A large body of literature has emerged over the last 50 years showing the inextricable link between psychological factors, such as stress, and the functioning of the hypo-pituitary-adrenal (HPA) axis. The HPA axis controls the release of cortisol, among other hormones, which is associated with psychological, physiological and physical health (Dickerson & Kemeny, 2004
). Psychological stress is characterized by the perception of threat of a situation or event in the environment. Threats can include environmental stressors such as work, home or neighborhood, major life events, trauma or abuse (McEwen & Seeman, 1999
). This perceived threat from the environment drives behavioral and physiological responses. Perception of threat is highly individualistic and influenced by the individual's experiences, genetics, prior stress exposure, social environment and individual characteristics such as social connection, social status, personality and coping style (Glei, Goldman, Chuang, & Weinstein, 2007
; McEwen & Seeman, 1999
; Olff, Langeland, & Gersons, 2005
). Determinants of salivary cortisol to challenge are also highly variable and include gender and sex steroids, genetics, nicotine, coffee and alcohol, and pre- and post-natal stress (Kudielka, Hellhammer, & Wust, 2009
“Allostatsis” refers to maintaining stability or homeostasis through change and was first introduced to describe how the cardiovascular system adjusts to resting and active states of the body (Sterling & Eyer, 1988
). McEwen and Seeman (1999)
expanded this concept to capture physiological responses, such as the secretion of cortisol, to environmental and psychosocial situations and demands. The body's response to psychological stress involves the activation of two systems: the HPA axis and the sympathetic nervous system (SNS), with the former releasing cortisol and corticotrophin-releasing hormone (McEwen, 1998
). When the HPA axis is functioning properly, elevated levels of these hormones are limited in magnitude and temporary in duration, and reduce to normal levels with the cessation of the stressor. However, under circumstances involving high intensity or chronic stress, hormonal dysregulation can lead to physical, psychosomatic, and psychological disorders (Ehlert & Straub, 1998
). Long-term effects of critical or traumatic life events also appear to be associated with distinct dysregulation of the HPA axis (Ehlert & Straub, 1998
) and, with this, vulnerability to disease and psychological dysfunction.
“Allostatic load” refers to the wear and tear of repeated neuroendocrine responses to chronic environmental challenges and is the “price” of allostasis, or adaptation (McEwen, 1998
). Biological systems that provide protection (“fight or flight”) against acute stressors can eventually become damaging if that adaptive response (allostasis) stays “on.” Physiologic response to an acute psychological stress can begin within seconds and peaks 15 to 20 minutes after the onset of the stressor (Kudielka et al., 2009
). However, over time, chronic stress (characterized by a prolonged state of or exposure to stress) leads to dysregulation of these protective systems. There is also debate over whether hyper- or hypo-responsivity to an acute stressor is influenced by the presence of chronic stress or exhaustion in an individual (Kudielka et al., 2009
). Dysregulation resulting from prolonged activation of protective systems – allostatic load – is characterized by elevated (or in some cases diminished) levels of biomarkers reflecting SNS, HPA axis, immune system and cardiovascular activity (Glei et al., 2007
). There is also evidence that acute and chronic stressors play an important role in the development of disorders having psychological features, such as depression, with the neuroendocrine system at the center of this causal pathway (Miller, Chen, & Zhou, 2007
; vanEck, Berkhof, Nicolson, & Sulon, 1996
Despite the importance of the adverse effects of stress on health, the study of HPA reactivity to stress in persons with various disabling conditions is very limited. In persons with spinal cord injury (SCI), the autonomic nervous systems involved in regulating the stress response is compromised by the direct effects of injury, such as sympathetic denervation and central neurotransmitter alteration (Palmer, 1985
). A vulnerable health status is common among many persons with SCI due to direct and indirect effects of injury. Changes in the body as a result of dysregulated cortisol and autonomic activity can have long-term health effects that include bone mineral loss and abdominal fat deposits (Gold, 2005
), increased cardiovascular risk (McEwen, 2003
), and insulin resistance (Bauman, 1997
), all of which are already compromised due to the effects of SCI. Stress-mediated immunity is also important in connection with infection, slow wound healing (Ebrecht et al., 2004
), and pressure sores, the latter of which are among the leading causes of re-hospitalization and morbidity in persons with SCI (Cardenas, Hoffman, Kirshblum, & McKinley, 2004
). The presence or absence of a concomitant brain injury may also affect cortisol response (Bay, Sikorskii, & Gao, 2009
) although this has not been reported in the SCI literature.
The study of cortisol secretion in response to an acute stressor or multiple stressors has been primarily conducted in laboratory settings. While this allows for the standardization of stressors (e.g., mental arithmetic, public speaking), generalization to real-life stress conditions is limited (Biondi & Picardi, 1999
). Field studies have focused on the occurrence of stressful events experienced in daily life, ranging from minor hassles to major traumatic events. Studies featuring momentary collection of cortisol in natural settings are predicated on the notion that more information is needed about psychoendocrinological responses to stressors encountered in daily life to better understand the mechanisms through which stress leads to disorders (Jacobs et al., 2007
Ecological momentary assessment (EMA) refers to a variety of methods that involve repeated sampling (usually multiple times during a day across several or more days) of current behaviors and experience in real time and in natural environments. This approach was developed, in part, in response to the limitations of retrospective recall. EMA maximizes ecological validity, reduces recall bias, and allows for the study of “microprocesses” that influence behavior in real-world settings (Shiffman, Stone, & Hufford, 2008
). Momentary assessment captures what the respondent is doing or feeling at the moment
. This approach is also very useful when examining biological factors, such as salivary cortisol, that change in response to factors such as daily stressors and time of day. Capturing behavior in context is particularly important for understanding the association of daily stressors with cortisol and mood. Because EMA involves the collection of data in the respondent's natural environment, generalization to the real-world and real-life is enhanced, in contrast to laboratory studies.
The study of HPA reactivity to stress has several important applications in the context of SCI. Although clinical experience suggests that persons with SCI experience elevated levels of stress related to the consequences and demands of injury, there are few studies that explicitly measure daily stressors, prospectively or otherwise; most studies typically measure perceived stress which is conceptualized as a global
perception of burden (Cohen, Kamarck, & Mermelstein, 1983
). Studies have shown that global perceived stress is not associated with injury characteristics or level of physical independence but, instead, to adjustment and coping (Gerhart, Weitzenkamp, Kennedy, Glass, & Charlifue, 1999
). Global perceived stress has been related to depressive symptoms and anxiety in men with SCI, with low levels of social support also implicated in increased vulnerability to the negative impact of stress on psychological well-being (Rintala, Robinson-Whelen, & Matamoros, 2005
). While global assessment of stress and its association with physiological parameters is valuable for understanding link between chronic stress and disease in the context of allostatic load and the consequence of chronic stress conditions, global assessments, in general, limit our understanding of dynamic changes in behavior across time and situations (Shiffman et al., 2008
) and their effect on physiological outcomes such as cortisol responsivity.
One of the most important yet unanswered questions with respect to SCI and cortisol secretion is: to what degree do the direct effects of the injury on bodily systems involved in the stress response alter cortisol secretion in response to stress, particularly in the natural environment? Studies of cortisol amplitude in SCI have produced conflicting results of low, normal, and high circulating concentrations of cortisol, with most studies collecting only one or two time points to establish diurnal variation (Zeitzer, Ayas, Shea, Brown, & Czeisler, 2000
). Differences in overall levels of cortisol between persons with and without SCI have not been consistently found (Campagnolo, Bartlett, Chatterton, & Keller, 1999
; Huang, Wang, & Chen, 2000
; Zeitzer et al., 2000
) and very few studies have compared diurnal variation among persons with SCI with that of their peers without SCI (Zeitzer et al., 2000
). In one of the few investigations of adrenal function and psychological outcomes in the context of SCI, in a small sample of persons with SCI meeting criteria for major depressive disorder (MDD), a dexamethasone suppression test, used to assess HPA function in psychiatric disorders, lacked adequate sensitivity and specificity for MDD in the sample (Frank, Kashani, Wonderlich, Lising, & Visot, 1985
). Examination of associations of cortisol secretion and the corresponding experience of acute psychological stressors in either laboratory or natural settings or the effects of chronic psychological stress for persons with SCI has not been reported.
Given how little is known about HPA responsivity to stress in the context of SCI and implications for the impact of dysregulation on health, the primary aim of this pilot study was to examine cortisol secretion in response to daily stressors in a natural setting among persons with SCI using EMA. The following questions (Q) were addressed in this study: 1) does the diurnal pattern of cortisol secretion of persons with SCI correspond with expected elevation in the morning and decline toward evening (Q1); 2) is this pattern significantly different from persons without SCI (Q2); 3) is there a difference in the mean level of cortisol secretion between persons with SCI and without SCI (Q3); and 4) is there a difference in the experience of daily stressors and their effect on cortisol secretion and mood between persons with SCI and without SCI (Q4).