Multiple sclerosis (MS) is a chronic autoimmune inflammatory disease of the central nervous system (CNS) whereby abnormal autoimmune responses cause damage to myelin, the lipid-rich layer that surrounds and insulates axons. This results in interruptions in communication within the CNS and between the CNS and peripheral nervous system (PNS). This dysfunction contributes to a variety of symptoms that negatively impact the lives of individuals with MS. Sleep disturbances and memory difficulties are two common challenges faced by MS patients, but are not comprehensively understood within the MS literature. Additionally, despite general consensus with regard to the important role that sleep plays in memory function, studies investigating the links between sleep disturbance and memory in MS are sparse. As such, the purpose of this dissertation was to determine whether sleep disturbance helps to explain differential memory functioning in individuals with MS, both cross-sectionally and over time.
A sample of 165 early MS patients participated in cognitive measures, gait assessments, sensorimotor assessments, and self-report questionnaires once at baseline, and again 3 years after their initial assessment. Magnetic Resonance Imaging (MRI) data were collected at baseline. The primary predictor variable for the present study was sleep disturbance, as measured by two validated self-report measures of sleep functioning, the Insomnia Severity Index (ISI) and Pittsburgh Sleep Quality Index (PSQI). This study’s primary outcome was memory function, which was assessed by the CANTAB Paired Associate Learning (CANTAB PAL), Brief Visuospatial Memory Test, Revised (BVMT-R), Selective Reminding Test (SRT), and Verbal Paired Associate Learning (VPAL). Additional predictors included mood, disease burden, estimated premorbid intelligence, and demographic variables (age, sex, BMI). As hypothesized, results revealed that changes in sleep significantly predicted changes in memory over time. Patients with stable sleep and worsened sleep demonstrated an average decline in memory z-score from baseline to follow up, whereas patients whose sleep improved demonstrated an average improvement in memory z-score.
Cross-sectionally, the presence of sleep disturbance significantly predicted worse memory performance when the ISI was used a measure of sleep disturbance, but not when the PSQI was used as a measure of sleep disturbance. Taken together, results highlight the importance of acknowledging sleep disturbance as an important predictor of memory function in individuals with early MS, paving the way for highly needed efforts toward prevention and intervention. However, findings should be extended to both objective and subjective sleep measures beyond the ISI.
