It’s estimated that 43%-70% of those with multiple sclerosis suffer from some level of cognitive impairment (yes, a very broad range! perhaps the finding of this study offers one clue why). Most commonly, this is seen in slower processing speed, impaired memory, impaired executive function, and poorer visuospatial processing. There are a number of factors that have been implicated in why some people suffer from cognitive impairment and others don’t, such as age of onset and male gender. As with dementia, depression also may be a factor, while cognitive reserve appears protective.
Another factor specific to MS may be temperature. Previous research has shown that people with MS tend to have more symptoms and greater lesion activity when the weather is warmer. More recently, this association has also been found with memory and processing speed.
I mentioned this research very briefly last year, when it was presented at conference. Here are a few more details now the journal article is out.
There were two parts to this study: cross-sectional and longitudinal. In the former, 40 patients with MS and 40 healthy controls were recruited throughout the calendar year, and cognitive performance and outdoor temperature were recorded for the day of testing. A different group of 45 patients with MS were recruited for the longitudinal analysis, in which cognitive status and outdoor temperature were recorded twice, six months apart.
In the cross-sectional analysis, warmer temperature was related to significantly worse cognitive performance in patients with MS, while controls were unaffected by temperature. Similarly, the longitudinal analysis found that an increased outdoor temperature from the first cognitive test to the second was related to a decline in cognitive performance within patients with MS.
 . Warmer Outdoor Temperature Is Associated with Worse Cognitive Status in Multiple Sclerosis. Neurology [Internet]. 2012 ;78(13):964 - 968. Available from: http://www.neurology.org/content/78/13/964