In this cohort of older adults from the community, usual gait speed measured over a short course predicted a decline in a test of attention and psychomotor speed over 5 years. A relatively small baseline difference in gait speed, 0.3 m/s, was associated with an almost twofold increased risk of declining in attention and psychomotor speed. The ability of gait speed to predict the decline in attention and psychomotor speed was independent of both baseline and 5-year change in global cognitive status as measured with the 3MS.
Prior related studies considered comprehensive cognitive assessments, such as the diagnosis of dementia or a persistent cognitive impairment, as outcomes [2
]. However, dementia is a complex diagnosis, reached often by consensus using criteria that are not entirely agreed upon, and represents a late stage in cognitive decline, when preventive strategies are no longer effective and treatment options are not yet available. The early detection of a decline in attention and psychomotor speed may represent an important goal for clinical practice: a lower DSST score predicts incident dementia [9
], cardiovascular events [8
] and death [10
], and white matter disease progression [12
] in middle-aged and older adults, and it is associated with activities of daily living disability [13
]. A recent investigation, conducted with computerized gait analysis, demonstrated that a summary measure derived from a factorial analysis and accounting for gait speed, step length and double support time was associated with a subsequent decline in attention, psychomotor speed and verbal fluency in community-dwelling seniors [29
]. We were able to demonstrate that decline in attention and psychomotor speed can be predicted by usual gait speed alone as measured with a stopwatch over a short course. This is a reliable and valid test which can be easily repeated in every clinical setting and even in a home environment. We were also able to control for various possible confounders of the association between gait and attention and psychomotor speed decline, including lifestyle habits and comorbidities previously demonstrated as predictors of cognitive decline in the elderly, such as the level of physical activity [20
] or diabetes, which have been reported to specifically predict a decline in the DSST [24
]. The predictive role of gait speed was also independent of baseline and concurrent change of global cognition.
Increasing evidence supports the notion that physical and cognitive functions decline concurrently over time [30
], and that both may share common etiologies, such as brain small-vessel disease [12
]. The findings of the present study suggest that gait impairment can become evident before cognitive function starts to decline or before this decline can be detected. Therefore, initial gait impairment might represent a useful tool for the early detection of the consequences of brain small-vessel disease in older adults.
Strengths of our study include the population-based sample, the longitudinal design and the large number of available covariates. The well-functioning population allowed us to focus on the association between physical and cognitive performances reducing the influence of concurrent declines in other physiologic systems. The DSST has a high sensitivity to changes in high levels of cognition [32
]. Some limitations have to be acknowledged. First, though the good level of global functioning of this cohort represents an advantage, it could also constrain the inference of our results. Second, the available cognitive battery was limited, which is however not uncommon in large epidemiologic studies where the focus is not specifically on cognition. Third, the definition of decline in DSST was somewhat arbitrary. Generally accepted cutoffs have never been set for this test, despite the large use in geriatric epidemiology. Our cutoff point had the advantage to be weighted on this specific population. Moreover, 1 SD of the concurrent decline in 3MS, in this cohort, was 5 points, which is generally the expression of global cognitive decline [28
]. Selecting a different cutoff for the DSST (1.5 SD) showed a consistently increased risk of decline in DSST in slow walkers, compared to participants in the highest quartile of gait speed. The lack of statistical significance of the trend is probably due to a reduced power, because only 7% of the sample had a decline in DSST greater than 1.5 SD. This low prevalence of participants who experienced such a relevant decline (1.5 SD) was not surprising, considering the initially physically and cognitively well-functioning population. Finally, considering the net change in DSST between baseline and follow-up as an outcome, the results consistently identified the independent role of usual gait speed in the prediction of attention and psychomotor speed decline.
In conclusion, usual gait speed over a short course predicted a decline in attention and psychomotor speed over 5 years in a well-functioning sample of older community dwellers. The predictive role of gait speed was independent of baseline and concurrent change in global cognition. In clinical practice, the measure of usual gait speed over a short course is easy, quick and inexpensive. It should therefore be considered as an essential tool in the routine clinical evaluation of older persons, independent of whether they report mobility complaints.