Cognitive impairment and magnetic resonance imaging correlates in primary progressive multiple sclerosis

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Cognitive impairment has been increasingly recognized as a frequent cause of disability in multiple sclerosis (MS) in all stages of the disease, with a profound impact on activities of daily living and quality of life.1 It is reported to be present in 15‐65% of patients with MS,3 depending on different cognitive impairment definitions and disease subtypes.1
Due to the relative rarity of primary progressive multiple sclerosis (PPMS),7 the impact of cognitive dysfunction on this MS subtype has not yet been fully investigated. The few studies assessing cognitive impairment in PPMS estimate a frequency of cognitive dysfunction ranging from 7% to 50% 2 and affecting several domains, including attention and working memory, verbal learning, spatial memory, spatial reasoning, and verbal fluency.4 Most of these studies rely on small single‐center samples, which may account for the widely varying frequency of cognitive impairment and equivocal results when comparing PPMS with other MS subtypes. In addition, data regarding magnetic resonance imaging (MRI) correlates of cognitive dysfunction in PPMS are scarce. In relapse‐onset MS, it is widely recognized that MRI measures of white matter (WM) and gray matter (GM) atrophy are correlated with cognitive dysfunction, generally with GM volume indices accounting for most of the variance.13 However, patients with PPMS have MRI characteristics that differ from those with relapse‐onset MS, with relatively low burden and activity of MRI‐visible lesions on T2‐weighted and gadolinium‐enhanced scans of the brain,14 and therefore, these results cannot be directly extrapolated. A few studies have investigated the relationship between MRI findings and cognitive impairment in PPMS. Ukkonen and colleagues15 found cognitive deficits in patients with PPMS to be correlated with T1‐ and T2‐lesion load, but not with global brain atrophy, while in a more recent study by Tur et al., GM damage as measured by magnetization transfer ratio was the main correlate of overall cognitive dysfunction.16 The influence of individualized subcortical GM structures and regional cortical atrophy on PPMS‐related cognitive impairment, however, remains to be determined.
The purpose of our study was to characterize cognitive impairment in patients with PPMS and to correlate the pattern of cognitive deficits with brain MRI volumetric data.

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