The influence of information processing speed on memory processes in patients with relapsing-remitting and primary progressive multiple sclerosis.

Slowed information processing speed (IPS) is the primary cognitive impairment in people with multiple sclerosis (pwMS). Although it is known to affect memory processes, the relationships between verbal learning (VL), verbal memory (VM) and IPS in different phenotypes of multiple sclerosis (MS) remain underexplored. This bicentric study aimed to investigate in detail IPS, VL, and VM as well as the relationships between these cognitive domains in a large sample of 92 pwMS (61 with early relapsing-remitting MS (pwRRMS), 31 with primary-progressive MS (pwPPMS)) and 61 age-matched healthy controls (HC). Compared with HC, pwPPMS performed significantly worse on all cognitive measures, whereas pwRRMS performed worse than HC only on IPS and VL. Irrespective of phenotype, IPS impairment emerged as the core cognitive deficit in MS, affecting 25% of the RRMS group and 45% of the PPMS group. PwRRMS outperformed pwPPMS in all domains, with the most pronounced differences observed in VL. Isolated VL or VM impairments were rare, as most pwMS with these deficits also had concurrent IPS impairment. PwPPMS had significantly higher rates of IPS and VL impairment than pwRRMS, but not in VM. Regression analyses showed that IPS significantly predicted both VL and VM performance in both MS groups. In HC, IPS was only associated with VL performance, hinting at different cognitive mechanisms underlying the performance in this domain. This study provides evidence that pwPPMS experience more pronounced cognitive changes in IPS and VL compared to pwRRMS. It also highlights the pervasive impact of IPS on VL and VM performance in both subtypes, underscoring the importance of IPS-focused interventions in the management of MS-related cognitive dysfunction.