Sleep problems in individuals with spinal cord injury: frequency and age effects.

Sleep problems occur frequently in the general population (Hays & Stewart, 1992), and are even more common in individuals with spinal cord injury (SCI; Biering-Sorensen & Biering-Sorensen, 2001). Although some of the sleep difficulties reported by individuals with SCI are likely related to the high incidence of central sleep apnea that is very common in this population (Biering-Sorensen, Norup, Jacobsen, & Biering-Sorensen, 1995; Burns, Kapur, Yin, & Buhrer, 2001; Burns, Little, Hussey, Lyman, & Lakshminarayanan, 2000), aging is also independently associated with decreases in sleep quality and efficiency and may exacerbate sleep problems associated with SCI. For example, Mellinger and colleagues reported that the rate of serious insomnia in people aged 65 to 79 years was 25%, as compared with only 14% in individuals aged 18 to 34 years (Mellinger, Balter, & Uhlenhuth, 1985). Moreover, a number of physiological measures of sleep quality, such as total sleep time, sleep efficiency, percentage of slow-wave sleep, percentage of rapid eye movement (REM) sleep, and REM latency, have all been shown to decrease with age (Ohayon, Carskadon, Guilleminault, & Vitiello, 2004). Similarly, measures of sleep dysfunction, such as sleep latency, percentage of lighter stages of sleep (stage 1 and stage 2 sleep), and wake after sleep onset all increase with age (Ohayon et al., 2004; Unruh et al., 2008). As a group, these findings suggest a linear (negative) relationship between age and sleep quality – that is, sleep quality tends to decrease as people get older. Research in individuals with SCI suggests that the joints and organ systems of these individuals “age” faster than those of individuals without SCI. As a result, individuals with SCI tend to report more health problems, and at a younger age, than do nondisabled individuals (Bauman & Waters, 2004; Capoor & Stein, 2005). Another age-related variable, time since original SCI (i.e., duration of SCI), has shown important relationships to health in individuals with SCI. Specifically, even after controlling for chronological age, by the time an individual has lived with an SCI for 20 or more years, significant declines in functional capacity are known to occur (Capoor & Stein, 2005). Additionally, the rate of decline increases with increasing age (Capoor & Stein, 2005). These findings suggest a curvilinear (quadratic) relationship between age and health problems in individuals with SCI. To the extent that these health problems include sleep difficulties, the findings would predict that increases in sleep problems may not begin to occur until a person reaches a certain age, but that once sleep problems begin, they may become more serious at an accelerated rate. The evidence that (1) sleep problems increase with age, (2) sleep problems are more frequent in individuals with SCI than in otherwise healthy individuals, and (3) persons with SCI show evidence of earlier physical decline (“aging”) than nondisabled individuals, suggests the possibility that middle-age and elderly individuals with SCI may be particularly vulnerable to experiencing sleep difficulties. However, to our knowledge, no study has yet examined the associations between multiple aging-related variables and sleep problems in individuals with SCI, and if such associations exist, whether they are linear (increased sleep dys-function with increased age) or quadratic (an increased rate of sleep dysfunction with age among older individuals than younger individuals). Lack of research in this area may be due, in part, to the many complexities involved in teasing out the effects of different aging-related variables. One of those complexities is the statistical overlap between age-related variables (Krause & Adkins, 2004). Three key age-related variables are chronological age, age at injury, and duration of injury. In a cross-sectional survey study, and relative to individuals who are younger, individuals with SCI who are older are both (a) more likely to have had the SCI for a longer duration, and (b) more likely to have been older when they were injured. This means that if a significant “chronological age” effect is found in an analysis, unless age at injury or duration of SCI are controlled, it is not possible to know if the significant age effect was due to the patient being older, the patient having a longer duration of SCI, the patient having been injured later in life, or some combination of these. Yet, because of their linear relationship, it is not possible to study the individual effects of these three variables in the same analysis, making it difficult to separate the effects of one variable from the others. However, it is possible to tease out the effects of at least some of these factors if one performs and then interprets a series of analyses, rather than attempting to examine these variables in a single analysis. One of the primary goals of this paper is to illustrate this statistical approach using actual data, to provide future researchers with a strategy for addressing this problem of linear dependence. Given the complexities involved in interpreting the findings from research examining aging variables, especially those that use data collected at one point in time, firm conclusions regarding the reasons and causes of any significant results cannot be made. However, this does not mean that analyses concerning age effects and age-related variables using concurrent survey data should not be performed. Such analyses can, in fact, be used to identify those aging-related variables in need of further study, and to determine if age or age-related variables might play an important predictive role in patient functioning. However, investigators need to be very careful when performing and interpreting analyses to examine the association between age-related variables and key measures of patient functioning. Given these considerations, this study sought to better understand the associations between age-related predictors and sleep problems in individuals with SCI, while taking into account some of the complexities associated with understanding the relationships between age-related variables and sleep quality. Specifically, the current study was designed to (a) replicate the previous finding of more severe sleep difficulties in a sample of individuals with SCI compared with normative samples; and (b) examine the associations between aging variables (specifically, chronological age, duration of SCI, age at SCI onset) and the severity of sleep difficulties. We tested two specific hypotheses in this study. First, we hypothesized, based on previous research (Biering-Sorensen & Biering-Sorensen, 2001; Biering-Sorensen et al., 1995), that the individuals with SCI in our sample would report a higher level of sleep dysfunction than normative samples of individuals who are not disabled. Second, based on the findings from non-SCI (otherwise healthy) samples (Ohayon et al., 2004; Unruh et al., 2008), we hypothesized a significant association between chronological age and sleep dysfunction, with increased dysfunction associated with increased age. In these analyses, we examined both linear (i.e., increased sleep dysfunction with increased age) and quadratic (i.e., faster rates of sleep dysfunction with increased age among older individuals compared with younger individuals) associations, given that quadratic relationships are hypothesized for most areas of decline with age in SCI models (Capoor & Stein, 2005). Analyses concerning the association between age at SCI onset and duration of SCI were considered exploratory, as there is not yet empirical evidence (one way or the other) to indicate that these variables play an important role in sleep dysfunction. As with chronological age, both linear and quadratic associations between sleep dysfunction and these aging variables were examined.