The relationship between cognitive impairment, cognitive fatigue, and visual evoked potential latency in people with multiple sclerosis.

BACKGROUND: Fatigue in people with multiple sclerosis (PwMS) can impact physical, cognitive, and psychosocial domains of daily life. The experience of fatigue in PwMS is thought to originate from the central nervous system, particularly for the domain of cognitive fatigue. Here, we tested the hypothesis that fatigue scores in PwMS would be significantly associated with an index of neural activity - the latency of the P100 of the visual evoked potential (VEP) - in line with the notion of a centralized origin of fatigue. We predicted that prolonged VEP latency would be associated with greater fatigue, and that this relationship would be the most pronounced within the domain of cognitive fatigue. METHODS: PwMS (n=249) completed the Modified Fatigue Impact Scale (Global, Physical, Cognitive, and Psychosocial scales of the MFIS) and Fatigue Severity Scale. VEP latency was obtained using an alternating checkerboard paradigm. We also examined the influence of depression (Beck Depression Inventory, second edition, BDI-II) and cognitive functioning (NeuroTrax testing battery) on the VEP/fatigue relationship. RESULTS: Surprisingly, we observed that earlier (not later) VEP latency was significantly associated with higher MFIS Cognitive score. The negative relationship between VEP latency and cognitive fatigue was evident in PwMS that had poor cognitive performance as measured by a latent variable that reflected attention, executive function, information processing speed, and motor skills; but a significant relationship was not observed in PwMS that exhibited good performance on this measure. CONCLUSIONS: These findings can be interpreted within a metacognitive framework - greater fatigue may be perceived when neural performance and the level of mental effort expended does not translate to efficient cognitive performance. Cognitive fatigue may be particularly salient in PwMS when neural resources are unable to compensate for cognitive difficulties. The mismatch between the expectation of what ought to occur and what does occur during cognitive performance may be a key feature of the experience of cognitive fatigue for some PwMS.

Longitudinal assessment of the relationship between visual evoked potentials and cognitive performance in multiple sclerosis.

OBJECTIVE: Visual evoked potentials (VEPs) can provide insight into disease activity in people with multiple sclerosis (PwMS). However, few studies have tracked concurrent changes in VEPs and cognitive functioning over time in MS. To address this, we examined the longitudinal relationship between VEP and cognitive performance in PwMS over a two-year period. METHODS: At baseline (T1) and follow-up (T2, 2.14 years after baseline, on average), P100 peak latency and inter-ocular latency (IOL) between eyes were calculated from the VEP elicited for checkerboard pattern-reversal stimuli. Cognitive performance was assessed for seven different domains (NeuroTrax battery). The potential for VEP variables to predict the T1-to-T2 change in cognitive performance was assessed in a series of multiple linear regression models. RESULTS: Baseline IOL and VEP latency were significantly associated with T1-to-T2 change in information processing speed. Post-hoc analyses indicated that PwMS that had both prolonged VEP latency and elevated IOL at baseline tended to exhibit greater information processing speed decline. Increase in VEP latency from T1-to-T2 was also associated with decline in psychomotor function over time. CONCLUSIONS: These findings provide evidence that VEP measures can serve as valuable prognostic indicators of longitudinal cognitive change in PwMS. SIGNIFICANCE: Visual system neurophysiology corresponds with changes in speeded cognitive performance in MS.

Prolonged visual evoked potential latency predicts longitudinal worsening of fatigue in people with multiple sclerosis.

BACKGROUND: Fatigue is a common problem experienced by people with multiple sclerosis (PwMS) and can impact physical, cognitive, and psychosocial aspects of daily living and quality of life. The tracking of meaningful longitudinal change in subjective fatigue that occurs as a result of MS activity may be enhanced by incorporating objective neurophysiological measures into longitudinal assessment. To examine this possibility, we examined the longitudinal relationship between visual evoked potential (VEP) measures and a variety of fatigue measures over an approximately two-year period in PwMS. METHODS: VEP measures were obtained using a checkerboard pattern-reversal paradigm. Fatigue was assessed with the Modified Fatigue Impact Scale (MFIS Global, Physical, Cognitive, and Psychosocial subscales) and the Fatigue Severity Scale (FSS) questionnaires. Multiple linear regression analyses were conducted in which the change in each fatigue scale score from baseline to follow-up (T1-to-T2) served as the outcome variables for separate models. Predictor variables included the peak latency of the P100 component of the VEP (maximum peak among the two eyes) and the inter-ocular latency (IOL) at T1, the T1-to-T2 change score for maximum VEP latency and IOL, and the fatigue score at T1 that corresponded to each outcome measure. RESULTS: Prolonged baseline VEP latency was a significant predictor of the T1-to-T2 increase in MFIS Global score, and increased VEP latency from baseline to follow-up was significantly associated with MFIS Cognitive score over the same time period. Furthermore, VEP latency measures in these two models were better predictors of changes in fatigue than baseline fatigue scores were, based on the magnitude of the standardized beta coefficients. Subsequent post-hoc analyses revealed that the relationship between change in VEP latency and change in MFIS Cognitive score was evident primarily for PwMS that had elevated MFIS Cognitive score at baseline. CONCLUSION: The present study provides novel evidence that prolonged VEP latency is predictive of worsening of global and cognitive fatigue in PwMS. VEP latency measures may therefore provide clinical utility for monitoring changes in fatigue in PwMS, when used in conjunction with other clinical tools.