Introduction
Life expectancy following spinal cord injury (SCI) has increased steadily in the past few decades, and is approaching 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. 1998; Krause et al. 2004). Due to advances in emergency, acute, and rehabilitation treatments, persons with SCI are now living many decades post-injury. There are increasing numbers of persons with long-term SCI who are over 55 years of age (Adkins 2001).
Initially, SCI was considered a relatively stable condition, and persons with SCI were thought to be able to maintain the same functional level for most of their lives (Trieschmann, 1987). However, this static view of aging with SCI has been replaced by one that acknowledges that aging is a multi-dimensional and complex process of physical, psychological, and social change (Aldwin & Gilmer, 2004).
McColl and colleagues (2002) described 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
One problem with research on aging after SCI is that the relationship between age at injury, current chronological age, and years post-injury (YPI) are all linearly dependent. This 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. Furthermore, historical changes in the treatment and rehabilitation of SCI, increases the complexity of the assessment of aging effects associated with this condition (Adkins 2004).
Despite these issues, the field continues to strive to attribute changes in health and wellbeing 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). This information can inform the creation of better health promotion strategies to mitigate declines in health and wellbeing since even slight changes in functioning after SCI can adversely affect a person’s level of independence.
Spinal Cord Injury: A Model of Premature Aging?
SCI has been described as a model of premature aging (Bauman & Spungen 1994). According to this theory, premature aging of certain body systems may occur because additional stresses, resulting from SCI, can exceed the capacity of those body systems to repair themselves (e.g., cardiovascular, musculoskeletal) (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 bodily functions reach a maximum capacity prior to or during early adulthood, and then begin 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). A classic study published in Science (Strehler & Mildvan 1960) used mathematical models to show that physiological function declines at a consistent rate of 0.5-1.3% per year beyond age 30. This physical peak can be measured by examining the functioning of individual organ systems. For example, we can assess cardiovascular capacity by how well the heart can pump blood. Similarly, we can assess the individual’s maximum functional capacities (e.g. 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 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 a SCI, physiological and functional changes potentially accelerate bodily declines for a period of time, after which the effect of aging is thought 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. 1994; 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, it may be possible to identify changes to systems that are 1) attributed to the SCI, 2) related to chronological age and the aging process, and 3) caused as a result of 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
Although some biological changes are unavoidable with aging, other aspects are more malleable. 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 2002). There are a multitude of factors that must be considered when evaluating how people age with SCI, including personality traits, economic factors, environmental barriers and facilitators, cultural issues, and social networks (intimate and remote) (Charlifue & Lammertse 2002). Given the complex interaction among these factors, it is not surprising that several 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). To ensure that people with SCI are not only living long, but that they are also living well, it is important to identify factors that lead to higher levels of quality of life that would be amenable to intervention.
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 that focus on analyses relevant to aging. 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 that included historical controls (from chart review or database) were graded with a level of evidence of 3. 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.
Although the use of longitudinal designs is 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, physiological aging, which includes health status and physical functioning, the cardiovascular and endocrine systems, immune system, musculoskeletal system, respiratory system , nervous system, skin and subcutaneous tissues, the genitourinary and gastrointestinal systems, secondary health complications, functional independence, and quality of life and community reintegration.
