Aging
Introduction
Numerous studies have found the life expectancy after spinal cord injury (SCI) has increased steadily in the past few decades, and is now comparable to that of the able-bodied population (Geisler et al. 1983; Whiteneck et al. 1992; Hartkopp et al. 1997; McColl et al. 1997; Frankel et al. 1998; DeVivo et al. 1999; Yeo et al. 2000; Krause et al. 2004). Due to advances in emergency, acute, and rehabilitation treatments, persons are now living many decades post-injury. Although still relatively small, there are increasing numbers of persons who have long-term SCI and who are over the age of 55 years of age (Adkins 2001), which has allowed for some appreciation and perspective on the types of changes that occur for an individual with SCI over time.
By definition, aging is a multi-dimensional process of physical, psychological, and social change, and gaining an understanding of aging is extremely complex (Aldwin & Gilmer 2004). Even in the general population, the study of aging is still a relatively new area that has experienced an upsurge in research, but is in the process of gaining consensus on many theoretical and methodological issues (Aldwin & Gilmer 2004). Initially, SCI was considered a relatively static condition, and that persons with SCI would be able to maintain their functional level for most of their lives (Trieschmann 1987). However, we now understand that the superimposition of impairment, such as SCI, further complicates the assessment of aging effects (Adkins 2004).
McColl and colleagues (2002) list five changes that persons with SCI undergo as they age: 1) the effects of living with SCI long-term (e.g. shoulder pain, chronic bladder infections); 2) secondary health conditions of the original lesion (e.g. post-traumatic syringomyelia); 3) pathological processes unrelated to the SCI (e.g. cardiovascular disease); 4) degenerative changes associated with aging (e.g. joint problems); and 5) environmental factors (e.g. societal, cultural) that may potentially complicate the experience of aging with a SCI. All of these factors have the potential to compromise a person with SCI’s ability to sustain independence and ability to participate in their communities at later stages in life.
Chronological Age, Years Post-Injury, and Age at Injury
A problem with aging research after SCI is that the relationship between age at injury, current chronological age, and years post-injury (YPI) are all linearly dependent, which limits the ability to assess the influence of all three factors at the same time statistically (Adkins 2001). Hence, investigators are limited to examine only three possible combinations of factors, which include: 1) current age and YPI; 2) current age and age at injury; and 3) age at injury and YPI. Despite this issue, the field continues to strive to attribute changes in health and well-being to these aging variables. Thompson and Yakura (2001) comment that “developing an understanding of the effect of advancing age versus longer durations of injury on the incidence and type of changes can help in the prediction of when people with SCI might be susceptible to changes in function” (p. 73). Doing so may lead to better health promotion strategies to avoid declines in health and well-being since even slight changes in functioning after SCI can adversely affect a person’s level of independence.
SCI: A Model of Premature Aging?
A growing body of evidence in the literature is suggesting that SCI represents a model for premature aging (Bauman & Spungen 1994). The premature aging of certain body systems may occur because of additional stresses that extend some physical systems beyond their ability to repair themselves, which then become systematic (Charlifue & Lammertse 2002). Although the aging process occurs at varying rates and at different ages for each individual (Charlifue 1993), it is generally accepted that our bodily functions reach a maximum capacity prior to or during early adulthood, and then begin to experience a gradual decline. This decline is thought to commence at approximately 25 years of age when the developmental process plateaus and biological capacity has peaked (Capoor & Stein 2005). At this point, the reserve capacity of our organ systems begins to drop at a rate of 1% of their function per year in able-bodied persons. This physical peak can be measured by examining the functioning of individual organ sytems. For example, we can assess cardiovascular capcity by how well the heart can pump blood. Similarly, we can assess the individual's maximum functional capacities (eg. how much weight an individual can lift). Hence, the average person at age 70 has approximately 50% of his/her capacity remaining in each organ system, which does not necessarily impact negatively on health or functioning since all organ systems have an 'excess reserve' (i.e. more cells, structure and supportive tissue than is required to meet daily life needs; Adkins 2004).
When the reserve capacity declines below 40% of original functioning, there is greater chance of becoming injured, and/or more susceptible to infection or disease (Kemp & Thompson 2002). With the occurrence of an SCI, there is disruption to the system. This disruption to physiological and functional changes potentially accelerates bodily declines for a period of time or capacity is reduced at approximately the time of injury, after which the effect of aging is said to proceed at a normal rate (Adkins 2004).
Age of injury may have important consequences on different aspects of health. Because there are increasing numbers of seniors incurring a SCI due to falls, a bi-modal age-of-onset distribution exists, with the prevalence of SCI peaking among individuals who are 30 and 60 years of age (Pickett et al. 2006). As a result, researchers have been able to investigate and compare age-related outcomes after SCI. For example, there are a number of studies showing that persons who incur a SCI at later ages have poorer functional outcomes than those injured at younger ages (DeVivo et al. 1990; Alander et al. 1997; Scivoletto et al. 2004), although in some instances, the impact of SCI may be minimized in older persons.
Within a reserve capacity model of biological aging that is disrupted by SCI, Adkins (2004) theorizes that the impact of injury “decreases the further out on the age continuum the injury occurs” (p. 5). However, if the injury occurs far enough along the continuum, then even a minimal change in rate will lower reserve capacity below 40% soon after injury since capacity is already low. Further, adults with older ages of SCI-onset may have other pre-existing or vulnerabilities to co-morbidities that affect outcomes compared to younger adults (Furlan et al. 2009).
Given the increasing mean age of SCI onset, along with increased life expectancy, we may be able to clarify which changes to systems are attributed to the SCI, those which are related to chronological age and the aging process, and those which result from their interaction. Adkins (2004), however, suggests that it may be prudent to establish age of onset exclusion criteria when studying biological aging with SCI. In addition, completeness and neurological level of SCI must also be taken into account since a person with a complete lesion may experience aging in a different manner than someone with an incomplete lesion (Charlifue 1993).
Aging and Quality of Life
In addition to issues of biological aging, the interaction of environmental and psychological factors with aging must be taken into account. Unlike physical aging, it may be that these aspects of a person’s life may actually improve, and may be more amenable to intervention to either delay, modify or eliminate their potential negative impact (Charlifue & Lammertse 2001). There are a multitude of individual factors that may affect, or be affected, by physiologic aging, which must be considered when evaluating how people age with SCI. This may include economic factors, environmental barriers and facilitators, cultural issues, and social networks (intimate and remote; Charlifue & Lammertse 2001). As a result of this complex phenomenon, there are issues that remain unclear with quality of life and aging with SCI, as some studies report contradictory findings, with life satisfaction and community integration decreasing with age, but increasing with years post-injury (YPI; i.e., Eisenberg & Saltz 1991; Krause & Crewe 1991; McColl & Rosenthal 1994; Pentland et al. 1995; Westgren & Levi 1998; Dowler et al. 2001; Tonack et al. 2008). Hence, it is imperative that we identify which factors lead to high levels of quality of life in order to ensure that people with SCI are not only living long, but that they are also living well.
Chapter Purpose
The chapter summarizes some key issues in the SCI aging literature, and evaluates the level of evidence provided by selected studies on aging with SCI. The selected research for evaluation includes longitudinal studies (duration of at least 2 years or more), case-control and cross-sectional comparative studies. As longitudinal studies inherently include at least a baseline and follow-up evaluation, these studies were graded with a level of evidence of 4 (at least equivalent to pre-post studies). Prospective longitudinal studies that also included a control group (e.g., able-bodied group) were graded with a level of evidence of 2 as they are considered cohort studies where one group is exposed to a particular condition (in this case, a spinal cord injury). Longitudinal studies which included historical controls (from chart review or database) were graded with a level of evidence of 3. Cross-sectional studies (or comparative studies) utilizing both individuals with SCI and able-bodied controls at one point in time were graded with a level of evidence of 5. Studies involving mixed populations in which < 50% of the subjects had a SCI were excluded as were articles not in English. As well, studies with small sample sizes (less than 5 SCI participants) and/or with limited age ranges (i.e. only persons in their twenties and/or early thirties, etc.) were excluded. Longitudinal studies that only reported cross-sectional analysis were also excluded from evaluation. The results presented from each study primarily focus on the analyses relevant to aging, and the p-values reported are those reported in the original articles.
Although the use of longitudinal designs are preferred, comparison studies with age-matched able-bodied (AB) controls is a useful approach for studying aging after SCI because it provides some awareness of the factors associated with the typical aging process (Charlifue 1993), while helping to illuminate whether changes are due to YPI rather than current age per se. After sustaining a SCI, age and YPI increase at the same pace, and so using age-matched AB controls allows us to determine the effects that might have occurred without SCI and those that occurred with SCI (Adkins 2004). This approach may offer some insight on whether changes after SCI are unique and/or accelerated in persons with SCI or if they are typical of the aging process.
The issues related to aging are described as mortality and life expectancy (see Table 1), physiological aging, which include the cardiovascular and endocrine systems (see Table 2), immune system (see Table 3), musculoskeletal system (see Table 4), respiratory system (see Table 5), nervous system (see Table 6), skin and subcutaneous tissues (see Table 7), and the genitourinary and gastrointestinal systems (see Table 8), and community reintegration and quality of life (see Table 9).
