The effects of multiple sclerosis: Exploring executive dysfunction and its linkswith other cognitive impairment
Multiple Sclerosis (MS) is one of the most common chronic diseases of the central nervous system, and in New Zealand an estimated 4,000 people are currently affected. Although the physical manifestations of the disease are well known, and memory impairments have been comparatively well researched, those cognitive abilities known collectively as executive functions, and their relationship with other cognitive processes, have seldom been examined. To fill this research gap, the present study examined the extent and nature of executive dysfunction and other cognitive impairments in a community based sample of participants with MS, and explored any links between executive dysfunction and other cognitive processes. Due to the well known heterogeneous nature of the effects of MS, impairment was assessed not only for the sample as a whole, but more informatively, and in contrast to much of the previous research on MS, within smaller more homogeneous groups formed on the basis of the extent and nature of the cognitive impairment of the participants. As the behavioural characteristics of executive dysfunction may have an influence on the ability to function appropriately on a daily basis in the community at large, evidence of typical 'dysexecutive' behaviour was also examined. Ninety five participants with MS were assessed on general ability (WAIS-III), memory (WMS-III), executive functioning (D.KEFS), information processing speed and simple attentional processes. While the extent of impairment remained low on general ability measures, some impairment became evident in over half of the participants with MS on the memory measures. Five categories of executive functioning were used to assess the extent and nature of executive dysfunction; Inhibition, Shifting, Reasoning, Planning and Fluency. Working memory was not included as an executive category, as the primary scores obtained from the D.KEFS assessment did not enable this process to be isolated. Working memory was however evaluated as part of the memory assessment (WMS - III). Over half the participants were found to be impaired in at least one category of executive dysfunction, and the most commonly impaired categories were fluency and reasoning. Although sixteen different profiles of executive dysfunction were evident amongst the participants, the majority of those impaired were assessed with only one category of impairment. Surprisingly, no impairment in information processing speed was evident, but some participants were impaired in relation to simple attentional processes. The participants with MS were subsequently divided into four groups based on the nature of their impairment, that is, depending on whether the participant had no impairment, memory impairment only, memory impairment and executive dysfunction or executive dysfunction only. The index scores for visual memory were found to be consistently lower than those for auditory memory for all the 'impairment type' groups, but little difference was found between long-term and short-term memory performance. Working memory was found to be the only cognitive process which differentiated between those groups with executive dysfunction and those without, and at all levels of performance, proved to be a reliable predictor of the degree of executive dysfunction. Working memory was also found to have a differing relationship with each of the other five specific categories of executive functioning. When the everyday behavioural characteristics of executive dysfunction, such as inhibition, intentionality, executive memory and positive and negative affect were assessed with the Dysexecutive Questionnaire, there was little evidence of any self reported problems. In particular, levels of insight were shown not to be compromised, therefore it seemed that if any behavioural difficulties did arise for the participants, they would have been able to recognize and subsequently address them. Thus, it appeared for this sample of participants with MS that although some executive dysfunction was more prevalent than had previously been suggested in the literature on MS, generally the living arrangements of these participants seemed appropriate in relation to their ability to function independently. Overall, it was recommended that for the initial cognitive assessment of those with MS, a comprehensive evaluation of working memory integrity should be included. This would provide valuable information regarding the likelihood of other executive dysfunction and would allow a more targeted assessment approach by limiting the more extensive testing of executive functions to only those people whose comparatively lower working memory performance indicated that further problems may exist.
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