Chronic active lesions preferentially localize in watershed territories in multiple sclerosis.

Objective: Paramagnetic rim lesions (PRLs) are a biomarker of chronic active lesions (CALs), and an important driver of neurological disability in multiple sclerosis (MS). The reason subtending some acute lesions evolvement into CALs is not known. Here we ask whether a relatively lower oxygen content is linked to CALs.

Methods: In this prospective cross-sectional study, 64 people with multiple sclerosis (PwMS), clinically isolated syndrome and radiologically isolated syndrome underwent a 7.0 Tesla (7 T) brain magnetic resonance imaging (MRI). The scanning protocol included a T₂-w fluid-attenuated inversion recovery (FLAIR), and a single echo gradient echo from which susceptibility-weighted imaging (SWI) was derived. WM lesions were identified on the T₂-w-FLAIR whilst PRLs were identified on the SWI sequence. T₂-lesions were classified as PRLs and rimless lesions (PRLs-). We registered a universal vascular atlas to each subject's T₂-w-FLAIR and classified each T₂-lesions according to its location into watershed- (ws), non-watershed- (nws), and mixed-lesion (m). Ws-lesions were defined as lesions that were fully located in a region between the territories of two major arteries.

Results: Out of 1,975 T₂-lesions, 88 (4.5%) were PRLs. Ws-regions had a higher number (p = 0.005) and proportion (p < 0.001) of PRLs- compared to nws-regions. Ws-PRL- were larger compared to nws-ones (p = 0.009). The number (p = 0.043) and proportion (p < 0.001) of PRLs was higher in ws-regions compared to nws-ones. Ws-PRLs were not significantly larger than nws-ones (p = 0.195).

Interpretation: We propose the novel concept of a link between arterial vascularization and chronic activity in MS by demonstrating a preferential localization of CALs in ws-territories.