The primary objective of this study was to investigate whether objectively measured DTC, calculated as the percentage change between the TUG and the TUG-C, was associated with falls status in people with MS, and the results suggest that there is no association. The subjective measure of DTC examined by asking the question “Do you have problems doing two things at once?” showed a significant difference between fallers and nonfallers and demonstrated that participants who report problems dual tasking are at twice the risk of falling when compared with those without problems dual tasking. Six distinct patterns of CMI were identified among this cohort of older people with MS and there was a significant difference between fallers and nonfallers and an increased risk of falls for those using pattern no. 6 (changes in gait, numbers correct). As the mean age of this study cohort is 52.6 years, they are already starting to experience the natural processes that occur with ageing and affect one's strength and mobility such as decreased muscle mass, decreased coordination, slower speed of movement, and increased movement variability.41 Overall, the age-related changes in the musculoskeletal system will be heightened in this MS cohort who already have dysfunction of the central and peripheral nervous system affecting mobility and activity performance, with falls being just one of the detrimental consequences. Problems such as falls and their sequela become more severe as the person with MS ages and these difficulties have been shown to be age related and age accelerated.42
The findings of this study regarding objectively measured DTC and falls status are consistent with previous findings from a comparable study population that similarly used a change in gait velocity during a mobility assessment to measure DTC.25 Another more recent prospective study that also used change in gait velocity to measure DTC but with a study population with relapse-remitting MS only and a milder disability level additionally found that DTC was unable to predict future falls.40 In contrast to these findings, a study using static posturography and electronic walkways found that the DTC of both cognitive performance and walking velocity was associated with an increased risk of recurrent falls among people with MS.26 However, the latter study had some noteworthy methodological differences when compared with the other studies. The study population was younger with a shorter disease duration and had significant methodological differences using more detailed and objective measures in the form of static posturography for balance assessment, electronic walkway for gait assessment, and correct response rate (response rate per second multiplied by percentage of correct responses) as the measure of cognitive performance whereas the other studies including this one used only simple timed walks when assessing DTC. However, it is often the more simple measures that can be easily performed without the use of expensive equipment that translate across to everyday clinical practice.
Our finding that DTC does not predict falls in MS differs to those of studies examining this concept in other populations. A study of more than 1000 older adults found that an objectively measured DTC of 18% or more prospectively predicted fallers (OR = 1.07; 95% CI: 1.04-1.10),43 but a recent systematic review has concluded that further prospective studies of older adults are needed to develop recommendations for dual-task testing as part of a multifaceted falls risk assessment.44 In subacute stroke patients, dual-task assessment of gait has demonstrated an altered stride length and step length in fallers versus nonfallers,45 and a longitudinal study of people with Parkinson disease demonstrated that motor DTC significantly predicted future fallers with a sensitivity of 71%, specificity of 77%, and 2.6 higher odds of being a future faller.46 A prospective study examining prediction of falls and near falls in people with mild Parkinson disease included a subjective question about dual tasking similar to our self-report measure of problems dual tasking, and simple logistic regression showed a self-report of balance problems while dual tasking had an OR of 4.0 for predicting falls and/or near falls.47
To the knowledge of the authors, this is the first study that investigated whether self-reported problems with dual tasking were linked to falls status in people with MS. Previous research among older people with MS has asked about problems with concentration and forgetfulness2,3 and qualitative interviews have identified situations that require divided attention as a factor related to falls,48 but no research to date has specifically asked about the participant's difficulties in doing 2 things at once. This concept of asking a simple question to help identify falls risk has been previously demonstrated in MS with regard to history of falls49 and in older people where health professionals are advised to routinely ask simple questions about fall history, fear of falls, and feelings of unsteadiness as a quick falls risk screening tool.50 In this study, a significant difference was identified between fallers and nonfallers in relation to subjectively reported problems with dual tasking. One possible reason for this finding may be that when individuals are subjectively reporting problems with dual tasking, they are considering all dual tasks that occur in their daily routine, whereas objective measures of dual-tasking ability look only at 1 dual-task activity in a controlled setting. A recent review21 determined that most dual-task tests in the MS population were conducted at a self-selected speed, similar to the task in this study. However, it is suggested that these tests are not representative of activities of daily living, when people are more likely to be carrying out tasks at a heightened speed and in novel environments, which is likely to place a higher demand on motor and cognitive resources. Therefore, asking people with MS whether they have problems doing 2 tasks at the same time may provide more accurate information regarding the patients' dual-tasking ability, and subsequently their falls risk, than attempting to objectively measure it.
While patterns of CMI have been examined in other neurological conditions such as stroke and Parkinson disease,22,51 this is the first study to research patterns of CMI and falls status in MS. Previous research examining the effect of dual-task activities in people with MS demonstrated significant deteriorations in gait with an added cognitive task, including altered swing time variability, increased double support time, and a large decrease in gait speed.52,53 Differences between fallers and nonfallers adopting each pattern of CMI in this study were significant for group 6 (changes in gait, numbers correct). This subgroup, along with group 3 (changes in gait and incorrect numbers), was one of the most common patterns utilized by participants and this may suggest that people with MS are not prioritizing the different components of a complex task appropriately similar to what has been observed in people with Parkinson disease.54 This may be a consequence of the limited processing capacity of the brain as highlighted in the attentional capacity theory and the bottleneck theory that have both been used to explain CMI behavior.55 However, these results should be interpreted with caution as there were small numbers in each of the subgroups using each type of pattern and a much larger sample would need to be analyzed to draw any strong conclusions.
There were several strengths to this study including a large sample size, a very low attrition rate, and use of the criterion standard recommendations of a specific falls definition and prospectively recorded falls data38 for a 3-month period. However, the findings from this study should be interpreted with consideration of its limitations. The progressive nature of this study cohort with median Expanded Disability Status Scale score of 6 limits the generalizability of the findings and results may not be applicable in a milder MS cohort. The cognitive task chosen was one of serial subtraction but it has been suggested that verbal fluency tasks may be more appropriate when assessing DTC in MS,56 and the investigators had no knowledge of baseline arithmetic competency levels of the participants. To fully investigate DTC, both the cognitive task and the motor task should be assessed under single- and dual-task conditions,57 but only the motor task was assessed under single-task conditions in this study. Indeed, this seems to be a common issue in MS studies examining DTC, as very few studies to date26,53,58 have actually reported the single-task performance of the cognitive task and examined the DTC of cognition as well as motor function. No instructions were given to participants regarding task prioritization and this study used the TUG as a measure of mobility while other studies measure objective balance control26 or different versions of timed walks.25,40 It has been advised that dual-task assessment needs to be standardized to facilitate comparison of results from different studies that will strengthen the evidence base and allow for clearer recommendations.56,57 Finally, the classifications of pattern of CMI used in this study may be considered a methodological limitation. Because of a lack of previously reported patterns of CMI for people with MS, the 6 patterns of CMI utilized were identified through observing the patterns of the first 12 study participants, but this method may have missed later characteristics that were not observed in that small initial sample.
The findings of this study suggest that objectively measured DTC is not associated with falls status in a more progressive MS cohort who are relatively “young” but presenting with falls at a rate greater than “older” people. The subjective measurement of dual-tasking ability is related to falls status and may be an easy method of screening for falls risk. Simply asking whether they have a problem performing 2 tasks at once is a quick, easy, and cost-free method of establishing dual-tasking ability and may be a useful adjunct in evaluating falls risk. In addition, differences appear to exist between fallers and nonfallers on the basis of whether they prioritize their cognitive or motor task under dual-task conditions. Future research involving more robust analysis and classification of patterns of CMI in a larger sample with a standardized method of DTC assessment will give a clearer insight into the role of DTC and CMI in falls risk assessment and its potential relevance as a component of falls interventions for people with MS.
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