Cognitive impairment affects 40-65% of people with MS. Why? In the past year, a number of studies have helped us build a better picture of the precise nature of cognitive problems that may affect multiple sclerosis sufferers:
- poorer performance on executive function tasks is fully explained by slower processing speed (which is presumably a function of the degradation in white matter characteristic of MS)
- slowing in processing speed is associated with weaker connections between the executive area and the brain regions involved in carrying out cognitive tasks
- cognitive reserve helps counter the decline in memory and cognitive efficiency
- brain reserve (greater brain volume, ie less shrinkage) helps counter the decline in cognitive efficiency
- working memory capacity explains the link between cognitive reserve and long-term memory
- subjective cognitive fatigue is linked to the time spent on the task, not on its difficulty
- mnemonic training helps protect against cognitive decline, but appears to be less helpful in those with slow processing speed.
What all this implies is that a multi-pronged approach is called for, involving:
- working memory training
- training in effective memory strategies
- practice in breaking down cognitive tasks into more manageable chunks of time
- practice in framing tasks to accommodate slower processing speed
- physical and mental activities that encourage neurogenesis (growing more neurons) and synaptogenesis (growing more connections).
Here's some more detail on those studies:
Slow processing speed accounts for executive deficits in MS
A study of 50 patients with MS and 28 healthy controls found no differences in performance on executive function tasks when differences in processing speed were controlled for. In other words, although MS patients performed more poorly than controls on these tasks, the difference was fully accounted for by the differences in processing speed. There were no differences in performance when there was no processing speed component to the task. Similarly, MS patients with a greater degree of brain atrophy performed more poorly than those with less atrophy, but again, this only occurred when there was a processing speed aspect to the task, and was fully accounted for by processing speed differences.
 . Does slowed processing speed account for executive deficits in multiple sclerosis? Evidence from neuropsychological performance and structural neuroimaging. Rehabilitation Psychology [Internet]. 2014 ;59(4):422 - 428. Available from: http://doi.apa.org/getdoi.cfm?doi=10.1037/a0037517
Functional connectivity factor in cognitive decline in MS
A brain imaging study involving 29 participants with relapsing-remitting MS and 23 age- and sex- matched healthy controls found that, as expected, those with MS were much slower on a processing speed task, although they were as accurate as the controls. This slowing was associated with weaker functional connections between the dorsolateral prefrontal cortex (the executive area) and the regions responsible for carrying out the task. It's thought that this is probably due to decreased white matter (white matter degradation is symptomatic of MS).
 Asynchrony in Executive Networks Predicts Cognitive Slowing in Multiple Sclerosis. Neuropsychology. 2015 .
Brain and cognitive reserve protect against cognitive decline in MS
A study compared memory, cognitive efficiency, vocabulary, and brain volume in 40 patients with MS, at baseline and 4.5 years later. After controlling for disease progression, they found that those with better vocabulary (a proxy for cognitive reserve) experienced less decline in memory and cognitive efficiency, and those with less brain atrophy over the period showed less decline in cognitive efficiency.
Cognitive efficiency is a somewhat fuzzy concept, but essentially has to do with how much time and effort you need to acquire new knowledge; in this study, it was assessed using the Symbol Digit Modalities Test and Paced Auditory Serial Addition Task, two tests commonly used to detect cognitive impairment in MS patients.
 . Brain reserve and cognitive reserve protect against cognitive decline over 4.5 years in MS. Neurology [Internet]. 2014 ;82(20):1776 - 1783. Available from: http://www.neurology.org/cgi/doi/10.1212/WNL.0000000000000433
Working memory capacity accounts for link between cognitive reserve & better memory
A study involving 70 patients with MS has found that working memory capacity explained the relationship between cognitive reserve and long-term memory, suggesting that interventions targeted at working memory may help protect against decline in long-term memory.
 . Working Memory Mediates the Relationship between Intellectual Enrichment and Long-Term Memory in Multiple Sclerosis: An Exploratory Analysis of Cognitive Reserve. Journal of the International Neuropsychological Society [Internet]. 2014 ;20(08):868 - 872. Available from: http://www.journals.cambridge.org/abstract_S1355617714000630
Cognitive fatigue linked to time on task, not difficulty
A study investigating cognitive fatigue in 32 individuals with MS and 24 controls has found that subjective and objective fatigue were independent of one another, and that subjective cognitive fatigue increased as time on task increased. This increase in cognitive fatigue was greater in the MS group. No relationship was found between cognitive fatigue and cognitive load. Fatigue was greater for the processing speed task than the working memory task.
In other words, the length of time spent is far more important than the difficulty of the task.
 . Subjective cognitive fatigue in multiple sclerosis depends on task length. Frontiers in Neurology. 2014 ;5:214.
Story mnemonic training helps some
A series of small studies have found cognitive benefits for MS patients from a 10-session training program designed to build their memory skills using a modified story mnemonic. The MEMREHAB Trial involved 85 patients with MS, of whom 45 received the training. In the most recent analyses of the data, the benefits were found to be maintained six months later, but unfortunately, it appears that those with processing speed deficits gain less benefit from the training.
The training consists of four 45-minute sessions focused on building imagery skills, in which participants were given a story with highly visualizable scenes and given facilitated practice in using visualization to help them remember the story. In the next four sessions, they were given lists of words and instructed in how to build a memorable story from these words, that they could visualize. The sessions employed increasingly unrelated word lists. In the final two sessions, participants were taught how to apply the technique in real-world situations.
 . The influence of cognitive dysfunction on benefit from learning and memory rehabilitation in MS: A sub-analysis of the MEMREHAB trial. Multiple Sclerosis (Houndmills, Basingstoke, England). 2015 .
 . A pilot study examining functional brain activity 6 months after memory retraining in MS: the MEMREHAB trial. Brain Imaging and Behavior [Internet]. 2014 ;8(3):403 - 406. Available from: http://link.springer.com/10.1007/s11682-014-9309-9