Imaging Cognitive Fatigue in Multiple Sclerosis: A Preliminary Quantification of Global Cerebral Blood Flow Using ASL Perfusion fMRI

Background:

Cognitive fatigue (CF) can be defined as an inability to maintain task performance throughout the duration of an attention-demanding task. Individuals with multiple sclerosis (MS) have been shown to be more susceptible to CF when compared to healthy controls (HCs); however, the neural correlates underlying this phenomenon are still under investigation. Arterial spin labeling (ASL) perfusion imaging provides a non-invasive method of objectively quantifying cerebral blood flow (CBF) during sustained attention tasks. To date, no study has yet evaluated CF in MS using this methodology.

Objectives:

To evaluate the neural correlates of CF in MS using ASL perfusion imaging during a sustained attention task.

Methods:

9 individuals with MS and 6 HCs completed a 20-min continuous psychomotor vigilance task (PVT) while undergoing ASL imaging. To evaluate CF, the task was divided into quintiles and performance was evaluated from the 1^st quintile to the last.  Mean reaction times (RTs) and number of lapses (RT > 500ms) were recorded. Whole brain CBF was evaluated throughout the duration of the PVT task as well as during task-free resting baselines both pre- and post- PVT.

Results:

Mean RTs on the PVT task significantly differed between groups, with the MS group demonstrating longer RTs across the task. In addition, the MS group produced significantly more lapses. Susceptibility to CF was noted for only the MS group with a significant difference observed between RTs at the 5th vs. 1st quintile. No global CBF differences were noted between groups nor between the PVT and Pre-/Post- resting baselines. Percent change in mean CBF values revealed HCs had a significantly larger decrease (-18.98%) in CBF during the 5^th quintile vs. the 1^st.

Conclusions:

As expected, the MS group displayed longer RTs on the task. The greater number of lapses for the MS group suggest an inability to process the stimuli within the 500ms window. Future studies should evaluate whether a longer RT window may be more appropriate. While HCs’s performance remained consistent, the MS group were susceptible to CF as evidenced by longer RTs across the quintiles. This susceptibility was not paralleled by CBF changes. HCs showed reduced CBF later in the task suggesting they may habituate to the task while still maintaining comparable performance. Future ROI analyses will evaluate quantitative regional CBF changes in areas comprising the fronto-parietal network believed to mediate sustained attention.