In this large sample of community-dwelling older women, baseline scores on a brief measure of executive function (Trails B) and a combination of impairment on Trails B and a test of global cognitive function (mMMSE) were associated with ADL and IADL difficulty both cross-sectionally and longitudinally. The cross-sectional results suggest that individuals with poor executive function, either with or without impaired scores on the mMMSE, were more likely to have prevalent functional difficulty when compared to women with no cognitive impairment. This finding is supported by prior studies that also found a strong cross-sectional relationship between executive function and functional dependence (7
In addition, the participants with Trails B impairment at baseline were more likely to develop incident as well as worsening of functional difficulty level, especially ADL difficulty, after 6 years, suggesting that a low score on an executive function test is also a risk for future functional decline. This association remained after adjusting for age, education, medical comorbidities, depression, and baseline functional difficulty level. This finding is important to demonstrate because other studies suggest that age, education, and medical comorbidities influence functional status (30
). Finally, women with low scores on Trails B, either with or without mMMSE impairment, also had an increased risk of mortality after 6 years, suggesting that low scores on a brief test of executive function are associated with poor outcomes on multiple measures.
Our results also suggested that the participants with Trails B impairment were at greater risk for changes in ADL than IADL dependence. This is a curious observation because most studies link executive dysfunction with IADL dependence. In our study, the ADLs included walking several blocks, dressing, getting in and out of bed, and bathing. Thus, the ADLs assessed in the current study may require a higher level of functioning than other ADLs assessed (such as toileting) and depend on executive function. A recent study suggests that dependence in bathing, which requires multiple steps, is associated with risk for nursing home placement in a community-dwelling sample (32
) and, thereby, may require some higher cognitive functions. It is also important to keep in mind that few studies assess both ADLs and IADLs and executive function in the same study, and several combine ADL and IADL scores. In one study, Wang and colleagues (12
) assessed both ADLs and IADLs but did not comment extensively about the differential effects. Most models of functional dependence [e.g., (33
)] predict that IADLs are more strongly associated with tests of higher cognitive function than ADLs. Future studies should directly compare the impact of global cognition and domain-specific cognition on ADLs and IADLs independently.
Executive functioning is a cognitive skill that involves the planning, initiation, and execution of goal-directed behaviors, mental flexibility, and problem solving (34
). Because the ability to perform ADL and IADLs, such as paying bills, dressing, preparing meals, and shopping, involves many of these skills, it is possible that even mild executive dysfunction could impact functional ability. Trails B, the measure of executive function in the current study, requires mental flexibility, set-shifting, and attention, which are needed for many ADLs, IADLs, and other daily functions (7
). It is important to note that using Trails B alone without the Trails A condition, which controls for the motor component, is not an optimal measure of executive function; however, several studies, particularly older epidemiological studies, administer only Trails B [e.g., (37
)]. Carlson and colleagues (20
) found that Trails B, but not Trails A, accounted for a significant proportion of the variance in IADL performance in a sample of community-dwelling older adults. Future studies should use more comprehensive measures of executive function.
Measures of executive function may be a more sensitive marker than global measures of cognition of functional difficulty (7
). In our study, Trails B was also a stronger predictor than the mMMSE of functional decline over time. This is not surprising because measures of global cognition test a wide range of skills in a superficial manner and do not adequately test executive function. It is also plausible that executive function is more sensitive than other cognitive domains. For example, Carlson and colleagues (20
) found that a factor score derived from four tests of executive function was more strongly associated with IADLs than with learning and memory performance in community-dwelling women. Another study with 27 community-dwelling elders found that executive measures were better predictors of functional status than memory, language, visuospatial, or psychomotor function (19
). Intact executive function, in particular, appears to be important for performing ADLs and IADLs. Our findings are also supported by a recent study (21
) that found that a decline in executive function, as measured by the Executive Interview (EXIT25), over time was related to a decline in functional status over 3 years in nondemented elderly persons. This study also found that the MMSE was not associated with a change in IADLs over time.
The presence of executive dysfunction despite a normal score on the MMSE may represent a continuum of normal aging or possibly a preclinical stage of dementia. The prefrontal cortex is particularly vulnerable to the effects of aging. Older individuals perform worse than younger individuals on tests of executive function (41
), and brain imaging studies document a preferential decrease in pre-frontal cortex volume with age (42
). Dysexecutive-like behaviors, such as difficulty with planning, impulsivity, and lapses of attention, have been described in normal, older adult populations (44
). However, some individuals may show preferential damage to the prefrontal cortex and develop executive dysfunction (47
). Grigsby and colleagues (29
) found that 9% of community-dwelling elders older than 60 years had impairment on the Behavioral Dyscontrol Scale despite normal performance on the MMSE, which is similar to the proportion of individuals with an isolated impairment on the Trails B test in the current study. An isolated impairment in executive function has also been documented in vascular cognitive impairment (48
) and in preclinical stages of frontotemporal dementia (49
) and Parkinson’s disease (50
). Thus, differentiating between normal, age-related changes and declines that hallmark a pre-clinical disease stage is important.
Cognitive impairment in older adults is a well-known predictor of mortality in both demented and nondemented populations, even after controlling for demographic and baseline characteristics. Although it is well-documented that moderate to severe cognitive impairment is associated with mortality, fewer studies evaluate the effect of subtle cognitive impairment on mortality. Several studies document a relationship between the MMSE (or other tests of general cognition) and mortality in nondemented samples (51
); however, other studies have not found this relationship (3
). Although few studies evaluate cognitive tests from multiple cognitive domains, earlier studies found that low performance on verbal fluency and episodic memory tasks are significant predictors of mortality (54
). Fried and colleagues (3
) found that the Digit Symbol Substitution task (which requires visuomotor coordination), but not the MMSE, predicted mortality after 5 years. In our study, low scores on Trails B were also associated with an increased risk of mortality.
There are several limitations of this study. First, the use of self-report functional questionnaires, and not performance-based measures, may underestimate functional dependence in elderly individuals (6
). In addition, tests from multiple cognitive domains were not available to compare the contribution to functional dependence. The use of Trails B (without using Trails A as a control) is also not ideal. Future studies should use more comprehensive measures of executive function. Although several studies suggest that executive function is a good predictor of functional decline, few studies compare multiple cognitive domains (19
) or use a comprehensive selection of executive function tests. Another weakness of this study is the absence of a comprehensive dementia evaluation. It is possible that women with mild cognitive impairment or possibly mild dementia were included in the sample. Although we can infer that the community-dwelling individuals were not severely demented, a comprehensive dementia evaluation is the only way to confirm the absence of dementia. A dementia evaluation was not possible due to the large sample size. Another limitation was the fact that the follow-up sample was 18% smaller than the original sample. The participants who completed the 6-year follow-up visit reported significantly less baseline difficulty on both ADL and IADL scales, and the longitudinal results are likely an underestimate of functional decline. Finally, the study population was composed of only women who were primarily Caucasian, making it difficult to generalize to men or other ethnic groups.
Executive dysfunction is a predictor of functional difficulty in community-dwelling elderly women both cross-sectionally and longitudinally. The findings from this study add to other studies suggesting that executive function is more strongly associated with functional difficulty than measures of global cognition. This study is unique in that it provides strong support that executive function is a predictor of future functional difficulty and decline over time. This result emphasizes the importance of screening for executive impairment, in addition to measures of global cognition, in elderly individuals. Future studies should also better investigate the clinical outcome of individuals who have executive impairment and preserved global cognition.