Symbol Digit Modalities Test score in patients with MS. Conclusions: The BCR is increased in MS and is more closely associated with cognitive dysfunction than are other magnetic resonance imaging surrogate markers including whole-brain atrophy. Increased BCR is best explained by frontal horn ventricular enlargement due to atrophy of deep frontal subcortical white matter. This highlights the close relationship between subcortical atrophy and cognitive impairment in patients with MS.