In this large, population-based, prospective cohort study of first-time ischemic stroke patients, we observed a modest annual decline in functional status over the long term after stroke, even after censoring those with recurrent stroke and MI and adjusting for risk factors and baseline stroke severity. There was a significant decline over years of follow-up even among those with early functional “recovery” (BI≥95 at 6 months). In the United States, there are 4 tiers of insurance access: private insurance (69% of the U.S. population in 200426
), Medicaid (a state-administered insurance only available to low income residents that has been associated with limited access to health care in New York State27, 28
; 13% of the population), Medicare (a federally-administered insurance primarily for residents aged ≥65 years; 14% of the population), and no insurance (16% of population). Functional decline in our study was seen particularly among those who were uninsured or insured with Medicaid. This decline began to be apparent at about 3 years when we used as a threshold for disability the BI threshold of 95, and at 2 years with a BI cutoff of 60.
Few community-based studies have examined long-term disability after stroke. One study reported that 37% of first stroke patients worsened in Rankin scores between 1 and 5 years after stroke in Rochester, Minnesota.29
Limitations of this study included comparison of mean Rankin scores without reporting of confidence intervals and the lack of multivariable analysis for predictors of outcomes. Furthermore, the GEE analysis that was used in our study is more sensitive to individual changes in disability over time. In another study, dependence in ADL tasks in 109 first stroke patients was analyzed, and a smaller percentage of women were independent at 5 years and there was a greater proportion of dependence among older patients.30
However, this study had a small sample, reported only percentages in each category without confidence intervals or significance tests, did not conduct multivariable analysis, and used a potentially insensitive ADL measurement scale. Another study examined modified BI at 5 years in 129 survivors of first stroke and found that 36% developed new major disability, with predictors including older age, drowsiness at baseline assessment, moderate hemiparesis, and recurrent stroke.31
This study was limited by small numbers of data available at 5 years and only one follow-up assessment of disability. Finally, none of these studies censored recurrent stroke or cardiac events, and hence it is unclear whether worsened disability over time was the result of recurrent stroke or long-term effects of a single index stroke. In contrast to these studies, our study collected data at multiple timepoints and excluded those with recurrent clinical vascular events and so was able to assess a linear change in function over time.
The delayed functional decline that we observed has not been well-described before and has several possible explanations. Non-stroke comorbidities that are known to affect functional status may contribute in the long-term, as prior research in non-stroke cohorts has shown.32
In particular, cognitive dysfunction has been shown to be associated with functional status after stroke.33, 34
In cognitive research, the concept of “cognitive reserve” refers to differing susceptibility to cognitive impairment that is related to variables such as education, literacy, intelligence quotient, and engagement in leisure activities.35
Considering the close correlation between cognitive status and functional status, there may be a similar phenomenon with regard to performing ADLs that we may term “functional reserve.” The deficit caused by a first stroke may result in a depleted functional reserve and a consequent failure to compensate for brain aging. We were unable to include detailed measurements of cognitive function in our analyses, however.
Another possible explanation for the observed delay in functional decline among uninsured and Medicaid patients is that the stroke caused a functional deficit that affected participants equally regardless of insurance status. Stroke, in other words, may serve as a sort of equalizer of function due to its direct biological effects on the brain, as well as the availability of acute rehabilitation and other therapies available to participants with all types of insurance. Over time after stroke, however, functional status among those with private insurance and Medicare and those with Medicaid or no insurance may diverge, due to disparities in care and more limited access to rehabilitative services, information about health, and ongoing management of risk factors and chronic conditions28, 36
that are known to have an impact on functional status.37
The delayed decline in functional status could also be due to the ceiling effect that has been observed with the BI.38
It is possible that there was a steady decline in functional status that began soon after ischemic stroke, but since the BI is insensitive to small changes in disability, this decline was not captured until 3 years after the stroke.
Another possible explanation for the observed decline in functional status is that participants may have experienced clinically silent recurrent strokes during follow-up. Studies have shown a prevalence of 6–18% of clinically silent strokes in different populations depending on risk factors and imaging protocols,39
and silent infarcts may be as much as 5 times as prevalent as symptomatic infarcts.40
A previous study in the Northern Manhattan population showed that 18% of 892 participants free of clinical stroke had subclinical infarcts.41
Despite the term “silent” stroke, these subclinical events may affect functional status.42
Previous studies with limited follow-up have shown predictors of functional status similar to the predictors that we found, in particular age,3, 4, 7, 8
DM,4, 8, 9
marital status (a marker of social support),5, 10
stroke severity,4, 7, 8, 11
side of stroke,11, 12
and urinary continence.13, 14
Most previous studies, however, have assessed these functional outcomes at single points in time, whereas in our analysis we were able to assess the associations for these risk factors with functional outcomes measured at multiple annual intervals. These risk factors thus continue to predict functional status over several years. The fact that these predictors were found to be significant in our study suggests that they represent robust associations with functional status, for selection bias is minimized in population-based studies and follow-up continued to 5 years post-stroke. The fact that DM was a significant predictor of functional outcome is notable considering the high prevalence of DM in this population, which is likely due to a combination of environmental (including dietary) and genetic factors.
In our multi-ethnic population-based study, race/ethnicity was not associated with functional outcome among all participants, in univariate or multivariate analyses. Socioeconomic status and access to care were primary mediating effects of functional status, perhaps mitigating the effect of race/ethnicity on outcome, which has been previously described.43
Limitations of this study include use of the BI to assess disability instead of a disability scale designed specifically for stroke patients. However, the BI is widely used in stroke research, which allows comparison of this study with prior studies. Second, detailed data on rehabilitation received after the stroke was not collected, and may have informed the relationship between late functional decline and insurance status. Third, since some of the data on baseline risk factors was obtained by self-report, uninsured participants who do not regularly get medical care may not be aware of undiagnosed conditions. Hence, in order to lessen bias, we used laboratory and blood pressure measurements and information about prescription medications to capture risk factors such as DM, HTN, and hypercholesterolemia. Fourth, neuropsychological data were not analyzed in this study, which could have informed the link between cognitive status and functional status. Future research will be directed towards clarifying the relationship between time and functional decline, comparing functional status over time in stroke patients and non-stroke controls matched for risk factors, and assessing the role of cognitive decline as a mediator of the change in functional status. Finally, the decline in functional status among those with less access to care highlights the need to improve the ways in which knowledge of risk factor control is translated in clinical practice to improve outcome following stroke.